1G (first-generation) wireless

The first generation of wireless communication, which is characterized by analog transmission systems. Key 1G standards included AMPS, TACS, JTACS, and NMT.



Thin coax Ethernet cable.



Thick coax Ethernet cable.



Two strands of multimode optical fiber Ethernet cable.



2-pair UTP Ethernet cable.


10GigE (10 Gigabit Ethernet)

An Ethernet standard that supports a data rate of 10Gbps. Sometimes referred to as 10Gbps Ethernet.



Two strands of multimode optical fiber Fast Ethernet cable.



2-pair Cat 3 UTP Fast Ethernet cable.



4-pair Cat 3 UTP Fast Ethernet cable.



2-pair Cat 5 UTP Fast Ethernet cable.



Coax patch cable/Gigabit Ethernet cable.



Long-wavelength single-mode optical fiber Gigabit Ethernet cable.



Short-wavelength multimode optical fiber Gigabit Ethernet cable.



4-pair Cat 5 or Cat 5e UTP Gigabit Ethernet cable.



2-pair Cat 6 (currently a TIA draft proposal) Gigabit Ethernet cable.


2B1Q (2 Binary 1 Quaternary)

A single-carrier modulation scheme that provides for 2 bits/Hz and is used in ISDN, HDSL, and IDSL.


2G (second-generation) wireless

The second generation of wireless communication, which introduced digital transmission and includes digital cellular and PCS systems, such as TDMA (ANSI-136), GSM, and IS-95.


2.5G (2.5-generation) wireless

The generation of wireless communication between 2G and 3G, which offers enhancements to the data services on existing second-generation digital platforms. 2.5G can support faster data rates, ranging from 64Kbps to 384Kbps, depending on the standard and the technology.



The North American and Japanese infrastructure for PRI, which provides 23 64Kbps B-channels for information and 1 64Kbps D-channel for signaling and additional packet data.


3DES (Triple DES)

A 168-bit encryption technique that uses three 56-bit keys. 3DES applies the DES algorithm to a plaintext block three times.


3G (third-generation) wireless

The third generation of wireless communication, which includes digital transmission; it also permits per-user and terminal mobility by providing a single mobile communication service, adjusted for broadband applications (including voice, data, and multimedia streams), that will support higher data speeds, 2Mbps, with the objective of ultimately supporting up to 155Mbps.


3.5G (3.5-generation) wireless

An intermediate third generation of wireless technology after 3G, which is introducing enhancements designed for services running over an IP backbone, including high-speed data and video. Key 3.5G standards include HSDPA, HSUPA, and HSOPA. 3.5G will offer data speeds up to 10Mbps and possibly up to 20Mbps.


3GPP (3G Partnership Project)

A group formed in 1998, under the ETSI, to make a globally applicable 3G mobile phone system specification within the scope of the ITU's IMT-2000 project.


3GPP2 (3G Partnership Project 2)

A group formed when ETSI refused to expand the scope of 3GPP to address CDMA2000. ANSI formed 3GPP2 to coordinate CDMA2000 developments.



The ITU infrastructure for PRI, which provides 30 64Kbps B-channels and 1 64Kbps D-channel.


4G (fourth-generation) wireless

The fourth generation of wireless communication, which will support a wide range of data rates, promising a maximum of 50Mbps to 100Mbps while moving and an average of 20Mbps and up to 1Gbps while standing still. The two key technologies involved with 4G are OFDM and MIMO.


5G (fifth-generation) wireless

The fifth generation of wireless communication, which will enable fast downloads of large chunks of data across the Internet and will support advanced multimedia applications, such as teleimmersion, virtual reality, and telerobotics.



An experimental network used as an environment for IPv6 research. More than 1,173 networks in some 60 countries were connected to the 6bone IPv6 network. 6bone ceased to operate in 2006, and IANA reclaimed all 6bone prefixes.



A technique for enabling IPv4-to-IPv6 interworking and coexistence that interconnects IPv6 networks over IPv4 networks without explicitly defined tunnels.


8-PSK (Eight Phase-Shift Keying)

A PSK modulation scheme in which 3 bits at a time are encoded.



A (access) link

A link that interconnects an STP with either an SSP or an SCP. The SSP and SCP, collectively, are referred to as the signaling endpoints. A message sent to and from the SSPs or SCPs first goes to its home STP, which in turn processes or routes the message.


AAA (Authentication, Authorization, and Accounting)

A network server used for access control. Authentication identifies the user, authorization implements policies that determine which resources and services a valid user may access, and accounting keeps track of time and data resources used for billing and analysis.


AAL (ATM Adaptation Layer)

The ATM layer that is responsible for the adaptation of the information of the higher layer to the ATM cells. It is composed of two layers, the Segmentation and Reassembly sublayer and the Convergence sublayer. ATM adaptation layer 1 supports CBR voice and video network services. ATM adaptation layer 2 supports VBR voice and video network services. ATM adaptation layer 3 supports VBR connection-oriented data services. ATM adaptation layer 4 supports VBR connectionless data services. ATM adaptation layer 5 supports connectionless VBR data (e.g., IP or signaling) over ATM. The native stream (whether it's real-time, analog, voice, MPEG-2 compressed video, or TCP/IP) goes through the AAL, where it is segmented into 48-byte cells.


ABR (Available Bit Rate)

One of the ATM service classes. ABR supports VBR data traffic with average and peak traffic parameters (e.g., LAN interconnection and internetworking services, LAN emulation, critical data transfer that requires service guarantees). Remote procedure calls, distributed file services, and computer process swapping and paging are examples of applications that would be appropriate for ABR.


access charge

A cost assessed to interexchange carriers for access to the local exchange network.


access concentrator

A device that can be used to concentrate local subscriber lines and multiplex them over high-speed transport to another point in the network, creating a virtual POP.


access line

A connection from the customer to the local telephone company for access to the PSTN, also known as the local loop; can also refer to the connection between the serving toll exchange and the serving office of the interexchange carrier used to access PSTN transport services.


ACK (Acknowledge Character)

A transmission control character transmitted by a station as an affirmative response to the station with which a connection has been set up. An acknowledge character may also be used as an accuracy control character.


adaptive equalization

Line equalization, sometimes known as impedance equalization, used for optimizing signal transmission to adapt to changing line characteristics.


adaptive routing

Routing that automatically adjusts to network changes such as traffic pattern changes or failures.



(1) A coded representation of the destination of data, as well as of its source. Multiple terminals on one communications line, for example, must each have a unique address. (2) A group of digits that makes up a telephone number. Also known as the called number. (3) In software, a location that can be specifically referred to in a program. (4) A name, label, or number that identifies a location in storage, a device in a network, or any other data source.


address signals

Signals that carry information that has to do with the number dialed, which essentially consists of country codes, city codes, area codes, prefixes, and the subscriber number.


ADM (Add/Drop Multiplexer)

A device that facilitates easy dropping and adding of payload by converting one or more lower-level signals, such as T-1 or E-1 signals, to and from one of the optical carrier levels.


administrative domain

A collection of hosts, routers, and networks governed by a single administrative authority.


ADPCM (Adaptive Differential Pulse Code Modulation)

An encoding technique, standardized by the ITU-T, that allows analog voice signals to be carried on a 32Kbps digital channel. The voice input is samples at 8KHz with 4 bits used to describe the difference between adjacent samples.


ADSL (Asymmetrical Digital Subscriber Line)

A technology for supporting high bandwidth over conventional twisted-pair local loop lines that enables subscribers to access multimedia-based applications such as video-on-demand. ADSL supports a downstream channel of up to 7Mbps, with an upstream channel of up to 800Kbps.



A technology standardized under ITU G.992.1 that supports a downstream channel of up to 12Mbps and an upstream channel of up to 1Mbps.



A technology standardized under ITU G.992.5 that supports a downstream channel of up to 24Mbps and an upstream channel of up to 1Mbps.



A reach-extended version of the ADSL2 specification that is standardized under ITU G.992.3. ADSL2-RE allows DSL systems to reach up to 3.75 miles (6 km). While it can support up to 8Mbps downstream and 1Mbps upstream, when taking advantage of its main feature, ADSL2-RE extends a 768Kbps downstream service by approximately 0.5 mile (1 km), to 3.5 miles (5.5 km).


AES (Advanced Encryption Standard)

An encryption algorithm for securing sensitive material, both unclassified and classified, by U.S. government agencies; replaces the previously sanctioned standard, DES. It may eventually become the de facto encryption standard for commercial transactions in the private sector. AES uses the Rijndael algorithm to specify three key lengths128 bits, 192 bits, and 256 bits.



Software that processes queries and sends responses on behalf of an application.


agnostic device

A device that supports multiple data protocols (e.g., IP, Frame Relay, ATM, MPLS) and supports multiple traffic types, such as voice, data, and video.


AIN (Advanced Intelligent Network)

The second generation of intelligent networks, pioneered by Bellcore (which is now Telcordia). An AIN is a service-independent network architecture that enables carriers to create and uniformly support telecom services and features via a common architectural platform, with the objective of allowing for rapid creation of customizable telecommunication services.


AIP (Application Infrastructure Provider)

A provider that manages the data center servers, databases, switches, and other gears on which applications run.


air interface

A term used in mobile communications to refer to the radio frequency portion of the circuit between the mobile station and the base station.


A-law encoding

Encoding, according to ITU-T Recommendation G.711, that is used with European 30-channel PCM systems that comply with ITU-T Recommendation G.732. It employs nonuniform quantization to obtain the desired compression characteristic.


alerting signals

The ringing tones, busy tones, and any specific busy alerts used to indicate network congestion or unavailability.


alternate routing

Routing of a call or message over a substitute route when an established route has failed, is busy, or is otherwise unavailable for immediate use.


AM (Amplitude Modulation)

Varying of a carrier signal's strength (amplitude) depending on whether the information being transmitted is a 1 bit or a 0 bit.


ambient noise

Communications interference that is present in a signal path at all times.


AMC (Adaptive Modulation and Coding)

A technique used in HSDPA that allows the modulation and coding schemes to be determined dynamically, depending on the conditions of the wireless channel at that time.


AMI (Alternate Mark Inversion)

An encoding method used with Dataphone Digital Service, the oldest data service still in use, which uses 64Kbps channels. AMI requires the use of 8Kbps of the 64Kbps of each channel to maintain synchronization, which means that in reality only 56Kbps is available for data transmission. The signal carrying the binary value alternates between positive and negative polarities; 0 and 1 values are represented by the signal amplitude at either polarity; no-value "spaces" are at 0 amplitude. Also called bipolar.



A device that boosts an attenuated signal back up to its original power level so that it can continue to make its way across the network.



A measure of the height of a wave, which indicates the strength of the signal.


AMPS (Advanced Mobile Phone System)

A standard for analog telephony that is deployed widely in the United States.



A signal that varies continuously (e.g., sound waves), along two parameters: amplitude (strength) and frequency (tone). The unit of measurement is the Hertz (Hz), or cycle per second.


analog loopback

A technique for testing transmission equipment and devices that isolates faults to the analog signal receiving or transmitting circuitry, and a device, such as a modem, echoes back a received (test) signal that is then compared with the original signal.


ANI (Automatic Number Identification)

A feature, often associated with SS7, that passes a caller's telephone number over the network to the receiver so that the caller can be identified. Also referred to as caller ID.


ANSI (American National Standards Institute)

A standards-forming body affiliated with the ISO that develops U.S. standards for transmission codes, protocols, media, and high-level languages, among other things.



A U.S. standard for electronic data interchange.


answer signal

A supervisory signal (usually in the form of a closed loop) from the called telephone to the exchange and back to the calling telephone (usually in the form of a reverse battery) when the called number answers.



In IP routing, the point-to-point flow of packets between a single client and the nearest destination server identified by an anycast address. Anycast involves a one-to-many association in which each destination address identifies a group of receivers; at any given time, only one of them is chosen to receive information from any given sender.


AODV (Ad Hoc, On Demand, Distance Vector)

The leading protocol for routing data across wireless mesh networks. It is designed with mobile wireless devices in mind. It is in the public domain and is therefore subject to no copyright protection.


API (Application Programming Interface)

A set of routines that an application program uses to request and carry out low-level services performed by the operating system.



Apple Computer's set of specifications for connecting computers and other devices to share information over LANs. It describes network hardware, software, and protocols and lets an assortment of Mac and non-Mac devices communicate over a variety of transceivers and communications media.



(1) Software with which the user interacts. (2) The use to which a system is put (e.g., e-mail, videoconferencing, high-speed data access, network management).


application layer

Layer 7 of the OSI model, which enables users to transfer files, send e-mail, and perform other functions that involve interaction with network components and services.


application-layer multicasting

A technique that ensures that just one stream goes across the backbone whenever possible.


application program

A software program that contains no I/O coding (except in the form of macro instructions that transfer control to the supervisory programs) and is usually unique to one type of application.



The physical interrelationship between the components of a computer or a network.


area code

A three-digit code designating a toll center that is not in the NPA of the calling party.


area code restriction

The capability of switching equipment to selectively identify three-digit area codes and to either permit or deny passage of long-distance calls to those specific area codes.


ARP (Address Resolution Protocol)

A protocol that determines the physical address of a node, given that node's IP address. ARP is the mapping link between IP addresses and the MAC address.


ARQ (Automatic Repeat Request)

An error control technique that requires retransmission of a data block that contains detected errors. A special form, called "go-back-n," allows multiple blocks to be acknowledged with a single response. "Stop and wait" requires an acknowledgment after each block.


artificial intelligence

The capability of a computer to perform functions associated with human logic such as reasoning, learning, and self-improvement.


ASCII (American Standard Code for Information Interchange)

The code developed by ANSI for information interchange among data-processing systems, data communications systems, and associated equipment. The ASCII character set consists of 7-bit coded characters (8 bits, including the parity bit), providing 128 possible characters. The ASCII character set consists of 34 control codes and 94 text characters, including the letters of the alphabet in both upper- and lowercase, the 10 digits, and a number of special characters.


ASIC (Application-Specific Integrated Circuit)

An integrated circuit that is customized for a particular use (e.g., to run a mobile phone), rather than intended for general-purpose use.


ASON (Automatic Switched Optical Network)

ANSI specification ITU G.8080, which describes a network based on a technology that enables the automatic delivery of transport services. ASON describes the set of control-plane components used to manipulate transport network resources in order to provide the functionality of setting up, maintaining, and releasing connections.


ASP (Application Service Provider)

A supplier that makes applications available on a subscription basis.


aspect ratio

The horizontal:vertical size ratio used for television. Traditional television has an aspect ratio of 4:3, and DTV has a 16:9 aspect ratio, which more closely resembles human vision.


ASTN (Automatic Switched Transport Network)

ANSI specification ITU G.807, which describes a network that allows traffic paths to be set up automatically through a switched network. ASTN allows the user to specify the start point, endpoint, and bandwidth required.



(1) Occurring without a regular or predictable time relationship to a specified event. (2) In data communications, a method of transmission in which the bits representing a character are preceded by a start bit and followed by a stop bit, which are used to separate the characters and to synchronize the receiving with the transmitting station. It does not use a regular time relationship between the sending and receiving devices.


asynchronous transmission

A transmission in which each information character, or sometimes each word or small block, is individually synchronized, usually by the use of start and stop elements. Also called startstop transmission.


ATM (Asynchronous Transfer Mode)

An international packet-switching standard that uses a cell-based approach in which each packet of information features a uniform cell size of 53 bytes. ATM is a high-bandwidth, fast packet-switching and multiplexing technique that allows the seamless end-to-end transmission of voice, data, image, and video traffic. It is a high-capacity, low-latency switching fabric that is adaptable for multiservice and multirate connections and offers an architected approach to QoS.


ATM layer

A layer that performs four main functions: multiplexing and demultiplexing of cells of different connections, translation at ATM switches and cross-connects, cell header extraction or addition before or after a cell is delivered to or from the adaptation layer, and flow control.


ATM physical layer

A layer that is composed of two sublayers: Physical Medium, which supports pure medium-dependent functions, and Transmission Convergence, which converts the ATM cell stream into bits to be transported over the physical medium.


ATM reassembly

Restructuring of data units from information contained in cells.


ATM segmentation

Parsing of the information units of the higher layers into ATM cells.


ATSC (Advanced Television Systems Committee)

An organization that establishes voluntary technical standards for advanced television systems, including DTV.



A decrease in the power of a received signal due to loss through lines, equipment, or other transmission devices. It is usually measured in decibels.


audible ringing tone

A tone received by the calling telephone indicating that the called telephone is being rung (formerly called ringback tone).


audio frequencies

Frequencies that correspond to those that can be heard by the human ear (usually 30Hz to 20,000Hz).


AUI (Attachment Unit Interface)

The connector used to attach a device to an Ethernet transceiver.



A technique that enables the receiver to automatically identify and reject messages that have been altered deliberately or by channel errors. Authentication also can be used to provide positive identification of the sender of a message.


autonomous system

A collection of TCP/IP gateways and networks that fall under one administrative entity and cooperate closely to propagate network reachability (and routing) information among themselves, using an interior gateway protocol. Sometimes abbreviated AS.



B8ZS (Bipolar 8-Zero Substitution)

An encoding method used on T-1 circuits (in the United States and Japan) that inserts two successive ones of the same voltage into a signal whenever eight consecutive zeros are transmitted. B8ZS is not compatible with older AMI equipment. In Europe, E-1 uses the encoding scheme HDB3 instead of B8ZS. Also called binary 8-zero substitution, clear channel, and clear 64.


B (bidirectional) frame

The part of the MPEG video compression process in which both past and future pictures/frames are used as references. B frames typically result in the most compression.


B/D link (Bridge/Diagonal link)

A link that interconnects two mated pairs of STP. It carries signaling messages beyond the initial point of entry to the signaling network, toward the intended destination.



A central network that connects several other, usually lower-bandwidth, networks so that those networks can pass data to each other. The backbone network is usually composed of a high-capacity communications medium, such as fiber optics or microwave.



The practice of routing telecommunications traffic beyond its intended destination and then back to the intended destination, usually to take advantage of tariffs or prices that are lower than those afforded by direct routing. In telecommunications, the term has evolved into a more generic meaning used to define a way to get data to a point from which it can be distributed over a network. In satellite technology, backhaul means to transmit data to a point from which it can be uplinked to a satellite. In the context of broadcast TV, backhaul refers to program content that is transmitted to a TV station or receiving entity, where it will be integrated into a finished show.



The board that contains a bus.



The provision of facilities, logical or physical, to speed the process of restart and recovery following failure.


backup copy

A copy of information, usually on a floppy disk, zip disk, or CD-ROM, that is kept and can be used if the original information is lost or destroyed.


bandpass filter

A circuit that is designed to allow a single band of frequencies to pass, neither of the cutoff frequencies being zero or infinity.



The transmission capacity of a telecommunications pathway, electronic or optical. Bandwidth refers to the range of frequencies, expressed in Hertz (Hz), that comprise a given transmission channel; in other words, it is the difference between the lowest and highest frequencies carried on the channel. The bandwidth determines the rate at which information can be transmitted through a circuit. The greater the bandwidth, the more data that can be carried, and in digital facilities, bandwidth is expressed in bits per second.


bandwidth efficiency

The efficiency with which a radio signal can be encoded and, therefore, how many bits per second can be carried on one cycle of radio bandwidth.


bandwidth exchange

An organization or a facility that functions as an exchange where bandwidth is the commodity. Some exchanges bring together buyers and sellers of bandwidth and facilitate contract negotiations and transactions; other exchanges actually switch traffic in real-time based on changes in bandwidth prices throughout the course of the day.



A concept in wide area networking in which the user can access additional WAN bandwidth as the application warrants. It enables users to pay for only the bandwidth they use, when they use it.


Barker Code

An 11-bit chipping code. The 1 bits are encoded as a particular sequence of 1s and 0s, and the 0 bits are the inverse of that sequence.


base station controller

An intermediate device in the cellular system that controls a group of base-station transceivers.


baseband signaling

Transmission of a digital signal without any additional modulation, or conversion. The term is typically used to refer to the pure digital side of a circuit when the other side is broadband, or frequency based, which means that the signal is modulated.


batch processing

A processing method in which a program or programs process data with little or no operator interaction.



The signaling rate of a transmission line, which refers to the number of transitions (voltage or frequency changes) made per second. The term is often confused with the number of bits per second that can be supported. At very low speeds and with modulation schemes, there is 1 bit per baud, so the number of baud can equal the number of bits per second. However, modern devices, using more efficient modulation schemes, carry multiple bits per signal; for example, with 64-QAM, there are 6 bits per baud, resulting in higher-speed transmissions.


Baudot code

A data code that uses a 5-bit structure and was used on vintage teleprinters (e.g., Telex).


BcN (Broadband Convergence Network)

An ETRI initiative designed to provide Internet access at speeds of 50Mbps to 100Mbps, about 50 times faster than current conventional services, with nationwide Korean coverage.


beacon frame

A frame sent by a token-ring adapter indicating that it has detected a serious problem. An adapter sending such frames is said to be beaconing.


BECN (Backward Explicit Congestion Notification)

A bit field in a Frame Relay header that the network uses to inform the transmitter of network congestion and the need to initiate a congestion avoidance procedure.


BER (Bit Error Rate)

In data communications testing, the ratio between the total number of bits transmitted in a given message and the number of bits in that message received in error. The BER is a measure of the quality of a data transmission, usually expressed as number as a power of 10 (e.g., 1 bit error in 105 bits transmitted, or 1 in 100,000).


best-effort QoS

QoS that is not guaranteed but is as good as possible under the circumstances.


beta test

The stage at which a new product is tested under actual usage conditions.


Beyond 3G

An enhancement to 3G that involves today's 3G technologies but supports bandwidths greater than 5MHz and adds smarter and more efficient IP-based back-end infrastructure and additional one-way or two-way airlinks to provide further capabilities. Also referred to as Super 3G and Ultra 3G.


BFWA (Broadband Fixed Wireless Access)

A wireless broadband technology that offers high capacity. There are two categories of BFWA standards: high-frequency BFWA (which is focused on broadband services, including real-time video, streaming video, and video transfer and operates on frequencies above 25GHz) and low-frequency BFWA (which is used mainly for data communications and operates on frequencies between 2GHz and 11GHz).


BGP (Border Gateway Protocol)

A gateway protocol that allows routers to communicate with each other. BGP is an exterior routing protocol used between autonomous systems and is of concern to service providers and other large or complex organizations. BGP4, the latest version of BGP, provides mechanisms for supporting (CIDR) and allowing aggregation of routes, including aggregation of autonomous system paths.



As described in RFC 2547, a VPN in which BGP is used to distribute VPN routing information across the provider's backbone and MPLS is used to forward VPN traffic from one VPN site to another.



A base-two system of numbers; the binary digits are 0 and 1.



The predominant signaling method used for digital transmission services, such as DDS and T-1, in which the signal carrying the binary values successfully alternates between positive and negative polarities. The 1 values are represented by the signal amplitude at either polarity, and no-value "spaces" are at 0 amplitude.


bipolar violation

An encoding method used on T-1 circuits that inserts two successive 1s of the same voltage. The device receiving the signal interprets the bipolar violation as a timing mark, which keeps the transmitting and receiving devices synchronized.


B-ISDN (Broadband ISDN)

A standard that was envisioned for use with advanced applications. SDH/SONET and ATM were both born out of the B-ISDN standard and a desire to deliver advanced applications.



The smallest unit of information in a digital device. In binary notation, a bit is either the character 0 or the character 1.


bit duration

The time it takes an encoded bit to pass a point on the transmission medium. In serial communications, a relative unit of time measurement used for comparison of delay times, where the data rate of a transmission channel can vary.


bit errors

Missing video elements, synchronization problems, or complete loss of picture.



A pixel-by-pixel description of an image in which each pixel is a separate element.


bit-oriented protocol

A communications protocol or transmission procedure in which control information in encoded in fields of one or more bits.


bit rate

The speed at which bits are transmitted, usually expressed in bits per second (bps).



A network in which there are connection sets that prevent some additional desired connections from being set up between unused ports, even with rearrangement of existing connections.



A 64-bit block code that has key lengths of 32 to 448 bits. Blowfish is used in more than 100 products, and it is viewed as one of the best available encryption algorithms.



A WPAN technology that is an open standard for short-range transmission of digital voice and data that supports point-to-point and multipoint applications, providing up to 720Kbps data transfer with a range of 33 feet (10 m) and up to 330 feet (100 m) with a power boost. A Bluetooth chip is a very low-cost chip that gives a device short-range wireless capability. PDAs, laptops, mobile phones, and other intelligent appliances embedded with Bluetooth chips can communicate and link with each other wirelessly.


BNC connector

A commonly used plug and socket that provides a tight connection for audio, video, and networking applications. Thinnet uses BNC connectors.



A circuit card on which integrated circuits are mounted.


BPL (Broadband Over Powerlines)

Another term for PLT.


bps (bits per second)

A measure of the data transfer rate possible on a digital communications line. For example, a 10Gbps backbone can support 10 billion bits per second. Bytes are used as a measure of storage, so a 10GB hard drive is not the same thing as a 10Gbps communications link. The abbreviation Kbps is used for thousands of bits per second; Mbps is used for millions of bits per second; Gbps is used for billions of bits per second; Tbps is used for trillions of bits per second; Pbps is used for 1,000Tbps; and Ebps is used for 1 billion Gbps.


BPSK (Binary Phase-Shift Keying)

The simplest and most robust form of PSK. BPSK uses a 180-degree phase shift and is a good solution under noisy conditions.


BRAN (Broadband-Compliant Radio Access Networks)

An ETSI technical committee for broadband radio access networks. BRAN is responsible for all aspects of standardization for present and future broadband radio access networks, using both existing and emerging technologies. BRAN's three main standards areas are HiperLAN, HiperAccess, and HiperMAN.


BRI (Basic Rate Interface)

In ISDN, the interface to the basic rate, which is 2B+D: two 64Kbps information-carrying channels plus one 16Kbps signaling channel. Also called Basic Rate Access (BRA).



An attaching device that connects two LAN segments to allow the transfer of information from one LAN segment to the other. Bridges operate by filtering packets according to their destination addresses. Most bridges automatically learn where these addresses are located and, thus, are called learning bridges. A bridge works at OSI Layer 2 and is transparent to upper-layer devices and protocols.



A method of transmitting data, voice, and video by using FDM, as with cable TV. Modems are required to modulate digital data streams onto the channel. Broadband in this context is used in contrast with baseband, which is all-digital transmission and uses TDM. In simple terms, broadband refers to a multichannel, high-bandwidth transmission line. According to the ITU-T, broadband means any transmission rate over 2Mbps. The most common use of the term today is to refer to a channel capable of high-speed transmission. The term is often used to refer to Internet access via cable modems, DSL, fiber options, and wireless alternatives, all of which are much faster than dialup.



A transmission to multiple receiving locations simultaneously. A broadcast can be made, for example, over a multipoint line to all terminals that share the line or over a radio or television channel to all receivers tuned to that channel.


broadcast storm

A pathological network condition in which an increasing and insupportable number of broadcast packets are generated.



A device that can transparently bridge protocols as well as route them. It is a hybrid of a bridge and a router.


brownout operation

An operation in response to heavy demand, in which main system voltages are lowered and power is reduced but not lost. Although conventional networking equipment is relatively immune to brownouts, the computer controlling the system is very sensitive to voltage variations and could fail under these conditions. Most equipment today has the capability to cope with these reductions, or a heavy-duty power supply or UPS can be furnished.


BSC (Binary Synchronous Communications)

A half-duplex, character-oriented data communications protocol originated by IBM in 1964. It includes control characters and procedures for controlling the establishment of a valid connection and the transfer of data. Also called bisync.



A storage device used to compensate for a difference in rate of data flow, or time of occurrence events, when transmitting data from one device to another.


buffered network

A real-time store-and-forward message-switching network, with computers at the switching points that act as buffers for the packets.


buffered repeater

A hybrid device of a repeater and a bridge. Entire packets are received and retransmitted (as with a bridge), but no address filtering is implemented (as with a repeater).



A pricing strategy in which a service provider or manufacturer includes all productshardware, software, services, training, and the likein a single price.



In data communications, a sequence of signals counted as one unit in accordance with some specific criterion or measure.


burst switching

A switching method for switching digitized voice and data characters in an integrated fashion.



(1) A physical transmission path or channel. A bus is typically an electric connection, with one or more conductors, wherein all attached devices receive all transmissions at the same time. (2) A LAN topology, such as that used in Ethernet and token bus, in which all network nodes listen to all transmissions and select certain ones based on address identification. A bus involves some type of contention control mechanism for accessing the bus transmission medium.


bus topology

A network architecture in which all the nodes are connected to a shared cable.



To establish a communications link without using the facilities of the local exchange carrier (e.g., the telephone company).



The amount of storage required to represent one alphanumeric character, or 8 bits. Bytes are used as a measure of storage, as in a 2GB hard drive. This is different from the measurement for transmission capacity, which is expressed in bits per second (e.g., a 10Gbps backbone).



C7 (Common Channel Signaling 7)

A signaling protocol specified by the ITU-T and used in high-speed digital networks to provide communication between intelligent network nodes. C7 is the European equivalent of SS7. Also sometimes called CCS7, for Common Channel Signaling System 7.


C (cross) link

A link that interconnects mated STPs.


CA (Certificate Authority)

A trusted third-party organization that issues digital certificates used to create digital signatures and publicprivate key pairs. The CA guarantees that the individual granted the unique certificate is, in fact, who he or she claims to be.


cable modem

A device designed to operate over cable TV networks to provide high-speed access to the Internet.



A CableLabs HAN standard. The CableHome 1.1 specification includes gateway security features; standardized prioritized QoS for HANs; support for home servers, teleworkers, and home offices; simple parental control; and LAN management messaging and LAN IP statistics monitoring.



A nonprofit research and development consortium that is dedicated to helping its cable operator members integrate new cable telecommunications technologies into their business objectives. CableLabs has developed industry specifications such as DOCSIS, OpenCable, PacketCable, VOD Metadata, and CableHome.


call processing

A sequence of operations performed by a switching system from the acceptance of an incoming call through the final disposition of the call.


campus network

A network that connects LANs from multiple departments in a single building or campus. Campus networks are LANs; although they may span several miles, they do not include WAN services.


CAP (Carrierless Amplitude Phase Modulation)

A single-carrier modulation scheme used in early deployments of ADSL.


carrier frequency

The frequency of the carrier wave that is modulated to transmit signals.


carrier system

A means of obtaining a number of channels over a single path by modulating each channel on a different carrier frequency and demodulating at the receiving point to restore the signals to their original frequency.


Cat 1 (Category 1) UTP

An EIA/TIA 568 standard for commercial building telecommunications wiring. This old-style UTP telephone cable is unsuitable for data transmission.


Cat 2 (Category 2) UTP

An EIA/TIA 568 standard of cable that can be used for data rates up to 4Mbps.


Cat 3 (Category 3) UTP

An EIA/TIA 568 standard of cable that can be used for data rates up to 10Mbps. This is the minimum cable requirement for 10BASE-T.


Cat 4 (Category 4) UTP

An EIA/TIA 568 standard of cable that can be used as the lowest-grade UTP, acceptable for data rates up to 16Mbps (token ring).


Cat 5 (Category 5) UTP

An EIA/TIA 568 standard of cable that can be used for data rates up to 100Mbps.


Cat 5e (Category 5e) UTP

An EIA/TIA 568 standard of cable that provides performance of up to 125MHz and is frequently used for 1000BASE-T Gigabit Ethernet.


Cat 6 (Category 6) UTP

An EIA/TIA 568 standard of cable that provides performance of up to 400MHz, more than double that of Category 5 and 5e, and is used for 10BASE-T/100BASE-TX and 1000BASE-T (Gigabit Ethernet) connections.


Cat 7 (Category 7) UTP

An emerging category of cable that is expected to operate up to 600MHz and will use STP or ScTP.


CATV (Community Antenna Television)

Signals that can be received at a selected site by sensitive, directional antennas and then transmitted to subscribers via a cable network. Additional channels, not normally available in that area, can also be transmitted. Traditional analog CATV is based on RF transmission, generally using 75-ohm coaxial cable as the transmission medium. CATV offers multiple frequency-divided channels, allowing mixed transmissions to be carried simultaneously.



A portion of the electromagnetic spectrum, approximately 4GHz to 6GHz, that is used primarily for satellite and microwave transmission.


CBQ (Class-Based Queuing)

A fully open, nonproprietary technology that brings bandwidth-controlled CoS to IP network infrastructures. It allows traffic to be prioritized according to IP application type, IP address, protocol type, and other variables. It allocates unused bandwidth more effectively than other QoS mechanisms do, and it uses priority tables to give critical applications the most immediate access to unused bandwidth.


CBR (Constant Bit Rate)

The highest ATM service class. CBR provides a constant, guaranteed rate to real-time applications such as streaming video, providing continuous bandwidth. It emulates a circuit-switched approach and is associated with minimum latencies and losses.


CCITT (Comité Consultatif International de Téléphonie et de Télégraphie)

An advisory committee to the ITU whose recommendations covering telecommunications have international influence among engineers, manufacturers, and administrators. It is now known as the ITU-T.


CCK (Complementary Code Keying)

A modulation scheme used with 802.11b WLANs. CCK was adopted to replace the Barker Code in wireless digital networks.


CCS (Common-Channel Signaling)

An electronic means of signaling between two switching systems, independent of the voice path. The use of CCS makes possible new customer services, versatile network features, more flexible call routing, and faster connections.


CDDI (Copper-Distributed Data Interface)

A version of FDDI that runs on UTP cable rather than on fiber-optic cable.


CDM (Code Division Multiplexing)

A form of multiplexing that encodes data with a special code associated with each channel and uses the constructive interference properties of the special codes to perform the multiplexing.


CDMA (Code Division Multiple Access)

A digital cellular technology that uses spread spectrum techniques. With CDMA, every channel uses the full available spectrum, and individual conversations are encoded with a pseudorandom digital sequence or frequency-hopping schedule.



A family of 3G mobile standards that use CDMA to send voice, data, and signaling data between mobile phones and cell sites. CDMA2000 devices include color displays, GPS, digital and video cameras, push-to-talk, support for streaming-type real-time VOD/audio-on-demand services, and voice recognition functions. Also known as IMT-2000 CDMA Multi-Carrier (CDMA-MC).



A TIA/EIA-IS-CDMA2000 (CDMA-MC) system deployed at 450MHz. CDMA450 provides a larger cell size compared to other bands, which translates to fewer cell sites and significantly lower capital and operating expenditures to service vast coverage areas.



A 2G CDMA technology (IS-95) that offers data rates of 9.6Kbps to 14.4Kbps. cdmaOne describes a complete wireless system based on the TIA/EIA IS-95 CDMA standard and represents the end-to-end wireless system and all the necessary specifications that govern its operation. cdmaOne provides a family of related services, including cellular, PCS, and wireless local loop.


CDPD (Cellular Digital Packet Data)

A North American standard for transferring packet data over cellular phone channels.


CD-ROM (Compact DiscRead Only Memory)

A storage device that is used in computer systems and typically contains multimedia information.



A fixed-length packet.


cell relay

A form of packet transmission used by ATM networks. Cell relay transmits 53 octet fixed-length packets over a packet-switched network. Because the cells are tiny and of fixed length, they can be processed and switched at very high speeds. ATM makes it possible to use a single transmission scheme for voice, data, and video traffic on LANs and WANs.



A communication service in which voice or data is transmitted by radio frequencies. The service area is divided into cells, each served by a transmitter. The cells are connected to a mobile switching exchange, which is connected to the worldwide telephone network.


CELP (Code-Excited Linear Prediction)

A vector-quantization-based compression scheme for speech. CELP can compress speech down to 4.8Kbps. There is also a low-end variant called LPC.



A local exchange carrier service in which switching occurs at a local exchange rather than at customer-owned PBXs. The telephone company owns and manages all the communications equipment.


CEPT (Conférence Européenne des Administrations des Postes et des Télecommunications [European Conference of Postal and Telecommunications Administrations])

An organization formed by the European PTTs for the discussion of operational and tariff matters. CEPT-0 (or E-0) is the basic increment, and it operates at 64Kbps. CEPT-1 is a 2.048Mbps 32-channel circuit; CEPT-2 is an 8.488Mbps 128-channel circuit; CEPT-3 is a 34.368Mbps 512-channel circuit; CEPT-4 is a 139.246Mbps 2,048-channel circuit; CEPT-5 is a 565.148Mbps 8,192-channel circuit.


CERN (Conseil Européen pour la Recherche Nucléaire [European Organization for Nuclear Research])

The world's largest particle physics laboratory and the birthplace of the World Wide Web.



A logical conversation path. A channel is the frequency band, time slot, or wavelength (also referred to as lambda) over which a given conversation flows.


channel bank

Equipment typically used in a telephone exchange that performs multiplexing of lower-speed, digital channels into a higher-speed composite channel. The channel bank also detects and transmits signaling information for each channel and transmits framing information so that time slots allocated to each channel can be identified by the receiver.


channel capacity

The maximum data traffic that a channel can handle.


CHAP (Challenge Handshake Authentication Protocol)

A protocol that uses a three-way handshake to periodically verify the identity of the peer throughout the life of the connection. The server sends to the remote workstation a random token that is encrypted with the user's password and sent back to the server. The server performs a lookup to see if it recognizes the password. If the values match, the authentication is acknowledged; if not, the connection is terminated. A different token is provided each time a remote user dials in, which provides additional robustness.


chirped-pulse WDM

A Bell Labs system in which a specialized mode-locked laser rapidly emits very wide pulses of light. Because each part of a fiber interacts differently with varying frequencies of light, the result of chirped-pulse WDM is unequal dispersion. The pulse is stretched out when it enters the fiber, and data can be put on the discrete frequencies that emerge.


CIDR (Classless Interdomain Routing)

An IP addressing scheme that replaces the older system based on Classes A, B, and C. With CIDR, a single IP address can be used to designate many unique IP addresses. The CIDR addressing scheme is hierarchical. Large national and regional service providers are allocated large blocks of contiguous Internet addresses, which they then allocate to other smaller ISPs or directly to organizations. Networks can be divided into subnetworks, and networks can be combined into supernetworks, as long as they share a common network prefix.


CIR (Committed Information Rate)

The amount of bandwidth that a user can expect from a Frame Relay carrier on a particular virtual circuit.



The physical path that runs between two or more points that can be used for two-way communication or to perform another specific function.


circuit grade

The data-carrying capability of a circuit. The grades of circuit are broadband, voice, subvoice, and telegraph.


circuit switching

The temporary direct connection of two or more channels between two or more points in order to provide the user with exclusive use of an open channel with which to exchange information. A discrete circuit path is set up between the incoming and outgoing lines, in contrast to message switching and packet switching, in which no such physical path is established.



In fiber-optic cable, a low-refractive-index material that surrounds the core and provides optical insulation and protection to the core.


clear-forward/clear-back signal

A signal transmitted from one end of a subscriber line or trunk, in the forward/backward direction, to indicate at the other end that the established connection should be disconnected. Also called a disconnect signal.


CLEC (Competitive Local Exchange Carrier)

A telephone company that competes with an ILEC. CLECs in the United States today focus mainly on delivering dial tone to business customers.



A computer that requests network or application services from a server. A client has only one user; a server is shared by many users.


client/server model

The model of interaction in a distributed system in which a program at one site sends a request to a program at another site and awaits a response. The requesting program is called a client; the program satisfying the request is called the server.


CLNP (Connectionless Network Protocol)

The OSI protocol for OSI connectionless network service. CLNP is the OSI equivalent to IP.



The use of clock pulses to control synchronization of data and control characters.


closed user group

A group of users in a network who are permitted to communicate with each other but not with users outside the group.



Two or more terminals connected to a single point or node.


cluster controller

A device that handles the remote communications processing for multiple (usually dumb) terminals or workstations.


CMIP (Common Management Information Protocol)

The OSI management information protocol for network management. CMIP is an alternative to SNMP and is not widely implemented.


CMIS (Common Management Information Services)

A service interface created and standardized by the ISO for managing heterogeneous networks.


CMTS (Cable Modem Termination System)

A device on which coax trunks terminate. CMTSs are linked by a common Ethernet hub, which, in turn, feeds into the IP router, which then develops the optimum path to take over an optical backbone onto the ISP.


CO (Central Office)

The physical location where local service providers terminate subscriber lines and locate the switching equipment that interconnects those lines. CO is used as a term in North America; elsewhere in the world, it is also referred to as a local exchange or Class 5 office.


coax (coaxial cable)

A transmission medium that consists of one (sometimes more) central wire conductor, surrounded by a dielectric insulator and encased in either a wire mesh or extruded metal sheathing. There are many varieties of coax, depending on the degree of EMI shielding afforded, voltages, and frequencies accommodated.



The conventions that specify how data may be presented in a particular system.


code character

A set of conventional elements established by code to enable the transmission of a written character (letter, figure, punctuation sign, arithmetical sign, and so on) or the control of a particular function (spacing, shift, line-feed, carriage return, phase corrections, and so on).


codec (coder-decoder)

A device used to convert analog signals, such as speech, music, or television, to digital form for transmission over a digital medium and back again to the original analog form. One codec is required at each end of a channel.


coding scheme

A pattern of bits used to represent the characters in a character set, as well as carriage returns and other keyboard functions. Examples of coding schemes are ASCII, EBCDIC, and Unicode.


COFDM (Coded Orthogonal Frequency Division Multiplexing)

A complex technique that combines OFDM with error-correcting codes, adaptive equalization, and reconfigurable modulation to provide many beneficial properties, including resistance to multipath, phase distortion, fading, and burst noise. COFDM is used in Europe and other places where the DAB standard has been adopted for digital radio broadcasting and for terrestrial DTV. It is also used in ADSL transmission.



Overlapping transmissions that interfere with one another. A collision occurs when two or more devices attempt to transmit at or about the same instant.


collision domain

A small cluster in a LAN where collisions occur. Collision domains are used to reduce collisions throughout a network.



A signal or group of signals that cause a computer to execute an operation or series of operations.



Programs requiring that the task to be performed be described in a special language with strict adherence to syntax.


common-battery signaling

A method by which supervisory and telephone address information is sent to an exchange by depressing and releasing the switch on the cradle of the handset.


common carrier

An organization in the business of providing regulated telephone, telegraph, Telex, and data communications services.


common control

An automatic switching arrangement in which the control equipment necessary for the establishment of connections is shared and is associated with a given call only during the period required to accomplish the control function.


communication adapter

A hardware feature that permits telecommunication lines to be attached to the processor.


communication line

A link (e.g., wire, telephone circuit, microwave, satellite) used to transmit data between computers and/or remote devices.


communications controller

(1) A hardware device for attaching either communication lines, ASCII devices, or a LAN to the processing unit. (2) A dedicated device with special processing capabilities for organizing and checking data and handling information traffic to and from many remote terminals or computers, including functions such as message switching. Also called a communications processor.


communications satellite

A satellite designed to act as a telecommunications radio relay and usually positioned in geosynchronous orbit 23,000 miles (35,800 km) above the equator so that it appears from earth to be stationary.



A compressor at one point in a communications path for reducing the volume range of signals, followed by an expander at another point for restoring the original volume range. A compander is designed to improve the ratio of the signal to the interference entering in the path between the compressor and expander.



The application of any of several techniques to reduce the number of bits required to represent information in data transmission or storage, thereby conserving bandwidth and/or memory.



An electronic device that compresses the volume range of a signal.



(1) The linking of transmission channels (e.g., phone lines, coaxial cable, optical fiber) end to end. (2) The linking of SONET STS-1 frames in order to carry a broadband information stream.



A device that connects a number of circuits that are not all used at once to a smaller group of circuits for economical transmission. A telephone concentrator achieves the reduction with a circuit-switching mechanism. In data communications, a multiport repeater or hub brings together the connections from multiple network nodes. Concentrators have moved past their origins as wire concentration centers and often include bridging, routing, and management devices.



A procedure for making transmission impairments of a circuit lie within certain specified limits, typically used on telephone lines leased for data transmission to improve the possible transmission speed. Two types are used: C conditioning and D conditioning. Also called line conditioning.



The devices and programs that make up a system, subsystem, or network. The term configuration may refer to a hardware configuration or a software configuration.



To describe to the system the devices and optional features installed on the system and describe their utilization.


connect time

The amount of time that a circuit, typically in a circuit-switched environment, is in use.


connectionless network

A network that treats each packet or datagram as a separate entity that contains the source and destination address. Connectionless services can drop packets or deliver them out of sequence, based on encountering various network conditions, such as congestion or outages.


connection-oriented network

A network in which the connection setup is performed before information transfer occurs. The path is conceived at the outset, and after the path is determined, all the subsequent information follows the same path to the destination. In a connection-oriented network, there can be some delay up front, while the connection is being set up, but because the path is predetermined, there is no delay at intermediate nodes after the connection is set up.



A term used to describe the physical interconnections of multiple devices/ computers/networks employing similar or different technology and/or architecture together to accomplish effective communication between and among connected members involving data exchange and/or resource sharing.


contactless identification

A technology in which a smart card has an antenna embedded inside it, enabling communication with a card reader without physical contact. The chip on the smart card stores data and programs that are protected by advanced security features. Contactless smart cards are passed near an antenna, or reader, to carry out a transaction.


content delivery network

A network with delivery services that are structured specifically for a client and are focused on streaming audio, video, and media, as well as the supporting e-commerce applications.



A method of line control in which the terminals request permission to transmit. If the channel in question is free, transmission proceeds; if it is not free, the terminal has to wait until it becomes free. A computer can build up a queue of contention requests; this queue can be organized in a prearranged sequence or in the sequence in which requests are made.


control character

A character inserted into a data stream for signaling the receiving station to perform a function to identify the structure of the message. Newer protocols use bit-oriented control procedures.


control network

A type of HAN network for home automation and control that is typically a low-speed powerline network. Control networks have relatively low cost and simple implementation, but they are not designed to support real-time, high-bandwidth, or mobility requirements.


control plane

An operational part of a network, including infrastructure and distributed intelligence, that executes various signaling, routing, and other control protocols (e.g., OSPF, RIP, RSVP). The control plane protocols exchange data with their peers and generate control information such as routing tables that is needed to forward data packets. Additional functions include managing user interfaces and monitoring system status and health.


control station

The station in a point-to-point or multipoint network that controls the sending and receiving of data. This station can poll or address tributary stations.


control unit

Circuitry or a device used to coordinate and control the operation of one or more I/O or storage devices and to synchronize the operation of such devices with the operation of the computer system as a whole.


controlled access unit

A managed MAU or a managed multiport siring hub for token-ring networks.



The trend for multiple network technologies and products to come together to form one network with the advantages of all the technologies and products, reducing costs and simplifying operations, administration, and management. In telecommunications, convergence can refer to network infrastructure convergence, device convergence, and applications convergence.



The process of changing from one method to another. It may refer to changing processing methods, data, or systems.


COPS (Common Open Policy Service)

An IETF query/response-based client/server protocol for supporting policy control. It addresses how servers and clients on a network exchange policy information, and it transmits information between a policy server and its clients, which are policy-aware devices such as switches.



The central part of a network.


CoS (Class of Service)

A categorization of subscribers or traffic according to priority levels. Network resources are allocated based on the CoS.


CPE (Customer Premises Equipment)

Equipment that is located at the customer premises and is owned and managed by the customer.


CR (Cognitive Radio)

A forthcoming device that can seamlessly switch call and transmission modes to whatever makes the most sense, given the location at a given time, whether cellular, Wi-Fi, or VoIP. CR is a smart system in which a radio device and its antenna can adapt their spectrum use in response to their operating environment, including the radio frequency spectrum, user behavior, and network state.


CRC (Cyclic Redundancy Check)

A powerful error-detecting technique. By using a polynomial, a series of two 8-bit block-check characters are generated that represent the entire block of data. The block-check characters are incorporated into the transmission frame and then checked at the receiving end.


CR-LDP (Constraint-Based Routed Label Distribution Protocol)

A protocol in the MPLS architecture that can set up paths to meet traffic-engineering requirements.


crossbar switch

A switch that has a crosspoint for each input/output pair, and only one contact pair needs to be closed to establish an input to output connection.



Interference or an unwanted signal from one transmission circuit detected on another, usually parallel, circuit.


CRTP (Compressed Real-Time Transport Protocol)

RTP compression that affects IP/UDP/RTP headers.


CSD (Circuit-Switched Data)

The original form of data transmission developed for TDMA-based mobile phone systems such as GSM.


CSMA (Carrier Sense Multiple Access)

A LAN access technique in which multiple stations connected to the same channel can sense transmission activity on that channel and defer the initiation of transmission while the channel is active. CSMA is similar to contention access.


CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)

A scheme for controlling network traffic that enables any of multiple nodes to send information over a shared network cable if the cable is free. It avoids collisions by having all nodes signal their intention to transmit before transmitting. If two nodes send intentions to transmit messages at the same time, both nodes wait for random amounts of time before trying again.


CSMA/CD (Carrier Sense Multiple Access/Collision Detection)

A LAN protocol that is a refinement of CSMA in which stations can detect the interference caused by simultaneous transmissions by two or more stations (collisions) and retransmit colliding messages in an orderly manner.


CSU (Channel Service Unit)

A component of CPE used to terminate a digital circuit, such as a leased line or T-1/E-1 facility, at the customer site. A CSU performs certain line-conditioning functions and responds to loopback commands from the local exchange. It also ensures proper 1s density in a transmitted bitstream and performs bipolar violation correction.


CTS (Clear To Send)

A control circuit that indicates to the DTE that data can or cannot be transmitted.



The process of designing or configuring a device, installation, or network to meet the requirements of particular users.



The physical changing of lines from one system to another, usually at the time of a new system installation.


CVSD (Continuous Variable Slope Delta Modulation)

A speech encoding and digitizing technique that uses 1 bit to describe the change in the slope of the curve between two samples rather than the absolute change between the samples.


CWDM (Coarse Wavelength Division Multiplexing)

A WDM system that was specifically developed for metro area applications. CWDM is based on the same principle as DWDM of accommodating multiple wavelengths on a single fiber, but it uses less expensive lasers, making it cost-effective for metro deployments, cable TV, and enterprise networks. The tradeoff is that greater spacing is required between wavelengths, reducing the total number of wavelengths that can be supported.


CXR (carrier)

A signal of known characteristics (e.g., frequency) that is altered (i.e., modulated) to transmit information.



One complete repetition of a regularly repeating electronic function. The number of cycles per second, measured in Hertz (Hz), is called the frequency.



DAB (Digital Audio Broadcasting)

A standard that enjoys wide use in Europe for radio broadcasts.


DAMA (Demand Assigned Multiple Access)

A system for allocation of communication satellite time to earth stations as the need arises.


dark fiber

Fiber-optic cable that has been installed but is not lit (i.e., that has no active light sources).


data center

The computer-equipped, central location within an organization. The data center processes and converts information to a desired form such as reports or other types of management information records.


data circuit

A communications facility that enables transmission of information in digital form.


data communication

The transmission and reception of data between computers and/or remote devices according to appropriate protocols.


data compression and coding

Techniques used to reduce bandwidth requirements for transmission of information over a particular communications link. These techniques may also be used in noncommunications applications, such as data storage and retrieval.


data exchange

The use of data by more than one program or system.


data line privacy

The evolving relationship between technology and the legal right to, or public expectation of, privacy in the collection and sharing of data.


data link

(1) The equipment and rules (protocol) used for sending and receiving data. (2) Any serial data communications transmission path, generally between two adjacent nodes or devices and without any intermediate switching nodes.


data link layer

OSI Layer 2, which defines how data is packetized and transmitted to and from each network device. It is divided into two sublayers: Media Access Control (MAC) and Logical Link Control (LLC). The MAC layer controls access to a communications link and shares it among many users, while the data link layer uses procedures and protocols to carry data across the link. The data link layer also detects and corrects transmission errors.


data management

Provision of access to data, monitoring or storage of data, and control of input/output devices.


data PBX

A switch that enables a user on an attached circuit to select from other circuits, usually one at a time and on a contention basis, for the purpose of establishing a through connection. A data PBX is distinguished from a PBX in that it supports data transmission and not voice.


data rate

The speed at which a channel carries data, measured in bits per second (bps).


data service

A digital service offered for data communications at subscriber locations.


data set

An infrequently used term for a modem.


data-switching exchange

The equipment installed at a single location to provide switching functions, such as circuit switching, message switching, and packet switching.


data system

A system for the storage and retrieval of data, its transmission to terminals, and controls to provide adequate protection and ensure proper usage.


data transmission

The movement of information from one location to another by means of some form of communication media.



A message of fixed maximum length, sent without network-provided facilities for assuring its accuracy, delivery, or correct sequencing with respect to related messages, that carries the full destination address used for routing.


dB (decibel)

A unit for measuring the relative strength of a signal parameter such as power or voltage. The number of decibels is 10 times the logarithm (base 10) of the ratio of the power of two signals, or the ratio of the power of one signal to a reference level.


DBPSK (Differential Binary PSK)

A form of phase-shift keying used for digital transmission in which the phase of the carrier is discretely varied in relation to the phase of the immediately preceding signal element and in accordance with the data being transmitted. The demodulator determines the changes in the phase of the received signal rather than the phase itself. DBPSK is used in low-speed 802.11 WLANs (operating at 1Mbps).


DBS (Direct Broadcast Satellite)

A satellite system that can transmit DTV signals directly to individual homes.


DCCP (Datagram Congestion Control Protocol)

A protocol in the TCP/IP protocol suite that provides congestion control for unreliable data flows. DCCP adds end host congestion control behavior to high-rate UDP streams such as streaming media.


DCE (Data Communications [or channel or circuit-terminating] Equipment)

Equipment that provides an interface between the DTE and the transmission channel (i.e., between the carrier's networks). It establishes, maintains, and terminates a connection between the DTE and the transmission channel and is responsible for ensuring that the signal that comes out of the DTE is compatible with the requirements of the transmission channel.


DCLEC (Data Competitive Local Exchange Carrier)

A company that is specifically focused on supporting data services in the local loop (e.g., providers that offer DSL services to end users).


DCS (Digital Cellular System)

A global system for mobile communications-based PCS networks used outside the United States.


DCS (Digital Cross-connect System)

A device that enables the reconfiguration of a digital network in response to congestion or failure in the network, as well as on-demand reconfiguration. DCSs add, drop, and/or switch payload as necessary across multiple links.


DDS (Digital Data Service)

A digital transmission service that supports speeds up to 56Kbps/64Kbps.



A proprietary suite of network protocols created by Digital Equipment Corporation to connect two minicomputers. It evolved into one of the first peer-to-peer network architectures.


dedicated line

An end-to-end communications line used exclusively by an organization. Also called a dedicated circuit.


delay distortion

The change in a signal from the transmitting end to the receiving end that results from the tendency of some frequency components within a channel to take longer to be propagated than others.


delta modulation

A method of representing a speech waveform (or another analog signal) in which successive bits represent increments of the waveform. The increment size is not necessarily constant. It produces digitized voice at 56Kbps.



The process of recovering data from a modulated carrier wave. It is the reverse of modulation.


DEN (Directory Enabled Networking)

An industry group formed by Microsoft and Cisco to create a common data format for storing information about users, devices, servers, and applications in a common repository. DEN describes mechanisms that enable equipment, such as switches and routers, to access and use directory information to implement policy-based networking.


DES (Data Encryption Standard)

A cryptographic algorithm that enciphers and deciphers data using a 56-bit key. As a secret-key, symmetric system, it requires the exchange of secret encryption keys between users.



Software routines or microcode used to check equipment malfunctions or to pinpoint faulty components.


dial tone

A signal, generated by a service circuit within the local exchange or PBX, that is sent to an operator or a user as an audible indication that the switch is ready to receive dialing digits.



The process of, or the equipment or facilities involved in, establishing a temporary connection via the PSTN.


dialup line

A circuit that is established by a switched-circuit connection. The term generally refers to the PSTN.



An AAA protocol for applications such as network access and IP mobility. DIAMETER provides a base protocol that can be extended in order to provide AAA services to new access technologies.


DID (Direct Inward Dialing)

A system in which incoming calls from the exchange network can be completed to specific station lines without attendant assistance. Also called direct dialing in (DDI).



A public key algorithm used mostly for exchanging keys. Its security rests on the difficulty of computing discrete algorithms in a finite field, generated by a large prime number.


DiffServ (Differentiated Services)

An approach to providing QoS in networks that use a small, well-defined set of building blocks from which a variety of services can be built. DiffServ evolved from IETF's IntServ. It is a prioritization model with preferential allocation of resources based on traffic classification.



Communications procedures, techniques, and equipment whereby information is encoded as either binary 1 or 0; the representation of information in discrete binary form, discontinuous in time, as opposed to the analog representation of information in variable, but continuous, waveforms.


digital certificate

A method for registering user identities with a third party, a CA. A digital certificate binds a user to an electronic signature that can be trusted like a written signature and includes authentication, access rights, and verification information.


digital loopback

A technique for testing the digital processing circuitry of a communications device. It can be installed locally or remotely via a telecommunications circuit. The device being tested echoes back a received test message, after first decoding and then reencoding it, the results of which are compared with the original message.


digital network

A network that incorporates both digital switching and digital transmission.


digital signal

A discrete or discontinuous signal, one whose various states are identified with discrete levels or values.


digital switching

The process of establishing and maintaining a connection, under stored program control, where binary-encoded information is routed between an input port and an output port. Generally, a virtual circuit is derived from a series of time slots (TDM), which is more efficient than requiring dedicated physical circuits for the period of time for which connections are set up.


Dijkstra algorithm

An algorithm that determines routes based on path length. It is used in OSPF.


directory service

A service that provides a white pageslike directory of the users and resources located on an enterprise network. Instead of having to know a device's or user's specific network address, a directory service provides an English-like listing for a user. The directory is being standardized collaboratively by the ITU (X.500 standards) and ISO.


distance-vector routing

Routing in which a router is aware only of routers directly connected to it. Each router sends its routing table to each of its neighbors; they in turn merge this routing table with their own.



The modification of the waveform or shape of a signal caused by an outside interference or by imperfections of the transmission system. Most forms of distortion are the result of the varying responses of the transmission system to the different frequency components of the transmission signal.


distributed computing environment

An architecture in which portions of the applications and the data are broken up and distributed among the server and client computers.


distributed data processing

Data processing in which some or all of the processing, storage, and control functions, in addition to I/O functions, are situated in different places and connected by transmission facilities.


distributed database

An application in which there are many clients as well as many servers. All databases at remote and local sites are treated as if they were one database. The data dictionary is crucial in mapping where all the data resides.


distributed system

A corporate system that can function independently from the host to provide local processing capabilities that meet end-user requirements yet can connect into the host network for file transfer, access to other applications, and host-specific functions.


distribution frame

A structure (typically wall-mounted) for terminating telephone wiring, usually the permanent wires from or at the telephone exchange, where cross-connections are readily made to extensions. Also called a connector block, a distribution block, an MDF, or an IDF.


DLC (Digital Loop Carrier)

A type of concentrator, also called a remote concentrator or remote terminal. Traditional DLCs are not interoperable with some of the DSL offerings, including ADSL and SDSL.


DLCI (Data Link Connection Identifier)

An identifier in a Frame Relay header that specifies the Layer 2 virtual circuit.


DLI (Data-Line Interface)

The point at which a data line is connected to a telephone system.


DMB (Digital Multimedia Broadcasting)

The first mobile TV standard to market, derived from the DAB standard. S-DMB (Satellite DMB) and T-DMB (Terrestrial DMB) are currently available in Korea, and other countries are considering adopting DMB as well.


DMB-T (Digital Multimedia Broadcasting-Terrestrial)

A new major broadcast standard that provides the best reception quality for the power required. DMB-T and DAB are the preferred Chinese standards.


DMT (Discrete Multitone)

A multicarrier modulation scheme used in ADSL.


DNS (Domain Name System)

A set of protocols and databases that translates between Web site names and physical IP addresses in the Internet or in any other TCP/IP-based internet.


DOCSIS (Data Over Cable Service Interface Specification)

An international standard developed by CableLabs that defines the communications and operation support interface requirements to permit the addition of high-speed data transfer to an existing cable TV system. There are several generations of DOCSIS standards: DOCSIS 1.0, 1.1, 2.0, and 3.0. The ITU has adopted DOCSIS 1.1 and 2.0 as international standards.


DOD (Direct Outward Dialing)

A system in which trunks are used specifically for outgoing calls (e.g., when you dial an access code such as the number 9 or the number 8 to get an outside-line dial tone before you can dial the actual number that you want to reach).



The portion of a satellite circuit that extends from the satellite to the earth station.



To receive data from a remote system.



The direction of transmission flow from the source toward the user.



The total time a system is out of service due to equipment failure.


DPNSS (Digital Private Network Signaling System)

The European standard for common channel signaling between PBXs.


DPSK (Differential Phase-Shift Keying)

A form of phase-shift keying used for digital transmission in which the phase of the carrier is discretely varied in relation to the phase of the immediately preceding signal element and in accordance with the data being transmitted.


DQDB (Distributed Queue Dual Bus)

The media access method of the IEEE 802.6 standard for MANs.


DQPSK (Differential Quadrature PSK)

A digital modulation scheme used with extended-rate 2Mbps 802.11 WLANs.



A connection point between a communicating device and a communications network.


DRR (Deficit Round Robin)

A queuing mechanism in which a maximum packet size number is subtracted from the packet length, and packets that exceed that number are held back until the next visit of the scheduler.


DS (Digital Signal) level

The increments of the PDH hierarchy (North American standard). DS-0 is a single channel with a capacity of 64Kbps; DS-1 is 24 DS-0 channels multiplexed into one 1.544Mbps T-1 digital trunk; DS-1C is a 3.152Mbps digital signal carried on a T-1 C facility; DS-2 is a 6.312Mbps digital signal carried over 96 DS-0 channels on a T-3 facility; DS-3 is a 44.736Mbps digital signal carried over 672 DS-0 channels on a T-3 facility; and DS-4 is a 274.176Mbps digital signal carried over 4,032 DS-0 channels on a T-4 facility.


DSCP (DiffServ Code Point)

A field in the packets transported over DiffServ networks that classifies the packets according to priority. DiffServ uses a DSCP to select the per-hop behavior of the packet at each DiffServ-capable node.


DSG (DOCSIS Set-top Gateway)

An extension to the DOCSIS standards that gives operators a standard method to deliver out-of-band data, such as channel lineups and program guides, and more advanced streaming applications via a DOCSIS channel to the digital cable set-top box.


DSI (Digital Speech Interpolation)

A system of digitized speech in which the speech can be cut into slices such that no bits are transmitted when a person is silent. As soon as speech begins, bits flow again.


DSL (Digital Subscriber Line)

A family of broadband technologies that use sophisticated modulation schemes to pack data onto copper wires. They are sometimes referred to as last-mile technologies because they are used only for connections from a telephone switching station to a home or office, not between switching stations. The family of technologies is often referred to as xDSL.


DSL bonding

The process of linking several DSL lines to configure bandwidth between the T-1/T-3 and E-1/E-3 rates.


DSLAM (DSL Access Multiplexer)

A device at a phone company's central location that links many customer DSL connections to a single high-speed ATM line.


DSP (Digital Signal Processing)

The study of signals in a digital representation and the processing methods for these signals.


DSSS (Direct Sequence Spread Spectrum)

A spread spectrum technique in which each data bit is converted to a series of 10 to 144 transmitted bits or chips.


DSU (Data Service Unit)

A synchronous serial data interface that buffers and controls the flow of data between a digital terminal and the CSU attached to a digital communications facility, converting between incompatible digital formats. DSUs can be considered modem replacements in digital networks.


DTE (Data Terminal Equipment)

Equipment (including any type of computer terminal, such as a PC, as well as printers, hosts, front-end processors, multiplexers, and LAN interconnection devices such as routers) that transmits data between two points without error. Its main responsibilities are to transmit and receive information and to perform error control. The DTE generally supports the end-user applications program, data files, and databases.


DTH (Direct To Home)

A satellite system that can transmit DTV signals directly to individual homes.


DTMF (dual-tone multifrequency) signaling

The basis for operation of pushbutton telephone sets. DTMF is a method of signaling in which a matrix combination of two frequencies, each from a group of four, is used to transmit numerical address information. The two groups of four frequencies are (a) 697Hz, 770Hz, 852Hz, and 941Hz, and (b) 1,209Hz, 1,336Hz, 1,477Hz, and 1,633Hz.


DTT (Digital Terrestrial Television)

An implementation of digital technology to provide a greater number of channels and/or better quality of picture (via HDTV) and sound through a conventional antenna instead of a satellite dish or cable connection.


DTV (Digital TV)

Television sent over a digital network. DTV is nearly immune to interference and degradation, and it can display a much better range of colors than can analog television.


Dual Stack

A technique for enabling IPv4-to-IPv6 interworking and coexistence that allows IPv4 and IPv6 to exist in the same host.



Communications in which data can be transmitted between two stations in both directions at the same time, with the use of a four-wire circuit. It is the same as full-duplex.


duplex circuit

A four-wire circuit used for transmission in both directions at the same time. It can be called full-duplex to distinguish it from half-duplex.


duplex signaling

A signaling system that occupies the same cable pair as the voice path yet does not require filters.


duplex transmission

Simultaneous, two-way, independent transmission. Also called full-duplex transmission.


duplexing technique

A procedure for separating incoming and outgoing conversations.


DVB (Digital Video Broadcasting)

A suite of internationally accepted, open standards for DTV.


DVB (Digital Video Broadcasting) Project

A European organization that has authored many specifications for satellite and cable broadcasting of digital signals.


DVB-C (DVB-Cable)

An open standard for digital video transmission over cable.


DVB-H (DVB-Handheld)

A mobile TV standard promoted by ETSI that saves batteries and offers mobility with high data rates, single-antenna reception, impulse noise tolerance, increased general robustness, and seamless handoffs.


DVB-MHP (DVBMultimedia Home Platform)

The DVB Project's open middleware system for DTV that enables the reception and execution of interactive, Java-based applications on a TV set.


DVB-S (DVB-Satellite)

An open standard for digital video broadcast over satellites. DVB-S supports only MPEG-2 encoded video streams.


DVB-S2 (DVB-Satellite 2)

An open standard for digital video broadcast over satellites that has improved quality over DVB-S.


DVB-T (DVB-Terrestrial)

A European consortium standard for the broadcast transmission of digital terrestrial television.


DWDM (Dense Wavelength Division Multiplexing)

An optical technology used to increase bandwidth over existing fiber-optic backbones. DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. In effect, one fiber is transformed into multiple virtual fibers.


dynamic routing

Routing that automatically adjusts to network topology or traffic changes.



The ITU-T's international public telecommunication numbering plan for the PSTN.


E (extended) link

A link that provides enhanced reliability by providing a set of links from the SSP to a second STP pair.


E&M signaling

A signaling arrangement that uses separate paths for signaling and voice signals. The M lead (derived from "mouth") transmits ground or battery to the distant end of the circuit, while incoming signals are received as either a grounded or open condition on the E (derived from "ear") lead.


EAP (Extensible Authentication Protocol)

A protocol that enables various external authentication methodsdigital certificates, usernames and passwords, secure tokens, and so onto be incorporated into wireless security environments.


earth station

An assemblage of communications equipment, including signal generator, transmitter, receiver, and antenna, that receives (and usually transmits) signals to/from a communications satellite. Also called a ground station.


EBCDIC (Extended Binary Coded Decimal Interchange Code)

A character set that consists of 8-bit code characters and is widely used for exchanging data between computer systems. It has 256 possible combinations: 17 are used for control purposes; 96 are used for text characters; and the remaining code combinations are unassigned.


eBIP (e-Business Infrastructure Provider)

A provider that saves small businesses time and money with Web-based solutions for human resources, accounting, marketing, group collaboration, and other services.


Ebps (exabits per second)

1 billion Gbps, or 1 billion billion bps.



A time-division-multiplexed, digital transmission facility that operates at an aggregate data rate of 2.048Mbps and above. E-carrier is a PCM system that uses 64Kbps for a voice channel. E-0 is the basic increment of the PDH hierarchy; it is a single channel with a capacity of 64Kbps. In E-1, 32 channels are multiplexed into one 2.048Mbps E-1 digital channel, also referred to as G.703; 30 channels are used for information, and 2 channels are reserved for signaling and control. Other E-carrier levels are E-2 (8.488Mbps over 128 channels), E-3 (34.368Mbps over 512 channels), E-4 (139.246Mbps over 2,048 channels), and E-5 (565.148Mbps over 8,192 channels).



A wave that has been reflected or otherwise returned with sufficient magnitude and delay for it to be perceived as a wave distinct from that directly transmitted.


echo cancellation

A process that allows full-duplex transmission to occur over a single electrical path. It relies on frequency splitting to derive separate voice and data channels from one wire. This feature is necessary for voice transmission but often interferes with data transmission.


Ecma International (formerly European Computer Manufacturers Association)

A European standards organization that creates ICT and consumer electronics standards.


e-commerce (electronic commerce)

The secure exchange of funds, executed over a network, for goods and services exchanged between parties.


EDFA (Erbium-Doped Fiber Amplifier)

An optical amplifier. Erbium is injected into fiber, and as a light pulse passes through the erbium, it is amplified; thus, it does not have to be stopped and processed as an electrical signal. The introduction of EDFAs opened up the opportunity to make use of fiber-optic systems operating at 10Gbps.



The network boundary between a customer and the core or central network.


EDGE (Enhanced Data Rates for Global Evolution)

An enhanced version of GPRS that combines digital TDMA and GSM to provide 48Kbps to 69.2Kbps per time slot on an aggregated basis, up to 384Kbps.


edge caching

A system in which Web content is duplicated on a machine close to the end user the first time the user requests the content. Subsequent requests for this content are satisfied from the nearby machine. This improves the speed and reliability of access because it avoids the Internet backbone and its peering points.


edge device

A device that can pass packets between a legacy type of network such as an Ethernet network and an ATM network, using data link layer and network layer information. An edge device does not have responsibility for gathering network routing information but simply uses the routing information it finds in the network layer using the route distribution protocol.


EDI (Electronic Data Interchange)

The asynchronous exchange from computer to computer of intercompany business documents (e.g., purchase orders, bills of lading, invoices) and information. EDI can be accomplished through OSI standards or through proprietary products.


EDTV (Enhanced-Definition TV)

Video with picture quality beyond what can be broadcast in NTSC or PAL but not sharp enough to be considered HDTV.


EFM (Ethernet First Mile)

IEEE 802.3ah, a standard that uses Ethernet to provide connectivity from the customer to the carrier.


EGP (Exterior Gateway Protocol)

A routing protocol used to exchange network reachability information among organizational networks. EGP indicates whether a network is reachable; it does not weight that decision. EGP has largely been replaced by BGP4.


EIA (Electronic Industries Association)

A U.S. organization that develops standards in the areas of electrical and electronic products and components.


EIA interface

A standardized set of signal characteristics (i.e., time duration, voltage, and current) specified by the EIA.


EIGRP (Enhanced Interior Gateway Routing Protocol)

A Cisco proprietary routing protocol based on IGRP. EIGRP has optimizations to minimize the routing instability incurred after topology changes and the use of bandwidth and processing power in the router.


EIR (Excess Information Rate)

The maximum amount of uncommitted data (in bits) in excess of committed burst size that a Frame Relay network can attempt to deliver.


elastic application

A traditional Internet application that can work without guarantees of timely delivery. Because it can stretch in the face of greater delay, it can still perform adequately when the network faces increased congestion and degradation in performance.


ELEC (Ethernet Local Exchange Carrier)

A competitive provider that specializes in providing Ethernet solutions in the local loop and metro area.


electromagnetic spectrum

The electromagnetic waves that can propagate through free space and are created when electrons move. It ranges from extremely low-frequency radio waves of 30Hzwith a wavelength of nearly the earth's diameterto high-frequency cosmic rays of more than 10 million trillion Hzwith wavelengths smaller than the nucleus of an atom. The electromagnetic spectrum is depicted as a logarithmic progression: The scale increases by multiples of 10, so that the higher regions encompass a greater span of frequencies than the lower regions. The greater the span of frequencies, the greater the bandwidth of the media operating over that portion of the electromagnetic spectrum.


electronic tandem networking

The operation of two or more switching systems in parallel.


e-mail (electronic mail)

An application that enables users to send and receive messages and files over their computer networks.


EMI (Electromagnetic Interference)

Noise on data transmission lines that reduces data integrity. Motors, machines, and other generators of electromagnetic radiation cause EMI, but shielding can reduce EMI.


EMS (Enhanced Messaging Service)

An extension of SMS that enables the sending of a combination of simple melodies, images, sounds, animations, and formatted text as a message to another EMS-compatible phone.



To imitate one system with another so that the imitating system accepts the same data, executes the same computer programs, and achieves the same result as the imitated system.



The process of encasing one protocol in another protocol's format. Also called tunneling.



The process of coding data so that a specific code or key is required to restore the original data. Encryption is typically applied for secure data transmission or to prevent unauthorized reception of broadcast material. Sometimes referred to as scrambling.


end office

The first point of access to the PSTN, or the point at which the subscriber loop terminates. Also referred to as a Class 5 office, local exchange, central office, or serving office.


end-to-end optical architecture

A network in which the optical signal never needs to be converted to an electronic signal.


enterprise network

A network that connects the computer resources throughout a company and supports a wide variety of the company's applications.


enterprise wiring hub

A hub that connects the PCs on a LAN and also provides the flexibility to perform a number of network functions that can benefit network administrators and network users in general.


ENUM (Electronic Number Mapping Standard)

A protocol that is the result of work of the IETF's Telephone Number Mapping working group, whose charter was to define a DNS-based architecture and protocols for mapping a telephone number to a uniform resource identifier (URI), which can be used to contact a resource associated with that number.


ephemeral port

A TCP or UDP port number that is automatically allocated from a predefined range by the TCP/IP stack software, typically to provide the port for the client end of a client/server communication.



The introduction of components to an analog circuit by a modem to compensate for signal attenuation and delay distortion. Generally, the higher the transmission rate, the greater the need for equalization.


ERL (Echo Return Loss)

Attenuation of echo currents in one direction caused by telephone circuits operating in the other direction.



In data communications, any unwanted change in the original contents of a transmission.


error burst

A concentration of errors within a short period of time as compared with the average incidence of errors. Retransmission is the normal correction procedure in the event of an error burst.


error control

A process of handling errors that includes the detection and correction of errors.


error correction code

A code that incorporates sufficient additional signal elements to enable the nature of some or all of the errors to be indicated and corrected entirely at the receiving end.


error rate

The ratio of the amount of data incorrectly received to the total amount of data transmitted.


ESCON (Enterprise Systems Connection)

A proprietary optical networking system.


ESP (Encapsulated Security Payload)

A security system in which IP datagram data is encrypted.


ESS (Electronic Switching System)

A system that uses computer-like operations to switch telephone calls.



A baseband LAN specification that operates at 10/100/1,000Mbps by using CSMA/CD running over thick or thin coaxial, twisted-pair, or fiber-optic cable. Standards have been developed for Gigabit Ethernet and 10Gbps Ethernet, and they are being developed for 100Gbps Ethernet as well. Ethernet is defined in IEEE 802.3.


ETRI (Electronics and Telecommunications Research Institute)

A nonprofit Korean governmentfunded research organization that has been at the forefront of technological excellence for more than 25 years.


ETSI (European Telecommunication Standards Institute)

A telecommunications standardization organization.



The European version of the U.S. DOCSIS standard. The goal of Euro-DOCSIS is to ensure correct and optimal performance of Euro-DOCSIS modems and CMTSs in European networks as well as being fully compliant with the European DVB standard in the downstream.


even parity check

A test of whether the number of digits in a group of binary digits is even.



The assembly of equipment in a communications system that controls the connection of incoming and outgoing lines and includes the necessary signaling and supervisory functions. Different exchanges, or switches, can be colocated to perform different functions (e.g., local exchange/central office, tandem exchange, toll/trunk/transit exchange).



A network between partnering organizations.


extranet VPN

A VPN that allows an external organization to have defined access into an enterprise's internal networks and resources.



F (fully associated) link

A link that directly connects to signaling endpoints, generally SSPs.



(1) Any or all of the physical elements of a plan used to provide communications services. (2) A component of an operating system. (3) A transmission path between two or more points, provided by a common carrier.



The reduction in intensity of the power of a received signal. In this phenomenon, which generally affects microwave or radio transmission, atmospheric, electromagnetic, or gravitational influences cause a signal to be deflected or diverted away from the target receiver.


Fast Ethernet

A standard for high-speed Ethernet that has a rate of 100Mbps.


fast hopping

A signal-processing scheme that supports high data rates with very low packet and delay losses (i.e., latencies) over a distributed all-IP wireless network.


fast packet switching

A packet-processing technology that has streamlined protocol handling, including Frame Relay and ATM.



A condition that causes any physical component of a system to fail to perform in acceptable fashion.


fault tolerance

The capability of a program or system to operate properly even if a failure occurs.


FCC (Federal Communications Commission)

A regulatory agency established by the Communications Act of 1934 that is charged with regulating all electrical and radio communications in the United States.


FDD (Frequency Division Duplex)

A full-duplex technique used when there is a significant contiguous spectrum allocated and when synchronization between the base stations is not possible. Each direction (incoming and outgoing) occupies a different portion of the frequency band, and a rather large portion of the spectrum is consumed.


FDDI (Fiber Distributed Data Interface)

A 100Mbps, fiber-based token-passing ANSI standard. FDDI consists of dual fiber-optic counter-rotating rings, each capable of supporting 100Mbps data rates. FDDI is defined for fiber-optic cable, but it has a twisted-pair alternative called CDDI. FDDI II is an enhanced version of FDDI that supports isochronous transmission (for voice and video) as well as the packet-oriented (both asynchronous and synchronous) traffic handling of FDDI.


FDM (Frequency Division Multiplexing)

A technique of dividing the bandwidth of a communications line into multiple smaller units of bandwidth, each of which supports an independent information stream.


FDMA (Frequency Division Multiple Access)

A multiple-access technique used in analog cellular systems, in which each user is assigned to a different frequency.


FEC (Forward Error Correction)

An error control system for data transmission that differs from standard error detection and correction in that the technique is specifically designed to allow the receiver to correct some errors without having to request a retransmission of data.


FEC (Forwarding Equivalence Class)

In MPLS, a term used to describe a set of packets with similar or identical characteristics that may be forwarded the same way.


FECN (Forward Explicit Congestion Notification)

A bit in the Frame Relay header by which the network can inform the receiver of network congestion. DTE receiving frames with the FECN bit set can request that higher-level protocols take flow control action, as appropriate.



0.000000000000001 (i.e., 1015) second.


FEP (Front-End Processor)

A dedicated communications system that intercepts and handles activity for the host. It may perform line control, message handling, code conversion, and error control, as well as such application functions as control and operation of special-purpose terminals.


FHSS (Frequency Hopping Spread Spectrum)

A spread spectrum technique in which the frequency hopping varies in a known pattern, and separate error correction must be included.


fiber-optic waveguides

Thin filaments of glass through which a light beam can be transmitted for long distances by means of multiple internal reflections. Occasionally, other transparent materials, such as plastic, are used.


fiber optics

A technology that uses light as a digital information carrier. Fiber-optic cables (light guides) are a direct replacement for coaxial cables and twisted-wire pairs. The glass-based transmission facilities occupy far less physical volume yet provide a tremendous amount of transmission capacity, which is a major advantage in crowded underground ducts. The fibers are immune to electrical interference, which is another advantage. Also called lightwave communications, photonics, or, simply, fiber.


Fibre Channel

A high-speed interface, standardized by ANSI, that supports up to 800Mbps over 6.2 miles (10 km) of fiber.


FIFO (First In, First Out)

A queuing technique in which the next item to be retrieved is the item that has been in the queue for the longest time. This ensures that cells remain in the correct sequence.


file server

In local networks, a station dedicated to providing file and mass data storage services to the other stations on the network.



To selectively forward data, based on criteria specified by the network manager.



A system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both.


five-nines reliability

A desired level of network or system reliability that equates to 99.999% uptime (i.e., about 5 minutes of downtime per year).


fixed wireless local loop

A stationary installation that dramatically cuts down on the cost of installing and maintaining the local loop. It uses a fixed antenna location, so it is relatively easy to engineer.


Flash-OFDM (Fast Low-Latency Access with Seamless Handoff OFDM)

A WMAN standard that is a variant of 802.20. Flash-OFDM uses a technique called fast hopping, a signal-processing scheme that supports high data rates with very low packet and delay losses (i.e., latencies) over a distributed all-IP wireless network. Flash-OFDM is not compatible with 2.5G or 3G technology.


flat network

A network that is constructed by using bridges or Layer 2 LAN switches. This type of network is easy to configure, and it promises better performance than hierarchical networks; it offers higher throughput and therefore also lower latencies. However, the scalability of a flat network is limited, and a flat network is subject to broadcast storms.


flat rate

A fixed cost for service. Additional charges may be applied for additional services or usage if so specified.


flow control

A system that uses buffering and other mechanisms, such as controls that turn a device on and off, to prevent data loss during transmission.


FM (Frequency Modulation)

One of three ways to modify a sine wave signal to carry digital bits. The sine wave, or carrier, has its frequency modified in accordance with the information to be transmitted. The frequency function of the modulated wave may be continuous or discontinuous.


FMC (Fixed-Mobile Convergence)

The vision of being able to use one phone with one number, address book, and voicemail bank while enjoying the benefits of low-cost, high-speed connectivity in the office or fixed-line residential environment, while still enjoying the freedom of mobility in the WAN. FMC features seamless handoff between fixed and mobile networks.


FOMA (Freedom of Mobile Multimedia Access)

The proprietary 3G standard used by Japanese mobile phone operator NTT DoCoMo.


forward channel

The communications path that carries voice or data from the call initiator to the network.


four-wire circuit

A circuit that contains two pairs of wire (or their logical equivalent) for simultaneous (i.e., full-duplex) two-way transmission. Two pairs of conductors, one for the inbound channel and one for the outbound channel, are connected to the station equipment.


FQ (Fair Queuing)

A queuing mechanism whose objectives are to provide fair access to bandwidth, resources, and routers and to ensure that no one flow receives more than its fair share.


FQDN (Fully Qualified Domain Name)

An unambiguous domain name that specifies a node's position in the DNS tree hierarchy absolutely.


fractional T-1/E-1

T-1/E-1 lines that have apportioned bandwidth for separate transmission channels (DS-0/64Kbps subchannels), generally in increments of four channels.



The process of splitting a packet into pieces when it is larger than the MTU it must transmit.



(1) In data transmission, the sequence of contiguous bits bracketed by and including beginning and ending flag sequences. (2) In a TDM system, a repetitive group of signals resulting from a signal sampling of all channels, including any additional signals for synchronizing and other required system information.


frame bandwidth allocation

The sum of the committed information rates associated with all the PVCs for a specific customer.


Frame Relay

A packet-switching technology that is simpler and more powerful than the X.25 standard. Frame Relay provides a multiplexed channel between a router and a T-1/E-1 nodal processor. It increases bandwidth utilization while reducing overall equipment costs. The Frame Relay standard addresses data communications speeds up to 45Mbps.



A control procedure used with multiplexed digital channels, such as T-1 carriers, in which bits are inserted so that the receiver can identify the time slots allocated to such subchannels. Framing bits may also carry alarm signals indicating specific alarm conditions.



An expression of how frequently a periodic (repetitious) waveform or signal regenerates itself at a given amplitude. Frequency can be expressed in Hertz (Hz), kilohertz (KHz), megahertz (MHz), and so on.


frequency-selective fading

A type of fading based on multipath time delay spread in which the bandwidth of the signal is greater than the coherence bandwidth of the channel or the delay spread is greater than the symbol period.


FSK (Frequency-Shift Keying)

A modulation method that uses two different frequencies to distinguish between a mark (digital 1) and a space (digital 0) when transmitting on an analog line. FSK is used in modems that operate at 1,200bps or slower.


FSO (Free Space Optics)

An optical wireless networking option that uses low-powered infrared lasers. There are two options in FSO: point-to-point products, used to provide high-speed connection between two buildings, and multiple high-speed connections through the air that operate over much shorter distances, either in a point-to-multipoint or meshed architecture.


FTAM (File Transfer Access and Management)

An ISO standard that describes how to create, delete, read, and change file attributes as well as transfer and access (at the file or record level) files stored at remote sites. It is an application-layer protocol.


FTP (File Transfer Protocol)

A protocol that enables a TCP/IP user on any computer to get files from another computer or to send files to another computer. Usually implemented as application-level programs, FTP uses the Telnet and TCP protocols. The server side requires a client to supply a login identifier and password before it will honor requests.


FTTC (Fiber-To-The-Curb)

A system in which fiber cable extends from a switching office to a curb.


FTTH/FTTP (Fiber-To-The-Home/Fiber-To-The-Premises)

A system in which fiber cable extends from a switching office to the subscriber's house/premises.


FTTN (Fiber-To-The-Node)

A system in which fiber cable extends from a switching office to a point near the premises, such as a curb.


FTTx (Fiber-To-The-x)

A system in which fiber cable extends from a switching office to the x (where x means premises, curb, building, or home).



A communication system or equipment that is capable of transmission simultaneously in both directions.


full-motion video

Moving images that the human eye perceives as being fully realistic. While there are no defined standards, full-motion video is frequently referred to as VHS-quality. Frame rates range from 24 frames per second (fps) in motion pictures, 25fps in the PAL system, and 30fps in the NTSC system.


FX (foreign exchange) line

A line that makes a toll call appear to be a local call.



GAN (Generic Access Network)

A telecommunication system that allows seamless roaming and handoffs between LANs and WANs, using the same dual-mode mobile phone. Formerly known as Unlicensed Mobile Access (UMA), it was adopted by the 3GPP in 2005.



A device or program (i.e., hardware or software) that connects two different networks that use different protocols and translates between these protocols, allowing devices on the two networks to communicate with each other.


gateway daemon

A program that runs under BSD UNIX on a gateway to allow the gateway to collect information from within one autonomous system using RIP, HELLO, or other IGPs, and to advertise routes to another autonomous system using the EGP.


Gateway-to-Gateway Protocol

The original IGP used by Internet core gateways (i.e., by routers).


GB (Gigabyte)

1 billion bytes, or 1,000MB.


Gen2 standard (UHF Generation 2 Air Interface Protocol)

An EPCglobal standard that forms the backbone of RFID tag standards.


GEO (Geosynchronous Orbit)

A circular orbit with a 24-hour orbital period approximately 22,300 miles (36,000 km) above the earth's equator. Because satellites in this orbit appear stationary relative to the earth's surface, GEO is especially useful for communications satellites that transmit to fixed earth stations.


GERAN (GSM EDGE Radio Access Network)

A second phase of EDGE that is planned to offer data rates of up to 1,920Kbps, to support packetized voice and real-time services. GERAN is a common evolution path for GSM and TDMA that intends to provide a cost-efficient means to deliver 3G services within the existing frequency bands


GFP (Generic Framing Procedure)

A protocol that allows mapping of variable-length, higher-layer client signals over transport networks such as SDH/SONET and OTN networks.


GGSN (Gateway GPRS Support Node)

A device that acts as a gateway between the GPRS network and public data networks such as IP and X.25 networks. GGSNs also connect to other GPRS networks to facilitate GPRS roaming.


GHz (Gigahertz)

1 billion cycles per second.


GigE (Gigabit Ethernet)

An Ethernet standard that supports 1Gbps. Sometimes referred to as 1Gbps Ethernet.


global information infrastructure

A vision of individual national information infrastructures joined together to form an international network.


GMPLS (Generalized Multiprotocol Label Switching)

An enhancement to MPLS technology that allows service providers to dynamically provision resources and provide the necessary redundancy for implementing various protection and restoration techniques for optical networks.


GMSK (Gaussian Minimum-Shift Keying)

A kind of continuous-phase frequency-shift keying modulation that produces one bit per symbol time. GSM uses GMSK.


GPRS (General Packet Radio Service)

An always-on nonvoice value-added service that enables information to be sent and received across a mobile telephone network via GSM phones.


GPS (Global Positioning System)

A navigation system in which satellites broadcast precise timing signals by radio to GPS receivers, allowing them to accurately determine their location in any weather, at any time of day, anywhere on earth.


GRE (Generic Routing Encapsulation)

A simple stateless protocol that allows for the tunneling of IP in IP. GRE tunnels can be used to form VPNs, connecting remote sites by using private IP addresses via a public network.



The process of selectively removing channels from a digital facility for routing to a designated remote location via another digital facility. Grooming basically enables the flexible dropping and adding of payload.


ground circuit

(1) A circuit in which energy is carried one way over a metallic path and returned through the earth. (2) A circuit connected to the earth at one or more points.


ground start

A signaling method whereby one station detects that a circuit is grounded at the other end.


Group 3 fax

An ITU-T standard for encoding an image and transmitting it over dialup lines.


Group 4 fax

An ITU-T standard for encoding an image and transmitting it over ISDN or other wideband digitized services.


G.SHDSL (Symmetric High-Speed DSL)

A member of the DSL family that offers symmetrical service with options to operate over one pair or two pairs of copper wires, and also has rate-adaptive capability. G.SHDSL supports data rates of up to 5.6Mbps in each direction over a distance of up to 2 miles (3 km). G.SHDSL was the first international standard for DSL. Also referred to as SHDSL.


GSM (Global System for Mobile Communications)

A European standard for 2G wireless digital communications that is globally implemented and supports both voice and data communications. GSM operates in several frequency bands: GSM 850 (850MHz), GSM 900 (900MHz), DCS 1800 (1.8GHz), and PCS 1900 (1.9GHz). New GSM data standards include HSCSD, GPRS, and EDGE and are referred to as 2.5G.


GTP (GPRS Tunneling Protocol)

An IP-based protocol used within GSM and UMTS networks.



An ITU standard that defines how audiovisual conferencing data is transmitted across networks. In theory, H.323 should enable users to participate in the same conference even though they are using different videoconferencing equipment.



Communications in which data can be transmitted between two stations in both directions but in only one direction at a time.


Hamming Code

A linear error-correcting code that can detect single- and double-bit errors.


HAN (Home Area Network)

A broadband network in a smart house that connects the various smart devices.



The transfer of duplex signaling as a mobile terminal passes to an adjacent cell in a cellular radio network.



The exchange of predetermined signals for control when a connection is established between two modems or other devices.


haptic interface

An interface that enables virtual touch.


hard wired

(1) Referring to a communications link, whether remote phone line or local cable, that permanently connects two nodes, stations, or devices. (2) Descriptive of electronic circuitry that performs fixed logical operations by virtue of an unalterable circuit layout, rather than under computer or stored-program control.



The physical equipment, as opposed to programs or procedures, of a computer system.


harmonic distortion

A waveform distortion that is usually caused by the nonlinear frequency response of a transmission.


HARQ (Hybrid Automatic Repeat Request)

A variation of the ARQ error control method that gives better performance than ordinary ARQ, particularly over wireless channels, albeit at the cost of increased implementation complexity.


hash function

The process of producing hash values for accessing data or for security. A hash value (or simply hash) is a number generated from a string of text. The hash is substantially smaller than the text itself and is generated by a formula in such a way that it is extremely unlikely that some other text will produce the same hash value.


HCF (Hybrid Coordination Function)

An IEEE MAC-layer protocol that is an important development in VoWLAN systems. In order to improve service for voice, the standard includes two operating modes: Enhanced Digital Control Access (EDCA) and Polled Access.



A class of high-speed ISDN channels. H-0 is 384Kbps, H-11 is 1.536Mbps, and H-12 is 1.920Mbps.


HCSD (High-Speed Circuit-Switched Data)

A standard for transferring high-speed data over aggregated GSM channels. HCSD provides data rates up to 64Kbps.


HDB3 (High-Density Bipolar 3)

A line interface standard for E-1 that is similar to B8ZS. HDB3 eliminates data streams with eight or more consecutive 0s, allows for 64Kbps clear channel capacity, and still assures the minimum 1s density required by E-1 lines.


HDLC (High-Level Data Link Control)

A form of communications line control that uses a specified series of bits rather than control characters to control data transmission over a communication line. HDLC is a bit-oriented protocol developed by the ISO.


HDSL (High-Bit-Rate DSL)

A symmetrical service that can be deployed over a distance of about 2.2 miles (3.6 km). HDSL is deployed over two twisted-pair cables, and it affords equal bandwidth in both directions. HDSL2 provides symmetrical capacities of up to 1.5Mbps or 2Mbps over a single twisted-pair cable.


HDTV (High-Definition Television)

A television format for which several competing standards exist but which normally requires a screen aspect ratio of 16:9 (versus 4:3 with analog TVs) and which is capable of reproducing at least four times more detail than in the analog broadcasting system.



The control center of a cable TV network.



The initial portion of a message or file, which contains statistical and control information.



The protocol used by a group of cooperative, trusting packet switches to allow them to discover minimal delay routes.



An exploratory method of problem solving in which solutions are discovered through an interactive, self-learning method.



A system of numbers in base 16. Hexadecimal digits range from 0 through 9 and A (10) through F (15). Each hexadecimal digit is represented by 4 binary bits.


HFC (Hybrid Fiber Coax)

A networking arrangement that supports a wide range of services, including traditional telephony, broadcast video, and interactive broadband services. It involves the use of fiber in the backbone and in the access network. The fiber terminates at a neighborhood node, and from that neighborhood node, coax (normally 750MHz or 1,000MHz) is run to the home, in a two-way subsplit system.


hierarchical routing

Routing that is based on a hierarchical addressing scheme. Most TCP/IP routing is based on a two-level hierarchy in which an IP address is divided into a network portion and a host portion. Routers use only the network portion until the datagram reaches a router that can deliver it directly. Subnetting introduces additional levels of hierarchical routing.


high frequency

The portion of electromagnetic spectrum that is typically used in shortwave radio applications. High frequencies are approximately in the 3MHz to 30MHz range.


HiperAccess (High-Performance Radio Access)

A fixed wireless broadband access network standard that gives broadband access to both the home and small- and medium-sized enterprises, as well as providing backhaul for mobile systems. HiperAccess was developed to provide a truly broadband system, with bit rates of up to approximately 100Mbps, although 25Mbps is expected to be the most widely deployed rate. HiperAccess is targeted at high frequency bands, especially the 40.5GHz to 43.5GHz band.


HiperLAN (High-Performance Radio LAN)

A mobile broadband short-range access network standard. This WLAN standard is defined by the BRAN project of ETSI and is a European alternative for the IEEE 802.11 standards. The first version of HiperLAN, called HiperLan1, was designed to provide faster data rates than the IEEE 802.11 standards. The second version of the standard, HiperLan2, will give consumers wireless access to the Internet and future multimedia, as well as real-time video services, at speeds of up to 54Mbps.


HiperMAN (High-Performance Radio MAN)

A fixed wireless access network standard for operating between 2GHz to 11GHz that is aimed at providing a broadband wireless solution for metropolitan area networks.


HIPPI (High-Performance Parallel Interface)

A gigabit-per-second OSI Layer 1 and 2 interface standardized by ANSI. HIPPI supports 800Mbps up to 82 feet (25 m) using a 32-bit parallel copper connector and can be extended up to several miles/kilometers by using fiber-optic technology. A higher-speed option uses 64 parallel lines to support operation at up to 1.6Gbps.


HMIPv6 (Hierarchical Mobile IPv6)

An enhancement to the Mobile IP technique that is being developed to improve mobile communications in certain circumstances by making the processes more secure and more efficient.


holding time

The length of time a communications channel is in use for each transmission. The holding time includes both message tone and operating time. Also called connect time.



An open standard for short-range transmission of digital voice and data between mobile devices.



A unit of network distance. The number of hops between a source and a destination is the number of nodes between them (e.g., number of routers between hosts on the Internet).


hop-by-hop retransmission

A system in which an intermediate device retransmits so that the retransmission travels a shorter path over a fewer number of hops and is therefore less delayed.


horizontal distribution frame

A hub for terminating cables run on a floor.



An end-user computer system that connects to a network. Hosts range in size from PCs to supercomputers.


host interface

The link between a communications processor or network and a host computer.


host system

(1) The computing system to which a network is connected and with which other devices can communicate. (2) The primary or controlling computer in a network.



A location that enables Internet access from mobile devices (such as laptops and PDAs) without connection cables to networked services, usually through Wi-Fi. Hotspots are often found in restaurants, train stations, airports, cafes, libraries, and other public places.


howler tone

The tone that alerts a subscriber when a telephone is off the hook.


HSCSD (High-Speed Circuit-Switched Data)

A high-speed transmission technology that enables users to send and retrieve data over GSM networks at transmission speeds between 28.8Kbps and 43.2Kbps by enabling the concurrent usage of up to four traffic channels of a GSM network.


HSDPA (High-Speed Downlink Packet Access)

A key 3.5G standard that provides data rates from 8Mbps to 14Mbps over a 5MHz bandwidth in W-CDMA downlink to support packet-based multimedia services. HSDPA enhances W-CDMA similarly to the way EDGE enhances GSM/GPRS.


HSOPA (High-Speed OFDM Packet Access)

A key 3.5G standard that incorporates OFDM and MIMO, promising to offer a 40Mbps download speed.


HSPA (High-Speed Packet Access)

The combination of HSDPA and HSUPA services.


HSSI (High-Speed Serial Interface)

A physical-layer interface between a DTE, such as a high-speed router or similar device, and a DCE, such as a DS-3 (45Mbps) or SDH/SONET OC-1 DSU.


HSUPA (High-Speed Uplink Packet Access)

A key 3.5G standard that enables users to transmit data upstream at a speed of 5.8Mbps.


HTML (Hypertext Markup Language)

A document standard that defines a simple logical structure including titles, headings, paragraphs, lists, forms, tables, and mathematical equations, as well as a language to specify hypertext links.


HTTP (Hypertext Transfer Protocol)

The standard mechanism used on the World Wide Web for the transfer of documents between server and client systems.



A Web protocol that encrypts and decrypts user page requests as well as the pages that are returned by the Web server.



A device that extends the maximum physical length of a network by cleaning and retransmitting signals among network segments. A hub provides the central connecting point in a star network topology. Also called a multiport repeater.


Huffman encoding

A statistical encoding technique for lossless compression. Statistical encoding is an entropy-encoding method. The Huffman algorithm calculates the frequency of occurrence of each octet for a given portion of the data stream. It then determines the minimum number of bits to allocate to each character and assigns an optimal code accordingly. The codes are stored in a codebook. This technique is used in sound, still, and moving image compression.


hybrid circuit

A circuit that has four sets of terminals arranged in two pairs designed so that there is high loss between the two sets of terminals of a pair when the terminals of the other pair are suitably terminated. Hybrid circuits are commonly used to couple four-wire circuits to two-wire circuits.


hybrid network

A network composed of both public and private facilities.


Hz (Hertz)

A unit of electromagnetic frequency that is equal to one cycle per second.


IAD (Integrated Access Device)

A device that consolidates voice, data, Internet, and video services using DSL, ATM, TDM, or MGCP over T-1/E-1 lines. The IAD, installed on the end-user's site, is a form of CPE.


IANA (Internet Assigned Numbers Authority)

An organization that oversees IP addresses and top-level domain name allocations.



An Australian WMAN technology that provides mobile wireless broadband Internet access. iBurst is a pure IP, end-to-end system.


ICANN (Internet Corporation for Assigned Names and Numbers)

A nonprofit corporation that manages the domain name and root server systems.


ICMP (Internet Control Message Protocol)

An integral part of IP that handles error and control messages. Routers and hosts use ICMP to send reports of problems about datagrams back to the original sources that sent the datagrams. ICMP also includes an echo request/reply that is used to test whether a destination is reachable and responding.


ICP (Internet Content Provider)

A service provider that specializes in providing content rather than infrastructure.


ICT (Information and Communications Technologies)

A broad field concerned with technology and other aspects of managing and processing information.


IDEA (International Data Encryption Algorithm)

An algorithm developed by ETH Zurich that is free of charge for noncommercial use. Viewed as a good algorithm, it is used in PGP and in Speak Freely, a program that allows encrypted digitized voice to be sent over the Internet.


IDF (Intermediate Distribution Frame)

A frame that has distributing blocks on both sides, permitting the interconnection of telephone circuitry.



A transmission medium that has a maximum loop length of 3.4 miles (5.5 km) and is deployed as a single twisted-pair cable that offers 128Kbps in each direction. IDSL is not used much anymore.


IEC (International Electrotechnical Commission)

An international standards organization that deals with electrical, electronic, and related technologies. Some of its standards are developed jointly with the ISO.


IEEE (Institute of Electrical and Electronics Engineers)

A scientific, engineering, and educational society that develops and publishes standards in a variety of electrical engineering and computer-related areas. IEEE membership is open to any dues-paying individual. The IEEE is responsible for 802 LAN standards.


IETF (Internet Engineering Task Force)

A nonprofit organization that produces the standards used in TCP/IP and the Internet.


I-frame (Intracoded frame)

A frame that is not reconstructed from another frame. An I-frame is also a reference frame; it serves as a reference to construct other frames.


IFRB (International Frequency Registration Board)

A board within the ITU that is responsible for the maintenance of an international list of radio frequency usage and the allocation of new frequencies.


IGMP (Internet Group Management Protocol)

A protocol that allows Internet hosts to participate in multicasting. It describes the basics of multicasting IP traffic, including the format of multicast IP addresses, multicast Ethernet encapsulation, and the concept of a host group (i.e., a set of hosts interested in traffic for a particular multicast address).


IGP (Interior Gateway Protocol)

Any protocol used to propagate network reachability and routing information within an autonomous system. RIP and IGRP are examples of IGPs.


IGRP (Interior Gateway Routing Protocol)

A proprietary network protocol developed by Cisco Systems that is designed to work on autonomous systems. IGRP is a distance-vector routing protocol.


IKE (Internet Key Exchange)

The key exchange protocol used by IPsec. IKE supports preshared keys, which is a simplified form of key exchange. It does not require digital certificates.


ILEC (Incumbent Local Exchange Carrier)

A telephone company that was providing local service in the United States when the Telecommunications Act of 1996 was enacted. For most residents in the United States, this would be one of the four "baby Bells"Qwest, SBC, BellSouth, and Verizon.


IMA (Inverse Multiplexing over ATM)

A specification that provides a way to combine an ATM cell stream over two or more circuits, thus allowing an organization to lease just the bandwidth it needs (e.g., more than T-1 and less than T-3).



In virtual reality, the user's subjective sensation of being inside the virtual world and not observing it from an outside perspective.



A proprietary protocol for transforming Internet information so that it can be displayed on the small screen of a mobile telephone or other portable device. i-mode is used in Japan and is also called DoCoMo (which means "anywhere").



A radio signal quality problem caused by degradations due to path loss, multipath, adverse weather, obstacles, interference, or other obstacles.


IMPS (Instant Messaging and Presence Service)

A mobile phone standard that enables instant messaging, presence, groups or chat, and shared content.


IMS (IP Multimedia Subsystem)

A standard that defines a generic architecture that offers VoIP and other multimedia services within wireline and wireless infrastructures. The aim of IMS is to provide all the services, current and future, that the Internet provides. IMS creates a telephony-oriented signaling network that overlays an underlying IP network. Using the IP protocol as its foundation, IMS supports data, video, SIP-based VoIP, and non-SIP packetized voice, such as H.323 and MGCP. IMS was designed to integrate with the PSTN and provide traditional telephony services such as 800 numbers, caller ID, and local number portability.


IMT-2000 (International Mobile Telecommunications 2000)

An evolving standard for 3G mobile communications that enables personal mobility and converging mobile and fixed networks.


IMUN (International Mobile User Number)

A number used to dial a subscriber in 3G mobile networks.


IN (Intelligent Network)

An architecture for providing advanced services in telecommunications networks.


in-band management

A system in which management information is communicated across the network.


induction coil

An apparatus for obtaining intermittent high voltage consisting of a primary coil through which the direct current flows, an interrupter, and a secondary coil with a larger number of turns in which the high voltage is induced.


information infrastructure

High-speed communications networks capable of carrying voice, data, text, image, and video (multimedia) information in an interactive mode serving an enterprise computing architecture.


information path

The functional route by which information is routed.


information signals

Signals associated with activating and delivering various enhanced features, such as call waiting.


information systems network

A network of multiple operating-level systems and one management-oriented system (centered around planning, control, and measurement processes). The network retrieves data from databases and synthesizes the data into meaningful information to support the organization.



The frequency range in the electromagnetic spectrum that is higher than radio frequencies but below the range of visible light.



The underlying structure or framework of the telecommunications system (e.g., switching, multiplexing, and transmission systems) that allows for the transmission of voice, video, and data.


InP (Indium Phosphide)

A semiconductor composed of indium and phosphorus that is used for making electronic and optoelectronic devices. InP is useful for optoelectronics devices such as laser diodes.


input queue

A holding area for packets that come to the input port more quickly than the router can process them.


Integrated IS-IS

A routing protocol that combines routing for TCP/IP and OSI protocols. It is a superset of IS-IS, the OSI routing technology that combines the functionality of both OSPF and IS-IS.


integrated photonic circuit

The optical equivalent of an integrated circuit. The goal of integrated photonic circuits is to consolidate large numbers of separate optical devices into a single chip, customizing them for different applications to improve performance.



A term that describes the mode of transaction for a particular information service. An interactive service allows for both input and output. It is sometimes referred to as a two-way, as opposed to a one-way, service.


interactive processing

A processing method in which each operator action causes a response from the program or system.


interconnected systems

Systems that are linked together in local and/or remote networks. The exchange of data between systems in a network is through standard channels or through communications lines. Communication between interconnected systems normally occurs without manual intervention; it is provided by combined hardware and software that support the interconnection.



The interworking of two separate wireline and/or wireless networks. Interconnection is used to refer both to the technical interface and to the commercial arrangements between two network operators providing service.



A boundary between two pieces of equipment across which all signals that pass are carefully defined. The definition includes the connector signal levels, impedance, timing, sequence of operation, and meaning of signals.


interior routing

Routing that occurs within an autonomous system.


international business service

A satellite-based service at up to 8Mbps. Services include data, fax, digital voice, and video- and audioconferencing.


international gateway

A device that connects calls between different countries.


international number

Digits that have to be dialed after the international prefix to call a subscriber in another country; that is, the country code followed by the subscriber's national number.



A collection of packet-switching networks interconnected by routers along with protocols that allow them to function logically as a single, large, virtual network.



The worldwide Internet consisting of large national and regional backbone networks, local Internet service providers, and IP networks.


internet 2

A network that replaces what the original Internet was forthe academic network. Internet 2 acts as a testbed for many of the latest and greatest technologies. Universities stress-test Internet 2 to determine how applications perform and which technologies suit which applications or management purposes best.


internet-based VPN

A VPN composed of multiple ISPs that provide local access services in defined geographical regions. Because it requires an enterprise to receive end-to-end services from multiple suppliers, performance is difficult to control and guarantee.


interoffice channels

A portion of a leased circuit between IXC exchanges.


interoffice trunk

A direct trunk between local exchanges (Class 5 offices) or between tandem, toll, or international exchanges. Also called interexchange trunk.



The ability to exchange information in a network that contains computers and additional devices that have dissimilar operating systems or protocols.



A network based on TCP/IP protocols (i.e., an internet) that belongs to an organization and is accessible only by the organization's members, employees, or others with authorization.


intranet VPN

A site-to-site connection whose key objective is to replace or reduce the use of leased-line networks, traditional routers, and Frame Relay services.


IntServ (Integrated Services)

The IETF's scheme to introduce QoS support over IP networks. IntServ provides extensions to the best-effort service model to allow control over end-to-end packet delays. IntServ is a per-flow resource reservation model that requires RSVP. Its key building blocks include resource reservation and admission control.


Inverse ARP

An extension to ARP that permits a station to request a protocol address (e.g., an IP address) given a hardware address (e.g., a Frame Relay DLCI).


inverse multiplexer

A device that spreads a high-bandwidth information stream over multiple lower-speed transmission channels (e.g., a 1.5Mbps signal transmitted over 24 64Kbps channels of a T-1).


I/O (Input/Output)

(1) A device or channel that may be involved in an input process, and, at a different time, in an output process. (2) A device whose parts can be performing an input process and an output process at the same time. (3) Pertains to either input or output, or both.


IP (Internet Protocol)

The protocol that specifies the exact format of all data as it travels through a TCP/IP network. In addition, IP performs the routing functions and selects the transmission path on which data will be sent. As part of these two functions, IP also provides a mechanism for dealing with unreliable data, specifying the manner in which network nodes will process data, how and when to generate error messages, and when to discard unreliable data.


IP address

The 32-bit address assigned to a host that wants to participate in a TCP/IP internet. IP addresses are the abstraction of physical hardware addresses just as an internet is an abstraction of physical networks. Actually assigned to the interconnection of a host to a physical network, an IP address consists of a network portion and a host portion.


IP backbone

A packet-switching network interconnected by routers along with protocols that allow them to function logically as a single, large, virtual network. IP backbones are operated by individual service providers, unlike the Internet, which is composed of more than 10,000 service providers.


IP Cablecom

The name by which the international community knows PacketCable. See also PacketCable.


IP datagram

The basic unit of information passed across a TCP/IP internet. An IP datagram is to an internet as a hardware packet is to a physical network. It contains a source and destination address along with data.


IP forwarding

The process of forwarding internet packets from one network to another.


IP fragmentation

A multibridge feature that handles packet size mismatch problems between FDDI and Ethernet endpoints. The maximum FDDI packet size is 4,500 bytes. The maximum Ethernet packet size is 1,548 bytes. Messages that are longer than 1,548 bytes must be fragmented into smaller packets to allow them to enter the Ethernet network.


IP long-distance wholesaler

A VoIP service provider that offers IP services to domestic and international carriers, corporations, and service providers to carry their traffic, particularly international transit.


IP Multicast

A protocol for transmitting IP datagrams from one source to many destinations in a LAN or WAN.


IP over DWDM

The use of IP on an intermediate architecture that runs on a DWDM network. Today's networks operate largely with IP running over ATM running over SDH/SONET running over DWDM.



A new-generation PBX that uses packet-switching technology and offers an attractive platform for the integration of voice and data in the enterprise.


IP switch

A switch that replaces slower, more processing-intensive routers. An IP switch is a router that provides connection-oriented services in the IP layer.


IP voice

The routing of voice conversations over an IP-based network such as the Internet. Voice traffic has a low bandwidth requirement but requires very high QoS.


IPDC (Internet Protocol Device Control)

A specification that creates flexible management of media gateway devices.


IPN (Interplanetary Internet)

A network project that defines the architecture and protocols necessary to permit interoperation of the Internet resident on earth with other remotely located internets resident on other planets or spacecraft in transit.


IPS7 (IP Signaling System 7)

A signaling protocol that works with SS7.


IPsec (IP Security)

A set of protocols developed by the IETF to support secure exchange of packets at the IP layer. IPsec has been deployed widely to implement VPNs.


IPT (IP Telephony)

The use of the Internet or a private IP network for telephony.


IPTV (IP Television)

A system in which DTV is delivered to subscribers using Internet Protocol over a broadband connection. IPTV is often provided in conjunction with VOD and Internet services such as Web access and VoIP.


IPv4 (Internet Protocol version 4)

The original generation of IP, in which an IP address has two parts: The first is the network ID and the second is the host ID. Under IPv4, there are five classes (Class A through Class E), which differ in how many networks and hosts are supported.


IPv6 (Internet Protocol version 6)

An IP addressing scheme that uses a 128-bit address, which allows a total of 340 billion billion billion billion unique addresses. IPv6 offers many benefits, but it requires a major reconfiguration of routers. Also called IPng (IP Next Generation).


IPX (Internetwork Packet Exchange)

The Novell equivalent of IP. IPX is used to route NetWare packets between LANs. IPX does not guarantee the delivery of messages; NetWare's SPX protocol handles that task.


IrDA (Infrared Data Association)

A short-range wireless technology that allows connection between devices using infrared links instead of wired cabling.


IRP (Interdomain Routing Protocol)

A protocol that provides routing for OSI-defined network environments and is similar to BGP in the TCP/IP network. In an OSI network, there are end systems, intermediate systems, areas, and domains. IRP is designed to provide routing among domains.



TIA's Interim Specification 54, also called NADC and Digital AMPS, and updated by IS-136. It is a TDMA-based wireless network that operates at 800MHz.



TIA's Interim Specification 95. It is a spread spectrum wireless network that operates in the 800MHz range, using a CDMA/FDD scheme.



TIA's Interim Specification 136. It is an updated TDMA-based standard that updates IS-54. Provides 3x spectral efficiency over analog AMPS systems.


ISC (International Switching Center)

An exchange used to switch traffic between different countries over international circuits.


ISDB (Integrated Services Digital Broadcasting)

A Japanese DTV and DAB format created to allow radio and TV stations to convert to digital. ISDB can be used for both fixed and mobile reception, and it encompasses several core standards: ISDB-S (satellite TV), ISDB-T (terrestrial digital TV), ISDB-C (digital cable TV), and ISDB-Tsb (terrestrial digital sound broadcasting in the 2.6GHz band).


ISDN (Integrated Services Digital Network)

A circuit-switched digital subscriber line service; an access technology. ISDN is part of the physical layer of the OSI reference model. ITU-T I.430 defines a 144Kbps Basic Rate Interface (BRI), and ITU-T I.431 defines a Primary Rate Interface (PRI) of 1.544Mbps in North America and Japan and 2.048Mbps in Europe.


IS-IS (Intermediate System to Intermediate System)

A protocol that uses a link-state algorithm to provide routing services for TCP/IP and OSI. It determines the best path for TCP/IP and OSI packets through the network and keeps routers informed of the status of the network and the systems available.


ISM (Industrial, Scientific, and Medical)

Unlicensed radio bands that operate at 900MHz, 2.4GHz, and 5.8GHz.


ISO (International Organization for Standardization)

An organization established to promote the development of standards to facilitate the international exchange of goods and services and to develop mutual cooperation in areas of intellectual, scientific, technological, and economic activity.


ISO Ethernet

An isochronous Ethernet standard designed to provide an efficient way to share normal Ethernet and isochronous traffic on a single twisted-pair cable in a local area environment.


ISOC (Internet Society)

A nongovernmental, nonprofit organization dedicated to maintaining and enhancing the Internet. Through its committees, such as the Internet Advisory Board and the IETF, ISOC is responsible for developing and approving new Internet standards and protocols.



A descriptor that signifies enabling network characteristics, including the ability to simultaneously transport disparate data types (voice, video, and data) across the same circuit. It also includes the capability to dynamically allocate bandwidth as the application warrants.


isochronous data stream

A nonpacketized data transmission. An isochronous data stream is a circuit-switched, fixed-rate, continuous data stream, such as voice, video, or real-time sensor data.


ISP (Internet Service Provider)

A company that gives users access to the Internet and related services.


ISV (Independent Software Vendor)

A vendor that develops the applications that ASPs then put up for sale or for rent.


IT (Information Technology)

A broad term that describes the computer hardware, software, and networking industry, including telecommunications and audiovisual equipment.


ITU (International Telecommunication Union)

A telecommunications agency of the United Nations that was established to provide standardized communications procedures and practices, including frequency allocation and radio regulations, on a worldwide basis. It is the parent group of the ITU-T (telecommunications), ITU-R (radio), and ITU-D (developing nations).


IV (Initialization Vector)

In cryptography, a block of bits that is required to allow a stream cipher or a block cipher executed in any of several streaming modes to produce a unique stream independent from other streams produced by the same encryption key, without having to go through a rekeying process.


IXC (Interexchange Carrier)

A long-distance telephone company that offers circuit-switched, leased-line, or packet-switched service.


IXP (Internet Exchange Point)

A physical infrastructure that allows different ISPs to exchange Internet traffic between their networks by means of mutual peering agreements, which allow traffic to be exchanged without cost.




A device used generally for terminating the permanent wiring of a circuit, access to which is obtained by the insertion of a plug.



The Japanese standard of the PDH, a TDM digital transmission system. J-carrier is a PCM system that uses 64Kbps per channel as the basis of the hierarchy. Higher levels reflect aggregation of the 64Kbps channels. J-1 is a 1.544Mbps 24-channel communications circuit; J-2 is a 6.312Mbps 96-channel communications circuit; J-3 is a 32.064Mbps 480-channel communications circuit; J-4 is a 97.728Mbps 1,440-channel communications circuit; and J-5 is a 565.148Mbps 8,192-channel communications circuit.



The slight movement of a transmission signal in time or phase that can introduce errors and loss of synchronization for high-speed synchronous communications.


JPEG (Joint Pictures Expert Group)

An international standard used primarily for still image compression.


JTACS (Japanese Total Access Communication Systems)

A Japanese wireless system that operates in the 800MHz to 900MHz band.



An extensible object-oriented API that supports telephony call control.



A patch cable or wire used to establish a circuit, often temporarily, for testing or diagnostics.




The portion of the electromagnetic spectrum allotted for satellite transmission. Its frequencies are approximately in the 20GHz to 30GHz range.


Kbps (Kilobits per second)

1,000 bits per second.


KHz (kilohertz)

1,000 cycles per second.



The portion of the electromagnetic spectrum that is being used increasingly for satellite communications. Frequencies are approximately in the 12GHz to 14GHz range.



L2TP (Layer 2 Tunneling Protocol)

A Layer 2 protocol that can work in a non-IP enterprise environment. L2TP is used primarily by service providers to encapsulate and carry VPN traffic through their backbones.


LAN (Local Area Network)

(1) A system for linking terminals, programs, storage, and graphic devices at multiple workstations over relatively small geographic areas. (2) A network that is limited to a small area, such as the premises of an office building or plant.


LANE (LAN Emulation)

An ATM Forum standard for emulating a LAN across an ATM network.


LAPB (Link Access Protocol Balanced)

A modified form of HDLC that the ITU-T chose as the link-level protocol for X.25 networks. LAPB provides for the reliable transfer of a packet from a host to an X.25 packet switch, which then forwards the packet to its destination.


LAPD (Link Access Protocol on the D Channel)

An ISDN data link layer protocol for the D channel. LAPD was derived from the LAPB protocol, designed primarily to satisfy the signaling requirements of ISDN basic access.


laser (light amplification by simulated emission of radiation)

A device that converts electrical energy into radiant energy in the visible or infrared parts of the spectrum, emitting light with a small spectral bandwidth. Lasers are widely used in fiber-optic communications, particularly as sources for long-haul links.


LATA (Local Access and Transport Area)

Geographic regions within the United States that define areas within which the RBOCs can offer exchange and exchange access services (local calling, private lines, and so on).



The delay associated with the time it takes a packet to travel from entry point to exit point.



In the OSI reference model, a collection of related network-processing functions that comprise one level of a hierarchy of functions.


Layer 1

The physical layer of the seven-layer OSI reference model, which deals with the physical means of sending data over a communications medium, defining physical characteristics such as media type, physical interfaces, electrical and optical coding schemes, and data transfer rates.


Layer 2

The data link layer of the seven-layer OSI reference model, which responds to service requests from the network layer and issues service requests to the physical layer. Layer 2 is sometimes split into two sublayers: the Logical Link Control (LLC) layer and the Media Access Control (MAC) layer. The MAC layer controls access to communications links and shares it among many users, and the LLC layer uses procedures and protocols to carry data across the link (and also detects and corrects transmission errors). Examples of Layer 2 protocols in a LAN environment include Ethernet, Token Ring, and FDDI. Examples of Layer 2 protocols in a WAN environment include Frame Relay and ATM. Examples of Layer 2 protocols in the PSTN are SS7 and MTP2.


Layer 3

The networking layer of the seven-layer OSI reference model, which identifies computers on a network and determines how to direct information transfer over that network. The key responsibility of Layer 3 is to add the addressing information and the control functions needed to move the data through the network and its intermediate nodes. It is involved in establishing, maintaining, and terminating connections, including packet switching, routing, data congestion, reassembly of data, and translation of logical addresses to physical addresses. Examples of Layer 3 protocols are X.25, IP, IPX, and MTP3.


Layer 4

The transport layer of the seven-layer OSI reference model, which corrects transmission errors and ensures that the information is delivered reliably. Layer 4 provides end-to-end error recovery and flow control capability. It deals with packet handling, repackaging of messages, division of messages into smaller packets, and error handling. Examples of Layer 4 protocols include TCP, UDP, and SPX.


Layer 5

The session layer of the seven-layer OSI reference model, which supports connections between sessions and handles administrative tasks and security. Layer 5 establishes and monitors connections between computers, and it provides the control structure for communication between applications. Examples of Layer 5 protocols include NetBIOS and LDAP.


Layer 6

The presentation layer of the seven-layer OSI reference model, which formats information so that a software application can read it. It performs transformations on the data to provide a standardized application interface and common communication services. Layer 6 offers services such as encryption, compression, and reformatting. It adds a field in each packet that tells how the information within the packet is encoded. It also indicates whether any compression has been performed and, if so, indicates what type of compression so that the receiver can decompress it properly. It also indicates whether there has been any encryption, and if there has, it indicates what type so that the receiver can properly decrypt it. Layer 6 ensures that the transmitter and receiver are seeing information in the same format.


Layer 7

The application layer of the seven-layer OSI reference model, which is responsible for exchanging information between the programs running on a computer and other services on a network. Layer 7 supports application and end-user processes. It acts as a window for applications to access network services. It handles general network access, flow control, error recovery, and file transfer. Examples of Layer 7 protocols include FTP, Telnet, SMTP, and HTTP.



The portion of the electromagnetic spectrum commonly used in satellite and microwave applications. L-band operates in the 390MHz to 1,550MHz range, and it supports various mobile and fixed applications.


LCD (Liquid Crystal Display)

A graphic display on a terminal screen using an electroluminescent technology to form symbols or shapes.


LDAP (Lightweight Directory Access Protocol)

The standard directory server technology for the Internet. LDAP allows retrieval of information from multivendor directories.


LDP (Label Distribution Protocol)

An MPLS signaling protocol.


leased line

A communications channel contracted for exclusive use from a common carrier, frequently referred to as a private line.


LEC (Local Exchange Carrier)

The incumbent local telephone company. There was originally no competition among LECs, but as soon as competition in the local loop picked up, LECs were segmented into ILECs, CLECs, and DCLECs.


LED (Light-Emitting Diode)

A semiconductor junction diode that emits radiant energy and is used as a light source for fiber-optic communications, particularly for short-haul links. LEDs are also used in alphanumeric displays in electronic telephones, calculators, and other devices.


LEO (low-earth-orbit) satellite

A satellite that orbits at about 400 to 1,000 miles (640 to 1,600 km) above the earth.


lightwave communications

A term sometimes used in place of optical communications to avoid confusion with visual information and image transmission, such as facsimile or television.


limited-distance modem

A device that translates digital signals into analog signals (and vice versa) for transfers over limited distances. Some limited-distance modems operate at higher speeds than modems that are designed for use over analog telephone facilities.



(1) The communications path between two or more points, including a satellite or microwave channel, also referred to as the transmission line. (2) In data communications, a circuit that connects two or more devices. (3) The transmission path from a nonswitching subscriber terminal to a switching system.


line hit

Electrical interference that causes the introduction of undesirable signals on a circuit.


line of sight

(1) A characteristic of some open-air transmission technologies (e.g., microwave, infrared, open-air laser-type transmissions) in which the path between a transmitter and a receiver must be clear and unobstructed. (2) A clear, open-air, direct transmission path that is free of obstructions such as buildings but may in some cases be impeded by adverse weather or environmental conditions.


line speed

The maximum data rate that can be reliably transmitted over a line.



(1) A physical circuit between two points. (2) A conceptual (or virtual) circuit between two users of a packet-switched (or other) network that allows them to communicate even when different paths are used.


link redundancy level

The ratio of the actual number of paths to the minimum number of paths required to connect all nodes of a network.


link state

A state in which each router is aware of the topology of the entire network. Each router sends out information about the links that the router has to all other routers on the network. The final routing table is based on the shortest path to each destination. Most new routing protocols are based on this algorithm.


link-state protocol

A generic class of routing protocols in which information about the status of the entire network is propagated to every node and used in routing decisions. OSPF, IS-IS, and NLSP are link-state routing protocols.


LLC (Logical Link Control)

A protocol developed by the IEEE 802, common to all of its local network standards, for data linklevel transmission control. It is the upper sublayer of the OSI Layer 2 protocol that complements the MAC protocol (IEEE 802.2). LLC 1 is a minimal-function LLC that supports connectionless link layer service. LLC 2 supports connection oriented data link service.


LMDS (Local Multipoint Distribution Service)

A technique for supplying broadband access via a point-to-point microwave digital system. Referred to as Multipoint Video Distribution service in Europe, it operates over a very large frequency allocation, a 1.3GHz band that is generally located in the range of 28GHz to 45GHz, depending on the country. LMDS is a popular technique for deploying wireless local loop.


LMI (Local Management Interface)

A Frame Relay specification for the method of exchanging status information between the user (e.g., bridge, router) and the network.


LMP (Link Management Protocol)

A protocol used in GMPLS networks that accommodates neighbor identification and link verification, as well as shared-risk groups, fault isolation, and topology-aware networks.


load balancing

A technique used to spread work between many processes, computers, disks, or other resources. Load balancing is a scalability problem for networks.



Adding loading coils to a transmission line to minimize amplitude distortion.


loading coil

An induction device used in local loops, generally those exceeding 3.4 miles (5.5 km) in length, that compensates for the wire capacitance and boosts voice-grade frequencies. Loading coils are often removed for new-generation, high-speed, local loop data services because they can distort data signals at higher frequencies than those used for voice.



Pertains to a device that is connected directly to a computer without using a WAN communication line.


local exchange

The switching center in which subscribers' lines terminate. The local exchange has access to the other exchanges and to national trunk networks. Also called a central office, end office, serving office, or Class 5 office.


local exchange trunk

A trunk between the CPE and the local exchange. Also referred to as a central office trunk.


local loop

A line that connects a customer's telephone equipment with the local telephone company exchange. Often referred to as a subscriber line, an access line, or the last mile.


local number portability

A service that enables a person to keep his or her own telephone number when moving to a new location.


local service area

The area within which the telephone operating company uses local rates for calling charges.



Apple Computer's proprietary 230.4Kbps baseband CSMA/CA network protocol.


location-based online services

Services provided over a wireless infrastructure that are based on the location of the user. The location of the user can be determined through GPSs or by cellular networks. Radio signals emitted from cellular phones can be tracked from cellular towers and triangulated, yielding locations nearly as accurate as those from a GPS receiver.


logical address

An address used to identify a communications program by name to the protocol stack. No matter where the program is put in the network, an individual's logical address will remain the same, even though the person's physical address may change.



Long-distance, describing (primarily) telephone circuits that cross out of the local exchange.



(1) A local circuit between an exchange and subscriber CPE, either residential (single-line telephone) or business (PBX). Also called a subscriber loop, local line, or local loop. (2) In programming, a sequence of computer instructions that repeats itself until a predetermined count or other test is satisfied.


loop circuit

A circuit that connects a subscriber's equipment with the local exchange switch. Also called a metallic circuit or local loop.


loop signaling systems

Any of the three methods of transmitting signaling information over the metallic loop formed by the trunk conductors and the terminating equipment bridges. Transmission of the loop signals can be accomplished by opening and closing the DC path around the loop, reversing the voltage polarity, or varying the value of the equipment resistance.


loop start

The most commonly used method of signaling an off-hook condition between an analog phone set and a switch, whereby picking up the receiver closes a wire loop, allowing DC current to flow, which is detected by a PBX or local exchange and interpreted as a request for service.



A diagnostic procedure used for transmission devices. In a loopback, a test message is sent to a device being tested, which then sends the message back to the originator for comparison with the original transmission. Loopback testing may be performed within a locally attached device or conducted remotely over a communications circuit.



A decrease in signal power in transmission along the circuit as a result of the resistance of impedance of the circuit or equipment.


lossless compression

In data compression, the process by which information is recovered without any alteration after the decompression stage. This technique is used for computer-based data or programs. It may also be required in certain multimedia applications where the accuracy of the information is essential, such as in medical imaging. Lossless compression is also called bit-preserving or reversible compression. Examples of lossless compression include run-length encoding and Huffman encoding.


lossy compression

In data compression, a situation in which the decompressed information is different from the original uncompressed information. This mode is suitable for most continuous media, such as sound and motion video, as well as for many images. That the decompressed information is different from the original in lossy compression does not imply that the perceptual response of an observer is different. Also called irreversible compression.


low frequency

Generally, frequencies between 30KHz and 300KHz.


low-level language

A programming language in which instructions have a one-to-one relationship with machine code.


LPC (Linear Predictive Coding)

A vector-quantization-based compression scheme for speech. LPC can compress speech down to 2.4Kbps.


LSP (Label-Switched Path)

A path through an MPLS network that is set up by a signaling protocol based on criteria in the forwarding equivalence class (FEC).


LSR (Label-Switching Router)

An MPLS-enabled router and/or MPLS-enabled ATM switch. As each packet enters the network, an ingress LSR assigns it a label, based on its destination, VPN membership, ToS bits, and other considerations. At each hop, an LSR uses the label to index a forwarding table.



m2m (machine-to-machine)

A term that refers to the concept of communications between a device holding some amount of data that is of interest to another and another device that desires the use of that data. Telemetry is the most common implementation of m2m. Also sometimes stands for man-to-machine.


MAC (Media Access Control)

The part of OSI Layer 2 that describes how devices share access to the network. Token Ring, Ethernet, and FDDI are MAC-layer specifications. Wiring hubs primarily deal with MAC-layer equipment.



A cell architecture in the cellular system that covered up to 8 miles (12.8 km) and used a lot of power, from 0.6 to 3 watts. This type of network was for fast-moving users, traveling distances of miles on their end-to-end journey. A macrocell could support up to about 60 users.


MAE (Metropolitan Area Exchange)

An interconnection and exchange point where public Internet backbones meet and exchange traffic. Also called a NAP.


mains powered

Being powered by a permanent source such as an AC electrical power supply.


MAN (Metropolitan Area Network)

A network that spans a geographical distance of up to a 62-mile (100-km) diameter; a citywide network.


managed object

A data-processing or data communications resource that can be managed through the use of an OSI management protocol. The resource itself need not be an OSI resource. A managed object may be a physical item of equipment, a software component, some abstract collection of information, or any combination of the three.


MAPI (Messaging Application Programming Interface)

A messaging API from Microsoft that consists of two components: Simple MAPI and Extended MAPI. Simple MAPI provides hooks to various messaging systems, so developers can create message-enabled applications by writing those applications to Simple MAPI rather than developing those hooks themselves. Extended MAPI is platform specific to Windows.



In network operations, the logical association of one set of values, such as addresses on one network, with quantities or values of another set, such as devices on a second network (e.g., nameaddress mapping, internet work-route mapping).



The signal (communications channel state) that corresponds to a binary 1.



A wired-logic control circuit that, among other functions, tests, selects, and establishes paths through a switching state(s) in response to external signals.



(1) An arrangement of elements (numbers, characters, dots, diodes, wires, and so on) in perpendicular rows. (2) In switch technology, the portion of a switch architecture where input leads and output leads meet, any pair of which can be connected to establish a through circuit. Also called a switching matrix.


MAU (Media Attachment Unit)

A transceiver that connects to the AUI port of an Ethernet interface card and provides attachments to some type of data communications medium.


MB (megabyte)

1,048,576 bytes; usually referred to as 1 million bytes.


MB-OFDM (Multiband OFDM)

A UWB standard that transmits data simultaneously over multiple, accurately spaced carrier frequencies and features high spectral flexibility as well as resilience to RFI and multipath.


mbone (multicast backbone)

A virtual network on top of the Internet that supports routing of IP multicast packets, intended for multipoint multimedia transmission. With mbone, a single packet can have multiple destinations and is replicated only at the closest node before the final receiver. This means that it can pass through several routers before it needs to be replicated to reach its final destinations. This leads to much more efficient transmission and also ensures that packets reach multiple destinations at roughly the same time.


Mbps (megabits per second)

1,048,576 bits per second; usually referred to as 1 million bits per second.


MBS (mobile broadband services)

Very high-bit-rate services (over 100Mbps) over wireless channels.


MCML (Multi-Class Multi-Link) PPP

A protocol that makes it possible to have multiple classes of latency-sensitive traffic carried over a single multilink bundle with bulk traffic.


m-commerce (mobile commerce)

Financial transactions that occur on mobile devices.


MCU (Multipoint Control Unit)

A device that supports multiparty videoconferencing between several individual-circuit videoconferencing systems. The MCU acts as a videoconference hub.


MD5 (Message Digest-5)

An algorithm used to create digital signatures. MD5 is intended for use with 32-bit machines. It is a one-way hash function, which means it converts a message into a fixed string of digits called a message digest.


MDF (Main Distribution Frame)

A structure that contains all the necessary power and test equipment to support terminal strip connections and wiring arrangements that connect outside and inside telephone exchange circuitry.


measured rate

A message rate structure that includes payment for a specified number of calls within a defined area, plus a charge for additional calls.



A system that uses mechanics, electronics, and computing to generate simpler, more economical, reliable, and versatile systems. Automotive applications of mechatronics, for example, include obstacle detection, door control, and climate control, as well as support for telematics and multimedia.


media filter

A filter used in token-ring lobe wiring to convert STP-only adapter cards to UTP wiring.


media gateway

A device that provides seamless interoperability between circuit-switched, or PSTN, networking domains and those of the packet-switched realm (i.e., IP, ATM, and Frame Relay networks). A media gateway interconnects with the SS7 network and enables the handling of IP services.



A mobile TV standard from Qualcomm. MediaFLO is a comprehensive end-to-end solution that simultaneously and cost-effectively delivers very high volumes of high-quality, streaming, or clipped audio and video multimedia to wireless subscribers.



(1) The material on which data is recorded (e.g., magnetic tape, CD-R). (2) Any material substance that is or can be used for the propagation of signals, usually in the form of modulated radio, light, or acoustic waves, from one point to another (e.g., optical fiber, cable, wire, water, air, free space).


medium frequency

Frequencies in the range between 300KHz and 3MHz.



1 million.


Megaco (Media Gateway Control)

An ITU standard that describes how the media gateway should behave and function. It is standardized under ITU-T H.248.



An area of a computer system that accepts, holds, and provides access to information.


MEMS (microelectromechanical system) switch

An optical switch that uses an array of microscopic mirrors to reflect light from an input port to an output port.



A displayed list of items from which a user can make a selection.



Programs that are run by instructions presented as a list of commands and available options. Unlike in a command-driven program, the user only has to select the desired option.


MEO (middle-earth-orbit) satellite

A satellite that orbits at an elevation of about 6,200 to 9,400 miles (9,900 to 15,000 km) above the earth.



A topology in which nodes are connected in an unconstrained way and may contain loops. A fully connected mesh has every pair of nodes directly connected.



An arbitrary amount of information whose beginning and end are defined. In data communications, a message consists of a header, a body, and a trailer.


message authentication

Authentication that verifies the integrity of an electronic message and also verifies that an electronic message was sent by a particular entity.


message format

Rules for the placement of such portions of a message as message heading, address, text, end-of-message indication, and error-detecting bits.


message numbering

Identification of each message within a communications system by the assignment of sequential numbers.


message switching

A technique that transfers messages between points that are not directly connected. The switching facility receives messages, stores them in queues for each destination point, and retransmits them when a facility becomes available. Synonymous with store-and-forward.


message unit

A unit of measure for charging local calls that details the length of call, distance called, and time of day.



Information that a routing algorithm uses to determine the best path to the destination. Some examples of metrics include path length, destination, next-hop associations, reliability, delay, bandwidth, load, and communication cost.


metro access product

A MAN product that brings fiber closer to the customer, to reduce deployment costs.


metro core product

A MAN product used in building citywide rings.


MGCP (Media Gateway Control Protocol)

A combination of the SGCP and IPDC specifications. In this protocol, softswitches provide the external control and management, making MGCP a good way to connect an IAD to a gateway.


MHz (megahertz)

A unit of frequency equal to 1 million cycles per second.


MIB (Management Information Base)

The specification for how data is stored, monitored, and managed in an SNMP device. MIB I and MIB II are revisions of the database used on TCP/IP networks.






An Internet browser specifically designed for mobile devices.



A small cell architecture in the cellular system that covers about 1 mile (1.5 km) and uses only about 100 milliwatts of power. Microcells allow greater frequency reuse than cells, increasing the overall network capacity and therefore the number of subscribers that can be served.



One-millionth of a second.



(1) The portion of the electromagnetic spectrum between 1GHz and 100GHz. (2) High-frequency transmission signals and equipment that employ microwave frequencies, including line-of-sight open-air microwave transmission and satellite communications.


MIDI (Musical Instrument Digital Interface)

A standard for defining how to code all the elements of musical scores, such as sequences of notes, timing conditions, and the instrument that is to play each note.






One-thousandth of a second.


MIME (Multipurpose Internet Mail Extensions)

An Internet standard that enables a message to contain textual, binary, or arbitrarily formatted data. An advantage of MIME is that it encodes the data into an SMTP-compatible format.


MIMO (multiple-input multiple-output) antenna

An advanced antenna technology that can carry several times more data traffic than today's most advanced 3.5G networks. MIMO enables a network to quickly deliver multimedia content. Multiple antennas are used at both the transmitter and receiver and are combined at each end to minimize errors and optimize data speed.


MIPS (Million Instructions Per Second)

A measure of a computer's processor speed.


MMD (Multimedia Domain)

The 3GPP2 name for IMS, which provides the technological basis for the provision of mobile multimedia services in next-generation networks.


MMDS (Multichannel Multipoint Distribution Service)

A technique for supplying broadband access via a point-to-point microwave digital system. It operates in the 2GHz to 3GHz band and can cover a fairly large area (approximately 30 miles [48 km]). It provides great capacity in that it enables 150 channels. Also called wireless cable.


MMS (Multimedia Messaging Service)

A presence-based technology for transmitting text messages and also multimedia content, such as images, audio, and video clips, over wireless networks, using WAP.


MMW (Millimeter Wave)

A technology that is often used in wireless local loops. MMW produces very narrow beams, called pencil beams. MMW usually covers the range between 10GHz and 300GHz.


mobile earth station

A radio transmitter and/or receiver situated on a ship, on a vehicle, on an aircraft, or in a briefcase that is used for satellite communications.


Mobile IP

An IETF standard that allows users to roam between IP networks without their mobile devices losing connections.


Mobile IPv6

An enhancement to the Mobile IP technique that is being developed to improve mobile communications in certain circumstances by making the processes more secure and more efficient.


Mobile Mesh

A wireless network implementation that uses all the nodes in the network to support communication. Because each node acts like a repeater, the greater the number of nodes, the greater the bandwidth and the better the signal that reaches the access point.


mobile TV

Television delivered over mobile devices. A number of standards are being promoted for mobile TV, including DMB, DVB-H, ISDB, and MediaFLO.


mobile wireless local loop

A last-mile solution that uses cellular telephone or cordless technology, along with satellites. This approach enables subscriber mobility, so it could replace a fixed line when in a home and could also be used to move outside those boundaries as a mobile line.



A standard that is optimized for IP and roaming in high-speed mobile environments. Also known as IEEE 802.20.


mobility network

A wireless network that allows a user to roam within the range of his or her home and the outdoor area surrounding it.


modal dispersion

The tendency of light to travel in a wavelike motion rather than a straight line. The greater the level of wave fluctuations, the greater the dispersion of the signal and the associated degradation of performance.


modem (modulator-demodulator)

A conversion device installed at each end of an analog communications line. The modem at the transmitting end modulates digital signals locally from a computer or terminal; the modem at the receiving end demodulates the incoming signal, converting it back to its original (i.e., digital) format and passing it to the destination business machine.



A design technique that permits a design or system to be assembled from interchangeable components. A modular system or device can be expanded or modified simply by adding another module.



The process of converting voice or data signals for transmission over a network. Also called line coding.


modulation division multiplexing

A multiplexing method in which the signals that modulate the optical carriers are multiplexed.



A device that converts a signal (voice or other) into a form that can be transmitted.



A hardware or software component that is discrete and identifiable.



(1) A software tool used to supervise, control, or verify the operations of a system. (2) A device used to display computer-generated information.



The largest permanently deployed multivendor IPv6 network in the world. Moonv6 is a global effort.


MP2 (MPEG-1 Audio Layer 2)

An audio codec that has largely been replaced by MP3 for PC and Internet applications but remains a dominant standard for audio broadcasting as part of the DAB and DVB standards.


MP3 (MPEG Audio Layer 3)

(1) A popular digital audio encoding and lossy compression format that was designed to greatly reduce the amount of data required to represent audio yet still sound like a faithful reproduction of the original uncompressed audio to most listeners. (2) Files of sound or music recordings stored in the MP3 format on computers or other devices.


MP4 (MPEG-4 Part 14)

A multimedia container format standard that is most commonly used to store digital audio and digital video streams, but also used to store other data, such as subtitles and still images. MP4 allows streaming over the Internet.


MP-BGP (Multiprotocol Border Gateway Protocol)

An IPv6-enabled routing protocol used to announce IPv6 routes across MPLS tunnels.


MPEG (Moving Picture Experts Group)

The ISO standards body responsible for the MPEG international video compression standards. MPEG-1 addresses VHS-quality images with a 1.5Mbps data rate. MPEG-1 can play back from a single-speed CD-ROM player at 352 x 240 (i.e., quarter-screen) at 30 frames per second (fps). MPEG-2, which today is the compression scheme of choice, addresses DTVor computer-qualityimages with a 6Mbps data rate. MPEG-2 offers resolutions of 720 x 480 and 1,280 x 720 at 30fps, with full CD-quality audio. MPEG-3 will address HDTV-quality images, at data rates up to 60Mbps. MPEG-4, an evolution of MPEG-2, features audio, video, and systems layers and offers VBR encoding for both narrowband and broadband delivery in a single file. It also uses an object-based compression method, rather than MPEG-2's frame-based compression. MPEG-4 allows objectssuch as two-dimensional or three-dimensional video objects, text, graphics, and soundto be manipulated and made interactive through Web-like hyperlinks and/or multimedia triggers. MPEG-4 Advanced Video Compression (AVC) is a digital video codec standard noted for achieving very high data compression. MPEG-7 is a multimedia content description standard for information searching. MPEG-21 is a framework for the all-electronic creation, production, delivery, and trade of content.


MPLS (Multiprotocol Label Switching)

An IETF initiative that integrates Layer 2 information about network links (e.g., bandwidth, latency, utilization) into Layer 3 (IP) within a particular autonomous system in order to simplify and improve IP packet exchange. MPLS gives network operators a great deal of flexibility to divert and route traffic around link failures, congestion, and bottlenecks.


MPλS (Multiprotocol Lambda Switching)

A variation of MPLS in which specific wavelengths serve in place of labels as unique identifiers. The specified wavelengths, like the labels, make it possible for routers and switches to perform necessary functions automatically, without having to extract instructions regarding those functions from IP addresses or other packet information.


MP-MLQ (Multipulse Maximum Likelihood Quantization)

An ITU compression standard that reduces voice to 4.8Kbps and can permit up to 10 voice channels on a single 64Kbps connection.


MPOA (Multiprotocol over ATM)

An ATM Forum standard for linking a number of local networks across an ATM backbone that caters to many different network protocols.


mrouter (multicast router)

A router that enfolds IP packets in special multicast packets and forwards them toward a destination mrouter.


MSAU (Multistation Access Unit)

A device that enables workstations on a LAN to be cabled in a star configuration. Also known as a token-ring hub.


MSO (Multiple-System Operator)

A cable TV operator of multiple cable systems.


MSP (Management Service Provider)

A provider that takes over the actual management and monitoring of a network.


MSPP (Multiservice Provisioning Platform)

A product designed to deliver convergence, revenue generation, and cost reductions on a single edge device. The multiservice edge is an emerging market segment of edge devices optimized for converging Layer 2/Layer 3 services over IP/MPLS. An MSPP can handle all the popular data protocols and interfaces on the access side, and it can interface to high-speed optical links on the transport side of the network.


MSS (Mobile Satellite Service)

A satellite network that is an IMT-2000 system that can be used worldwide. MSS is intended to take over when a user is out of range of terrestrial base stations. MSS is expected to offer services similar to those of terrestrial cellular networks, including data at ISDN speeds, toll-quality voice, video, and multimedia messaging. However, given the level of multinational cooperation required to achieve global availability, MSS is not likely to be realized as a solution anytime soon.


MSU (Modem Sharing Unit)

A device that permits two or more terminals to share a single modem.


MTBF (Mean Time Between Failures)

The average length of time for which a system, or a component of a system, works without fault.


MTP (Message Transfer Part)

Part of SS7 used for communication in the PSTN. MTP is responsible for the correct and reliable end-to-end data transport of SS7 messages between communication partners. MTP Level 2 (MTP2) corresponds to OSI Layer 2, and MTP Level 3 (MTP3) corresponds to OSI Layer 3.


MTS (Mobile Telephone Service)

A telephone service provided between mobile stations and the PSTN in which radio transmission provides the equivalent of a local loop.


MTSO (Mobile Telephone Switching Office)

A component in the cellular radio system that links the base transceiver stations with the terrestrial local exchanges to complete calls over the PSTN.


MTTR (Mean Time To Repair)

The average time required to perform corrective maintenance on a failed device.


MTU (Maximum Transmission Unit)

The maximum packet size that can be transmitted over a LAN or an internet.


mu-law encoding

Encoding according to ITU-T recommendation G.711 that is used with 24-channel PCM systems in the United States and Japan. It is similar to A-law encoding, but the two differ in the size of the quantizing intervals.


multicarrier modulation

A modulation scheme that uses and aggregates a certain amount of bandwidth and then subdivides it into subbands. Each subband is encoded by using a single-carrier technique, and bitstreams from the subbands are bonded together at the receiver.



Simultaneous distribution of data to a defined subset of all receive points in a network. The subset may be redefined for each transmission and range from one to all receive points.



Pertaining to broadcasting media capable of carrying multiple different television and radio channels.



A communications arrangement in which multiple devices share a common transmission channel, although only one may transmit at a time.



The presentation of more than one mediumtypically images, sound, and textin an interactive environment.


multimode fiber

A fiber-optic cable with a core diameter large enough to allow light to travel on different paths, supporting propagation to multiple nodes.



A propagation phenomenon that results in radio signals' reaching the receiving antenna by two or more paths. Multipath affects the quality of communication because it can create secondary and tertiary signals that compete or interfere with the primary signal.


multiple-access technique

A method that enables sharing of spectrum by multiple users. FDMA, TDMA, and CDMA are examples of multiple-access techniques.


multiple trunk groups

Groups that indicate that the switching system is capable of being equipped for more than one group of trunk circuits.



To interleave or simultaneously transmit two or more messages on a single channel.



A device that enables more than one signal to be sent simultaneously over one physical channel. A multiplexer, sometimes called a mux, combines inputs from two or more terminals, computer ports, or other muxes and transmits the combined data stream over a single high-speed channel. At the receiving end, the high-speed channel is demultiplexed, either by another mux or by software.



Pertaining or referring to a communications line to which three or more stations are connected. Multipoint implies that the line physically extends from one station to another until all are connected.


multipoint network

In data communications, a configuration in which more than two terminal installations are connected to a single port.



The simultaneous execution of two or more computer programs.



See multiplexer.


MVNO (Mobile Virtual Network Operator)

Companies that do not own licensed spectrum of their own but instead make use of another mobile operator's network, while reselling the wireless services under their own brand names.


MWS (Multimedia Wireless System)

A European term for high-frequency BFWA that is focused on broadband services, including real-time video, streaming video, and video transfer.



NACK or NAK (Negative Acknowledgment)

A message that indicates an error in transmission and says the previous block needs to be resent before anything else can happen.


NADC (North American Digital Cellular)

A TIA standard that uses TDMA and TDD schemes and offers a total of three time slots. NADC operates on the 800MHz frequency band, uses AMPS for signaling to reserve resources, and transfers speech in digital form; therefore, it is a digital overlay that is interoperable with analog AMPS infrastructure.


nailed-up connection

A slang term for a permanent, dedicated path through a switch. A nailed-up connection is often used for lengthy, regular data transmission going through a PBX.


name resolution

The process of mapping a name to a corresponding address. DNS provides a mechanism for naming computers in which programs use remote name servers to resolve machine names into IP addresses for those machines.



One-billionth of a second.


NAP (Network Access Point)

The point at which backbones interconnect to exchange traffic between providers. Bottlenecks at NAPs greatly affect the ability to roll out new time-sensitive, loss-sensitive applications, such as Internet telephony, VoIP, VPNs, streaming media, and TV over Internet.



A service occupying low bandwidth (64Kbps or below).



Transmission of specific programming to predetermined users of a telecommunications network. With narrowcast, only some users of the network are receiving the same information.


NAS (Network Attached Storage)

Storage devices that can be accessed over a computer network rather than being directly connected to the computer. This enables multiple computers to share the same storage space, which minimizes overhead.


NAT (Network Address Translation)

An Internet standard that enables a LAN to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. A NAT box located where the LAN meets the Internet makes all necessary IP address translations.


NCP (Network Control Program)

Software that represents a centralized database that stores a subscriber's unique VPN information. The NCP screens every call and applies call processing according to customer-defined requirements.


near-end crosstalk

Unwanted energy transferred from one circuit usually to an adjoining circuit. It occurs at the end of the transmission link where the signal source is located, with the absorbed energy usually propagated in the opposite direction of the absorbing channel's normal current flow. Near-end crosstalk is usually caused by high-frequency or unbalanced signals and insufficient shielding.


NetBIOS (Network Basic Input/Output System)

A session-layer interface that is widely used in PC networks.


NetBIOS extended user interface

A transport-layer protocol designed to support NetBIOS over 802.2 LANs.



(1) A collection of devices connected by communication lines for data processing or information interchange. (2) A series of points connected by communications channels. (3) A collection of telephone lines normally used for dialed telephone calls. (4) A group of computers and peripherals that are interconnected so that they can communicate with each other.


network architecture

The philosophy and organizational concept for enabling communications between data-processing equipment at multiple locations. The network architecture specifies the processors and terminals and defines the protocols and software that must be used to accomplish accurate data communications.


network control

In a network, the establishment, authorization, and maintenance of logical and physical connections between stations and applications, plus the synchronization, routing, integrity, and recovery of data transmitted during the established connections.


network layer

In the OSI model, the logical network entity that services the transport layer. The network layer is responsible for ensuring that data passed to it from the transport layer is routed and delivered through the network.


network node

A point on a network where communications lines interface. Thus, a network node might be a PBX, a local exchange, a multiplexer, a modem, a host computer, or one of several other devices.


network redundancy

Including in a communication pathway additional links in order to connect all nodes.


network topology

The physical and logical relationship of nodes in a network; the schematic arrangement of the links and nodes of a network, typically either a star, ring, tree, or bus topology, or some hybrid combination thereof.



Communication between stations in a network.


next-generation gateway switch

A switch designed to support a wide variety of trafficdata, voice, fax, multimedia, and other emerging sensory formsover a data backbone. It provides seamless interoperability between the circuits that network the PSTN and packet-switching networks.


next-generation network

A high-speed packet-based network that is capable of transporting and routing a multitude of services, including voice, data, video, and multimedia. A next-generation network is a common platform for applications and services that the customer can access across the entire network as well as outside the network. Next-generation networks offer unrestricted access by users to different service providers and support generalized mobility, which will allow consistent and ubiquitous provision of services to users.


NFC (Near Field Communication)

A technology standard for very-short-range (typically 1 to 2 inches) wireless connectivity that enables quick, secure, two-way interactions among electronic devices. NFC technology, which operates in the 13.56MHz range, typically takes the form of a small chip embedded in a phone or a plastic card (such as a credit card). With NFC-enabled mobile phones, transactions can be conducted by simply touching a point-of-sales device, entry point, or ticket gate.


NGI (Next-Generation Internet)

A U.S. government project that is intended to drastically increase the speed of the Internet. Note that there is a distinct difference between NGI and NGi; the former is a U.S. government initiative, and the latter is a generic movement.


NGI (Next-Generation Internet)

A generic movement toward the future Internet, which will be so pervasive, reliable, and transparent that it will be a seamless part of lifelike electricity or plumbing. NGi projects that are under way include Internet 2, Abilene, HOPI, NLR, MAN LAN, GÉANT2, and TEIN2.


NIC (Network Interface Card)

A component that connects a station to a network (e.g., LAN). Also called a network adapter card.


NIOD (Network Inward/Outward Dialing)

A system that provides the capability for dialing both ways between a toll network and a local network.


N-ISDN (Narrowband ISDN)

A network architecture and set of standards for an all-digital network. N-ISDN was intended to provide end-to-end digital service using the public telephone networks worldwide and, therefore, to provide high-quality, error-free transmission.


NIST (National Institute of Standards and Technology)

A nonregulatory agency that promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards, and technology.


NLSP (NetWare Link Services Protocol)

A link-state protocol that offers low network overhead and fast convergence.


NMC (Network Management Center)

A center used for control of a network. An NMC may provide traffic analysis, call detail recording, configuration control, fault detection and diagnostics, and maintenance.


NMT (Nordic Mobile Telephone)

A Scandinavian wireless system that originally operated at 450MHz, offered around 220 channels, and had a very large coverage area.



(1) A device on a network that can independently send or receive information and that has a network address. (2) The point at which a device is linked to a network.



Unwanted electrical signals, introduced by circuit components or natural disturbances, that tend to degrade the performance of a communications channel.



A switch that has a through-traffic path for each attached station. A nonblocking switch or switching environment is designed to never experience a busy condition due to traffic volume.


nonswitched line

In data communications, a permanent connection between computers or devices that does not have to be established by dialing.


nonvolatile storage

A storage medium whose contents are not lost when the power is removed.


NPA (Numbering Plan Area)

A geographic subdivision of the territory covered by a national or integrated numbering plan. An NPA is identified by a distinctive area code.


NRZ (Nonreturn to Zero)

A digital signaling technique in which the signal is at a constant level for a duration of time.


NSIS (Next Steps In Signaling)

A network architecture that addresses the introduction of QoS on an end-to-end basis. The design goals of NSIS include applicability across different QoS technologies, such as DiffServ and MPLS, as well as resource availability upon request prior to a reservation request.


NSP (Network Service Provider)

A very large, global backbone carrier that owns its own infrastructures (e.g., AT&T, Sprint, Verizon).


NTSC (National Television Standards Committee)

A television broadcasting system that uses 525 picture lines and a 60Hz field frequency. It is used primarily in the United States, Canada, Mexico, and Japan. See also PAL and SECAM.


NTU (Network Termination Unit)

A device that connects the PSTN with CPE. An NTU marks the final interconnect between the public network and a customer's private equipment. It is owned by the service provider and typically has communication standards, such as voltages and protocols, that allow specific types of equipment to communicate with the PSTN.


null attached

The operation of an FDDI concentrator without being attached to the backbone network. This configuration establishes a small, autonomous, single-ring FDDI network consisting of a limited number of directly connected single-attached stations.



The current exchange numbering plan, in which N is any digit from 2 to 9 and X is any digit from 0 to 9.



OADM (Optical Add/Drop Multiplexer)

An optical multiplexing device that uses special filters to extract the wavelengths that need to be dropped off at a given location. It eliminates the costly electronics used to convert between light and electricity in a nonoptical multiplexer.


OC-1 to OC-768 (Optical Carrier-1 to Optical Carrier-768)

Optical carrier levels used for the categories of bandwidth in a SONET fiber-optic network. OC-1 is the base optical carrier transmission speed of 51.840Mbps. To calculate OC-2 to OC-768 speeds, simply multiply the OC-1 base by the desired magnitude. Common OC levels include OC-1 (51Mbps), OC-3 (155Mbps), OC-12 (622Mbps), OC-48 (2.5Gbps), OC-192 (10Gbps), and OC-768 (40Gbps).


OCAP (OpenCable Applications Platform)

The OpenCable software specification, which creates a common platform on which interactive services can be deployed. OCAP is intended to enable the developers of interactive TV services and applications to design such products so that they will run successfully on any cable TV system in North America, regardless of the set-top or television receiver, hardware, or operating system software choices.



8 bits. An octet is equivalent to a byte, as long as the byte is also 8 bits. Whereas bytes can range from 4 to 10 bits, octets are always 8 bits.


odd parity check

A test of whether the number of digits in a group of binary digits is odd.


OFDM (Orthogonal Frequency Division Multiplexing)

A multicarrier modulation scheme that broadcasts on many frequencies, reducing interference from collisions with walls and objects.


OFDMA (Orthogonal Frequency Division Multiple Access)

A multiple-access scheme for OFDM systems that defines either 2,048 or 4,096 subcarriers. In current OFDM systems, only a single user can transmit on all the subcarriers at any given time. OFDMA allows multiple users to transmit simultaneously on the different subcarriers per OFDM symbol.



A state in which a telephone set is in use (i.e., the handset is removed from its cradle).



(1) Pertaining to equipment or devices not under direct control of the central processing unit. (2) Used to describe terminal equipment that is not connected to a transmission line. (3) Not controlled directly by or communicating with a computer.



To move data or programs out of storage.


OGT (Outgoing Trunk)

A one-way trunk that carries only outgoing traffic.


OLT (Optical Line Termination)

A switch that sends traffic downstream to subscribers and that also handles the upstream traffic.


ones density rule

A scheme that allows a CSU/DSU to recover the data clock reliably. The CSU/DSU derives the data clock from the data that passes through it. In order to recover the clock, the CSU/DSU hardware must receive at least one 1-bit value for every 8 bits of data that pass through it. Also called pulse density.


one-way trunk

A trunk between a switch (e.g., a PBX) and an exchange or between exchanges, where traffic originates from only one end.



A state in which a telephone set is not in use (i.e., the handset is resting in the cradle).



(1) Being controlled directly by or directly communicating with a computer. (2) Connected to a computer so that data can pass to or from the computer without human intervention. (3) Directly in the line loop.


online services

Computer functions offered to end users who do not own host computers. Online services include time sharing, archival storage, and prepared software programs.


ONP (Open Network Provision)

A pan-European standard that ensures the provision of the network infrastructure by European telecommunications administrations to users and competitive service providers on terms equal to those for the administrations themselves.


ONU (Optical Network Unit)

A device in which optical-to-electrical conversions take place.


open system

A system that facilitates multivendor, multitechnology integration based on publicly available standards for subsystem interaction. Three characteristics of an open system are portability, scalability, and interoperability.



A CableLabs initiative to publish specifications that define digital cable network interfaces and the nature of next-generation cable set-top boxes. The goal of OpenCable is to help the cable industry deploy interactive services over cable, create a common standard for digital cable TV within the United States, and promote competition among licensed device manufacturers.


optical carrier

Specifications that define line speeds and transmission encoding and multiplexing methods for the SDH/SONET fiber-optic backbone network.


optical fiber

Any filament, or fiber, made of dielectric materials that is used to transmit laser- or LED-generated light signals, usually for digital communications. An optical fiber consists of a core, which carries the signal, and cladding, a substance with a slightly higher refractive index than the core, which surrounds the core and reflects the light signal back into it. Also called lightguide or fiber-optic.


optical switch

A device that resides at a junction point in an optical backbone and enables signals in optical fibers or integrated optical circuits to be selectively switched from one circuit to another. An optical switch links any of several incoming lines to any of several outgoing lines and automatically reroutes traffic when a network path fails.


OPX (Off-Premises Extension)

A telephone extension located other than where the main switch is.


OSI (Open Systems Interconnection) model

A seven-layer logical network architecture used to define network protocol standards to enable an OSI-compatible computer or device to communicate with any other OSI-compliant computer or device for a meaningful exchange of information. Layer 7, the application layer, is responsible for exchanging information between the programs running on a computer and other services on a network. Layer 6, the presentation layer, formats information so that a software application can read it. Layer 5, the session layer, supports connections between sessions and handles administrative tasks and security. Layer 4, the transport layer, corrects transmission errors and ensures that the information is delivered reliably. Layer 3, the network layer, identifies computers on a network and determines how to direct information transfer over that network. Layer 2, the data link layer, groups data into containers to prepare that data for transfer over a network. Layer 1, the physical layer, defines how a transmission medium connects to a computer, as well as how electrical or optical information is transferred on the transmission medium.


OSP (Online Service Provider)

A provider that organizes online content and provides intuitive user navigation.


OSPF (Open Shortest Path First)

A routing protocol used on TCP/IP networks in which routers maintain an internal map of the network and exchange information about the current state of each network link. OSPF's features include least-cost routing, multipath routing, and load balancing.


OSS (Operational Support System)

A set of programs that help a communications service provider monitor, control, analyze, and manage problems with a telephone or computer network.


OTDR (Optical Time Domain Reflectometer)

A device that can be used in fiber networks to detect potential leaks that could be the result of unwanted intrusion.


OTN (Optical Transport Network)

A network that is composed of a set of optical network elements connected by optical fiber links and is able to provide functionality of transport, multiplexing, routing, management, supervision, and survivability of optical channels carrying client signals, according to the requirements given in ITU-T Recommendation G.872.


out-of-band management

A system in which management data is communicated through a link, outside the network, typically through a modem or some other serial connection.


out-of-band signaling

Signaling in which the conversation and the signaling take place over different paths. A separate digital channel (called a signaling link) is created, where messages are exchanged between network elements at 56Kbps or 64Kbps. Out-of-band signals run no danger of interference from speech or data, which allows signaling to take place during the conversation. However, the out-of-band signal needs extra bandwidth and extra electronics to handle the signaling band.



Data that has been processed.



Excess traffic, on a particular route, that is offered to another (alternate) route.


overlay network

A high-performance digital network that interconnects with the main public network but has its own lines, exchanges, and, often, a separate international gateway.






To seize a circuit even though the circuit is already occupied.



PABX (Private Automatic Branch Exchange)

See PBX.



A group of binary digits, including data and call control signals, that is switched as a composite whole. The data, call control signals, and error control information are arranged in a specific format. Also called block, frame, cell, or datagram.


packet loss

A problem that occurs when there is congestion at the packet switches or routers. Packet loss can considerably degrade real-time applications.


packet overhead

A measure of the ratio of the total packet bits occupied by control information to the number of bits of data, usually expressed as a percentage.


packet radio

A data network that uses licensed bandwidth and is specifically built for two-way data, not for voice communications.


packet switching

A method of transmitting messages through a communication network in which long messages are subdivided into short packets. Each packet contains the data and a destination address and is passed from source to destination through intermediate nodes. At each node, the packet is received, stored briefly, and then passed to the next node. The packets are then reassembled into the original message at the receiving end.



A CableLabs initiative that defines standards for the cable TV industry. It develops interoperable interface specifications for two-way cable networks in order to deliver advanced real-time multimedia services. PacketCable interconnects three networks: the HFC access network, the PSTN, and IP networks. Known internationally as IP Cablecom.


packet-switched network

A network that consists of a series of interconnected switches that route individual packets of data over one of several redundant routes. Packet-switched networks include X.25, Frame Relay, IP, ATM, and MPLS.


PAD (Packet Assembler/Disassembler)

A protocol conversion device that accepts characters in a serial data stream and converts them into packets to send across a packet-switched network, such as an IP network.


PAL (Phase Alternating Line)

The color television broadcasting system developed in West Germany and the United Kingdom that uses 625 picture lines and a 50Hz field frequency. See also NTSC and SECAM.


PAM (Pulse Amplitude Modulation)

A form of modulation in which the amplitude of the pulse carrier is varied in accordance with successive samples of the modulating signal.


PAN (Personal Area Network)

A network that surrounds an individual and provides networking between badge-based computers and other input/output devices.


PAP (Password Authentication Protocol)

A protocol that uses a two-way handshake for the peer to establish its identity upon link establishment. The peer repeatedly sends the password to the authenticator until verification is acknowledged or the connection is terminated.


parallel transmission

The simultaneous transmission of all the bits making up a character or byte, either over separate channels or on different carrier frequencies on the same channel.



The state of being even numbered or odd numbered. A parity bit is a binary digit appended to a group of binary digits to make the sum of the digits either all odd (odd parity) or all even (even parity).


parity check

A procedure in which noninformation bits are added to data to make the number of 1s in a grouping of bits either always even or always odd. This procedure allows detection of bit groupings that contain single errors. It can be applied to characters, blocks, or any specific bit grouping. Also called vertical redundancy check (VRC).


passive optical splitter

A device used in a PON that splits the power of the optical signal and distributes it to multiple customers and in the reverse direction combines multiple light streams onto a single fiber. Passive optical splitters don't need any power, hence the term passive; they work like a prism, splitting light into the colors of the rainbow. This means that a passive optical splitter is not sensitive to temperature or other elements that would be problematic for electrical components. Optical splitters can be located in a telco building, outside plant, or any other building along the fiber route.



A word or set of characters that must be given to satisfy security needs.


patch panel

A passive wiring device that facilitates manual patching of end-user machines onto ports on one or more network hubs.



(1) In a network, any route between any two nodes. (2) The route traversed by the information exchanged between two attaching devices in a network.


Pbps (Petabits per second)



PBX (Private Branch Exchange)

A telephone switch located on a customer's premises that primarily establishes voice-grade circuits between individual users (extensions) and the switched telephone network. Typically, the PBX also provides switching within a customer's premises and usually offers numerous other enhanced features, such as least-cost routing and call-detail recording. Also called a PABX.


PCM (Pulse Code Modulation)

A scheme used to convert an analog voice signal into a digital bitstream for transmission. PCM is a digital transmission technique that involves sampling of an analog information signal at regular time intervals and coding of the measured amplitude into a series of binary values, which are transmitted by modulation of a pulsed, or intermittent, carrier. PCM is a common method of speech digitizing by using 8-bit codewords, or samples, and a sampling rate of 8,000 times per second.


PCS (Personal Communication Services)

A digital service that operates in the 1.8GHz to 2GHz band and uses both microcell and picocell architectures.


PDC (Personal Digital Cellular)

Also known as Japanese Digital Cellular (JDC), a 2G standard for digital wireless communications deployed widely in Japan.


PDH (Plesiochronous Digital Hierarchy)

The first generation of digital hierarchy, which defines the available digital transmission rates and number of channels. PDH is used by telecommunications operators and implemented according to three standards: T-carrier in North America, E-carrier in ITU-T countries, and J-carrier in Japan. PDH is defined by the ITU-T in its G.703 standard.


PDN (Public Data Network)

A generic term for the collection of packet-switching networks that provide public data services. Well-known examples are the Internet and X.25.


PDU (Protocol [or packet] Data Unit)

A message of a given protocol, comprising payload and protocol-specific control information, typically contained in a header. PDUs pass over the protocol interfaces that exist between the layers of protocols (per the OSI model).


peering agreement

An arrangement in which operators agree to exchange with one another the same amount of traffic over high-speed lines between their routers so that users on one network can reach addresses on the other.



The interaction by which computers and other network devices communicate with each other as equals and on their own initiative (as opposed to a client/server environment).



A major factor on which the total productivity of a system depends. Performance is largely determined by a combination of several other factors: throughput, latencies, response time, and availability.


pervasive computing

An environment in which computers are taken out of standalone boxes to which people are tied and put into ordinary things, in everyday objects. Also called ubiquitous computing.


P-frame (Predicted frame)

In MPEG terminology, a frame that is only reconstructed from preceding reference frames. It can also be a reference frame because it reconstructs other frames in some instances.


PGP (Pretty Good Privacy)

A technique for encrypting messages. Using PGP is one of the most common ways to protect messages on the Internet because it is effective, easy to use, and free. PGP is based on the public key method, which uses two keys: a public key disseminated to individuals who should receive a message and a private key used to decrypt messages that are received.


phantom circuit

A third voice circuit that is superimposed on two 2-wire voice circuits.



The angle of a waveform at a given moment.


phase jitter

A random distortion of signal lengths caused by the rapid fluctuation of the frequency of the transmitted signal. Phase jitter interferes with interpretation of information by changing the timing.


phased-array antenna

A small, flat antenna that is steered electronically. It is actually a group of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. It provides great agility and fast tracking, as well as the ability to form multiple antenna beams simultaneously. It allows for very fast and precise steering of the communications beam, which is very important for high-bandwidth communication because the data rate is inversely proportional to the angular offset.


PHS (Personal Handyphone System)

A Japanese standard for 2G PCS wireless networks.


PHY (Physical Layer)

Layer 1 of the OSI model. It defines the electrical, optical, mechanical, and procedural characteristics of the interface.


physical address

The address of a physical communications device in a system.


physical interface

The definition of the number of pins in a connector, the number of wires in a cable, and what signal is being carried over which of the pins and over which of the wires, to ensure that the information is being viewed compatibly.



A tiny cell architecture in the cellular system that covers about 150 feet (46 m). Compared to the microcell design, the picocell design offers even better frequency reuse, even lower power, even smaller subscriber units, and even better, longer battery life.



An ad hoc computer network of devices that use Bluetooth protocols to allow a master device to interconnect with slave devices.


ping (packet Internet groper)

The name of a program used with TCP/IP internets to test reachability of destinations by sending them an ICMP echo request and waiting for a reply.


pixel (picture element)

In computer graphics, the smallest element of a display space that can be independently assigned color and intensity.


PKE (public key encryption)

A message authentication mechanism that is part of most Web browsers.


PKI (Public Key Infrastructure)

A process that secures e-business applications such as private e-mail, purchase orders, and workflow automation. It uses digital certificates and digital signatures to authenticate and encrypt messages and a CA to handle the verification process.



The physical equipment of a telephone network that provides communications services.



A device that converts computer output into drawings on paper or displays the output on display-type terminals instead of printing a listing.


PLP (Packet Layer Protocol)

A standard in the network layer of X.25.


PLT (Powerline Telecommunications)

A wireline technology that provides the transmission of data to users over the same lines that bring electric power to homes and businesses. PLT could allow electrical utilities to provide high-speed Internet, voice, data, and video services to customers of all classes via power transmission and distribution lines. Current technologies offer speeds ranging from 1Mbps to 45Mbps. Also known as powerline communications (PLC) and broadband over powerlines (BPL).


PM (Phase Modulation)

A way to modify a sine wave signal to make it carry information. The sine wave, or carrier, has its phase changed in accordance with the information to be transmitted.


PNAP (Private Network Access Point)

A private point of access to the Internet, also called a peering point, that bypasses public NAPs.


PoC (Push-to-Talk over Cellular)

An open standard for push-to-talk (i.e., walkie-talkie) technology that defines how push-to-talk can be deployed over a cellular operator's packet data network using VoIP. PoC uses VoIP technology to stream voice over data networks such as GPRS. The key to PoC is IMS.


point-to-point line

A circuit that connects two points directly, where there are generally no intermediate processing nodes, although there could be switching facilities. Synonymous with two-point and always-on.


policy-based management

A system in which information about individual users, groups, organizational units, and entire organizations, as well as events (e.g., the beginning of the accounting department's month-end closing), can be associated with various network services or classes of service.



A host-system-controlled method for determining whether each of the stations on a communication line has data to send.


PON (Passive Optical Network)

A network in which one access line is shared among multiple buildings. Optical splitters and couplers are used at each fiber connection in the network.


POP (Point Of Presence)

The physical access location into a network.


POP (Post Office Protocol)

An application-layer protocol that e-mail clients use to retrieve e-mail from a remote server over a TCP/IP connection.



(1) A point of access into a communications switch, a computer, a network, or another electronic device. (2) The physical or electrical interface through which a person gains access to a computer, a network, or another electronic device. (3) The interface between a process and a communications or transmission facility.


port number

A number in the range 1 to 65,535 that identifies a port. The port number does not represent a physical port, such as the serial port to which a modem or a mouse might be attached; instead, it is like a regional memory address.


port speed

The maximum signaling rate on a digital access line.


PoS (Packet over SDH/SONET)

A communications protocol for transmitting packets over the circuit-switched protocols SDH and SONET. Much of the traffic on the Internet is carried over PoS links.


POTS (Plain Old Telephone Service)

The standard analog telephone service that most homes have traditionally used.



Broadband data transmission over electrical powerlines.


PPP (Point-to-Point Protocol)

A protocol that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP is a successor to SLIP.


PPTP (Point-to-Point Tunneling Protocol)

A Layer 2 protocol that can work in a non-IP enterprise environment, which is one of its strengths for customers who use multiple protocols rather than using only IP. PPTP provides low packet overhead and good compression, but it has security weaknesses.


presence-based services

The ability to see in real-time where someone is, how that person prefers to be reached, and even what the person is doing. Presence-based services involve the ability, willingness, desire, and capability of a user to communicate across media end devices and even time and space.


presentation layer

Layer 6 in the OSI model, which provides services to the application layer, enabling it to interpret the data exchanged, as well as to structure data messages to be transmitted in a specific display and control format.


preventive maintenance

The routine checking of components to keep a system functioning.


PRI (Primary Rate Interface)

A bundle of ISDN circuits, primarily a PBX interface. The United States and Japan use 23B+D, and the ITU uses 30B+D. Also called Primary Rate Access (PRA).


primary station

On a point-to-point communication line, the station that gains control of the line first. On a multipoint line, the station that controls communications.


private key

The decryption (reception) or encryption (signature) component of an asymmetrical key set.


private line

The channel equipment furnished to a customer as a unit for exclusive use, generally with no access to or from the PSTN. Also called a leased line.


private network

A network based on leased lines or other facilities that provides telecommunication services within an organization or within a closed user group, as a complement to or a substitute for the public network.



Hardware or software that is privately owned.



A set of rules that govern network communications. Low-level protocols define transmission rates, data encoding schemes, physical interfaces, network addressing schemes, and the method by which nodes contend for the chance to transmit data over a network. High-level protocols define functions such as printing and file sharing.


protocol header

Information in a packet that the protocol needs in order to do its work.


protocol stack (or protocol suite)

A collection of protocols that computers use to exchange information.


provisioned VPN

A packet-switched VPN that runs across the service provider's backbone, generally using Frame Relay or ATM.


proxy ARP (proxy Address Resolution Protocol)

A technique in which one machine, usually a router, answers ARP requests intended for another by supplying its own physical address. By pretending to be another machine, the router accepts responsibility for routing packets to it. The purpose of proxy ARP is to allow a site to use a single IP network address with multiple physical networks.


proxy server

A server that provides firewall functionality, acting as an intermediary for user requests and establishing a connection to the requested resource either at the application layer or at the session or transport layer.


PSK (Phase-Shift Keying)

A modulation technique for transforming digital information to analog whereby that information is conveyed as varying phases of a carrier signal.


PSTN (Public Switched Telephone Network)

The complete traditional public telephone system, including telephones, local and interexchange trunks, transport equipment, and exchanges.


PTO (Public Telecommunications Operator)

An incumbent carrier. This term is used in countries other than the United States.


PTT (post, telegraph, and telephone) organization

Usually a governmental department that acts as its nation's common carrier. Also called PT&T.


public key

A device used by algorithms that encrypt and decrypt using asymmetric yet mathematically linked keys. Each security module is assigned a pair of keys: The encryption key is public and does not require distribution by secure means. The decryption, or private, key cannot be discovered through knowledge of the public key or its underlying algorithm. Public key algorithms can apply to one or more of the following: key distribution, encryption, authentication, or digital signature.



A momentary, sharp alteration in the current or voltage produced in a circuit to operate a switch or relay that can be detected by a logic circuit. A pulse is a sharp rise and fall of finite duration.


punchdown block

A common termination point in the wiring closet for wires going out to the individual offices and wall sockets.


push technology

A program that updates news, weather, or other selected information on a computer user's desktop interface through periodic and generally unobtrusive transmission over the World Wide Web.



A mobile application that turns a mobile phone into a walkie-talkie by giving it trunked radio functionality. Sometimes abbreviated PTT.



A mobile application that is based on push-to-talk and combined with other applications, including presence, picture sharing, and video clips.


PVC (Permanent Virtual Circuit)

A defined path that provides essentially a dedicated private line between users in a packet-switching network. The network is aware of a fixed association between two stations, permanent logical channel numbers are assigned exclusively to the permanent circuit, and devices do not require permission to transmit to each other.


PWM (Pulse Width Modulation)

The process of encoding information based on variations of the duration of carrier pulses. Also known as pulse duration modulation (PDM).



The ITU-T H.320 standard for interoperability in videoconferencing over ISDN.



QAM (Quadrature Amplitude Modulation)

A single-carrier scheme that modulates both the amplitude and phase to yield higher spectral efficiency. Various levels of QAM exist, and they are referred to as nn-QAM, where nn indicates the number of states per Hertz (Hz). The number of bits per symbol time is k, where 2k = nn. So, 4 bits/Hz is equivalent to 16-QAM, 6 bits/Hz is equivalent to 64-QAM, and 8 bits/Hz is equivalent to 256-QAM.


QoS (Quality of Service)

In networking, a concept by which applications can indicate their specific requirements to the network before they actually start transmitting information data. Implicit QoS means that the application chooses the required levels of QoS. With explicit QoS, the network manager controls that decision.


QPSK (Quadrature Phase-Shift Keying)

A single-carrier modulation scheme that supports 2 bits per symbol time.



The Ecma International standard for common-channel signaling between PBXs.


quadruple-play services

A service provider's bundle of voice, data, video/TV, and wireless/mobile services, delivered over the same infrastructure.


quantization noise

Signal errors caused by the process of digitizing a continuously variable slope.



A request for information that is entered while a computer system is processing.



An ordered accumulation of data or transactions stored for later processing.



A process in which communications calls, processing requests, processes, and so on are stacked or held so that they can be worked with in sequence.


QVGA (Quarter Video Graphics Array)

A computer display with 320 x 240 resolution, such as on a mobile phone, PDA, or handheld gaming console.


radio channel

The frequency band allocated to a service provider or transmitter.


radio wave

An electromagnetic wave of frequencies between approximately 20KHz and 3GHz.


RADIUS (Remote Authentication Dial-in User Services)

An authentication and access control server used to determine whether a user is allowed access to corporate resources.


RADSL (Rate-Adaptive DSL)

A symmetrical or asymmetrical transmission medium that has a maximum loop length of 18,000 feet (5.5 km) and is deployed as a single twisted-pair cable. RADSL adapts the data rate dynamically, based on any changes that may be occurring in the line conditions and based on the loop length. With RADSL, the rates can vary widely, from 600Kbps to 7Mbps downstream and from 128Kbps to 1Mbps upstream.


RAM (Random-Access Memory)

A type of computer memory that can be accessed randomly (i.e., any byte of memory can be accessed without touching the preceding bytes).


Raman amplifier

An optical amplifier that uses a powerful laser source to boost the signal power in standard optical fiber. In this type of amplifier, the amplification effect is achieved by a nonlinear interaction between the signal and a pump laser within an optical fiber.


RAN (Radio Access Network)

Part of a mobile telecommunication system that sits between the mobile phone and the core network. It is possible for a single handset to be simultaneously connected to multiple RANs (e.g., both GSM and UMTS RANs).


RARP (Reverse Address Resolution Protocol)

The TCP/IP protocol that a diskless machine uses at startup to find its address. The machine broadcasts a request that contains its physical hardware address, and a server responds by sending the machine its IP address.


RBOC (Regional Bell Operating Company)

One of several independent telephone companies created from the breakup of AT&T.


RBS (Robbed Bit Signaling)

A type of channel-associated signaling used in North America on T-1 trunks in which one of the bits of digital voice is stolen and replaced with the proper signaling bit.


RC4 (Rivest Cipher 4)

A streaming cipher technique. A stream cipher adds the output of a pseudorandom number generator bit by bit to the sequential bits of the digitized plain text.


RED (Random Early Detection)

A queue management algorithm and a congestion avoidence algorithm that monitors the average queue size and drops packets based on statistical probabilities.



(1) The portion of the total information contained in a message that can be eliminated without loss in essential information. (2) The provision of duplicate, backup equipment to immediately take over the function of equipment that fails. (3) In a database, the storage of the same data item or group of items in two or more files.


redundancy check

An automatic or a programmed check based on the systematic insertion of components or characters used especially for checking purposes.


regenerative repeater

(1) A repeater used in telegraph applications to retime and retransmit the received signal impulses and restore them to their original strength. These repeaters are speed and code sensitive and are intended for use with standard speeds and codes. (2) A repeater used in PCM or digital circuits that detects, retimes, and reconstructs the bits transmitted. (3) A LAN interconnect relay device that copies electrical signals from one LAN to another. Also called a regenerator.



The first unit in the assembly of common control equipment in an automatic exchange. The register receives address information in the form of dial pulses or DTMF signals and stores it for possible conversion or translation.



The measure of a network's availability. Reliability is often measured in terms of the number of nines; for example, five-nines reliability means that the network is available 99.999% of the time.



Pertains to a computer or device that is connected to another computer or device over a communication line.


remote access

Communication between a computer or PBX in one location and a device that is physically removed from the location of the computer or PBX.


remote access server

A server that provides network access to remote users, generally via analog POTS lines, or perhaps ISDN connections, including dialup protocols and access control for authentication.


remote access software

Sometimes called remote control software, a program that is a superset of the asynchronous communications software market. It allows a PC to have complete control over another PC at a different site.


remote data concentration

A communications processor used for multiplexing data from low-speed lines or terminals onto one or more high-speed lines.


remote maintenance

A feature or service in which a service technician can dial into a system and be connected to the system and the system processor to run diagnostics and perform system administration.


remote monitoring MIB (remote monitoring Management Information Base)

The MIB that enables any SNMP management console to extract information from a protocol analyzer running remote monitoring.



(1) In analog transmission, equipment that receives a pulse train, amplifies it, and retimes it for retransmission. (2) In digital transmission, equipment that receives a pulse train, reconstructs it, retimes it, and then amplifies the signal for retransmission. (3) In fiber optics, a device that decodes a low-power light signal, converts it to electrical energy, and then retransmits it via an LED or a laser-generating light source, often including some form of signal amplification.


resale carrier

A company that redistributes the services of another common carrier and retails the services to the public.


Resilient Packet Ring

A standard for the optimized transport of data traffic over fiber rings. Its main goal is to increase the efficiency of IP services and Ethernet.


reverse channel

A simultaneous data path in the reverse direction over a half-duplex facility. Normally the reverse channel is used for positive/negative acknowledgments of previously received data blocks.


RF (Radio Frequency)

A frequency that is much higher than the audio frequencies but below the infrared frequencies. RF is usually above 20KHz.


RFI (Radio Frequency Interference)

EMI that causes unwanted signals (interference or noise) in radio communications.


RFID (Radio Frequency Identification)

At a simple level, a technology that involves tags that emit radio signals and devices called readers that pick up the signal. RFID is a method of remotely storing and retrieving data by using RFID tags and readers. RFID can be used to identify and track any movable item or asset. RFID is an alternative to bar coding. Its advantages include data capacity, read/write capability, and a lack of line-of-sight requirements.


RFID encoder

A read/write RFID reader. RFID encoders can be used to read information from an RFID tag and to program information into a blank RFID tag.


RFID reader

A device that processes the data from RFID tags according to the application requirements. RFID readers are used to query RFID tags in order to obtain identification, location, and other information about the object the tag is embedded in.


RFID tag

A small object that can be attached to or incorporated into a product, an animal, or a person and then read by an RFID reader. An RFID tag is a tiny microchip composed of a processor, memory, and a radio transmitter that is mounted onto a substrate or an enclosure. Also referred to as an RFID transponder.


Rijndael algorithm

The algorithm used for AES. The algorithm was designed by two Belgian cryptologists, Vincent Rijmen and Joan Daemen.



(1) A ring-shaped contact of a plug, usually positioned between but insulated from the tip and sleeve. (2) An audible alerting signal on a telephone line. (3) A network topology in which stations are connected to one another in a closed logical circle, with access to the medium passing sequentially from one station to the next by means of polling from a master station or by passing an access token from one station to another. Also called a loop.


ring in/ring out

A connection on a token-ring MAU that is used to tie multiple MAUs into a larger ring.


ringing signal

Any AC or DC signal transmitted over a line or trunk for the purpose of alerting a party at the distant end of an incoming call. The signal can operate a visual- or sound-producing device.


RIP (Routing Information Protocol)

A routing protocol used on TCP/IP networks that distributes the addresses of reachable networks and metrics reflecting the degree of difficulty involved in reaching particular networks form particular locations.



A standard four-wire modular connector used with telephones.



An eight-wire modular connector used with UTP.


RO (Receive Only)

(1) A printer terminal without a keyboard for data entry. (2) A satellite earth station that can receive but not transmit signals.


ROADM (Reconfigurable Optical Add/Drop Multiplexer)

An add/drop mux that is software based and adds the capability to switch between different networks. ROADMs can switch traffic at both the SDH/SONET and wavelength layers.



An easily reprogrammable, computer-controlled device that can physically manipulate its surroundings.


rotary dial calling

A system that accepts dialing from conventional rotary dial sets that generate pulses.



A device that connects two LAN segments that use similar or different architectures at the OSI network layer (Layer 3). A router determines the most efficient route for passing data through an internet. Packets that contain a network address different from the originating PC's address are forwarded to an adjoining network. Multiprotocol routers can handle this job for two or more protocols simultaneously.


routing algorithm

A formula that uses metrics (e.g., path length, destination, next-hop associations, reliability, delay, bandwidth, load, communication cost) to determine the best path to the destination.


routing protocol

A protocol that enables routers to communicate with each other. Routing protocols include RIP, IGP, OSPF, EGP, and BGP.


routing table

A database that tells a router how to send packets to various destinations.


RPC (Remote Procedure Call)

A system that enables an application programmer to distribute programs between computer systems interconnected by a network. RPC development tools eliminate the need for in-depth knowledge of diverse network protocols and computing platforms, enabling a programmer to concentrate on developing the application itself.



A technical specification published by the EIA that establishes mechanical and electrical interface requirements between DTE and DCE, employing serial binary data interchange and operating at speeds up to 19.2Kbps.



An EIA standard for interfaces between DTE and automatic calling equipment for data communication.



An EIA specification for electrical characteristics of balanced-voltage digital interface circuits.



An EIA specification for electrical characteristics of unbalanced-voltage digital interface circuits.



An EIA specification for general-purpose, 37-position and 9-position interfaces for DTE and DCE, employing serial binary data interchange and operating at speeds up to 2Mbps.


RSA (Rivest, Shamir, and Adelman)

A public key algorithm whose security derives from the difficulty of factoring large prime integers.


RSVP (Resource Reservation Protocol)

A protocol that enables an internet to support specified levels of QoS. By using RSVP, an application can reserve resources along a route from source to destination. RSVP-enabled routers then schedule and prioritize packets to fulfill the QoS.


RSVP-TE (Resource Reservation ProtocolTraffic Engineering)

A router-based RSVP modification for MPLS traffic engineering that supports the instantiation of explicitly routed label-switched paths with or without resource reservations. It also supports smooth rerouting of label-switched paths, preemption, and loop detection.


RTCP (Real-Time Transport Control Protocol)

An ITU protocol that provides status feedback from senders to receivers.


RTCP XR (RTCP Reporting Extension)

An emerging media transport standard that allows information hidden by SRTP to be extracted directly from the digital signal processor software and IP phones and gateways and reported directly in the RTCP XR message.


RTMP (Routing Table Maintenance Protocol)

A communication protocol used by AppleTalk to ensure that all routers on the network have consistent routing information.


RTP (Real-Time Transport Protocol)

An Internet protocol for transmitting real-time data such as audio and video. RTP itself does not guarantee real-time delivery of data, but it does provide mechanisms for the sending and receiving applications to support streaming data.


RTSP (Real-Time Streaming Protocol)

A protocol that runs on top of IP multicasting, UDP, RTP, and RTCP.




A statistical procedure whereby generalizations are drawn from a relatively small number of observations.


SAN (Storage Area Network)

A network designed to attach computer storage devices such as disk array controllers and tape libraries to servers. SANs are common in enterprise storage.


satellite communications

The use of orbiting satellites to relay transmissions from one earth station to another or to multiple earth stations.



A cause of lightwave signal loss in optical fiber transmission. Diffusion of a light beam is caused by microscopic variations in the material density of the transmission medium.


SCP (Service Control Point)

A centralized node that contains service logic for the management of a call.



A coding device that is applied to a digital channel to produce an apparently random bit sequence. A corresponding device is used to decode the channel (i.e., the coding is reversible).


ScTP (Screened Twisted-Pair)

STP cabling with metal shielding also covering the group of shielded copper pairs. This type of cabling offers improved protection from interference from external sources.


SCTP (Stream Control Transmission Protocol)

A transport protocol that supports multistreaming and multihoming.


SDH (Synchronous Digital Hierarchy)

An ITU-T standard for digital broadband communications.


SDLC (Synchronous Data Link Control)

An IBM data linklayer protocol associated with SNA. In contrast to BSC, SDLC provides for full-duplex transmission and is more efficient.


SDP (Session Description Protocol)

A protocol used to describe and encode the capabilities of multimedia session participants. This description is then used to negotiate the characteristics of the session so that all the devices can participate.


SDR (Software-Defined Radio)

A forthcoming application of multiradio technology that is a collection of hardware and software technologies that enable reconfigurable system architectures for wireless networks and user terminals. An SDR device can become a cellular phone, a GPS receiver, an amateur packet radio, or any other sort of radio transmitting or receiving device.


SDSL (Symmetrical [or Single-Line] DSL)

A symmetrical service that has a maximum loop length of 18,000 feet (5.5 km) and is deployed as a single twisted-pair cable. It can be deployed in various capacities, in multiples of 64Kbps, up to a maximum of 2Mbps in each direction.


SDTV (Standard-Definition Television)

A DTV format that provides a picture quality similar to that of DVD.


SECAM (Systeme Electronique Couleur Avec Memoire [Sequential Color with Memory])

A TV system used in France and the former French colonies, as well as in parts of the Middle East. Russia and the former Soviet-allied countries use a modified form of SECAM. There are two versions of SECAM: SECAM vertical and SECAM horizontal. See also PAL and NTSC.


secondary channel

A low-speed channel established on a four-wire circuit over which diagnostics or control information is passed. User data is passed on the primary, high-speed channels of the circuit.



Protection of information against unauthorized access or use.


Segmentation and Reassembly sublayer

A sublayer of the AAL that supports mapping between variable-length data units and ATM cells.


selective ringing

A system that has the capability of ringing only the desired subscriber's telephone on a multiparty line. Ringers tuned to one of five possible frequencies are used to achieve this effect.


Semantic Web

Tim Berners-Lee's latest project, which is focused on creating a universal medium for information exchange using refined indexing and searchingin other words, providing meaning (semantics) to the content of documents in a manner that is understandable to machines.


sensor mesh network

A type of wireless micromesh network that uses many connected sensors to monitor a variety of factors. The sensors are all connected to each other and then to a base station. The sensors synchronize with each other, collect data, and sleep until the next iteration.



A processor that provides a specific service to the network. A routing server connects nodes and networks of like architectures, a gateway server connects nodes and networks of different architectures, and so on.


server-based network

A network in which one computer is the repository (i.e., the server) and the other computers request information from and store information on the server.



A period of time in which an end user engages in dialog with an interactive computer system.


session layer

Layer 5 of the OSI model. It manages a logical connection between two communication points.


set-top box

A locally powered piece of cable equipment that resides in the subscriber's home and provides tuning, descrambling, and pay-per-view capabilities.


SGSN (Serving GPRS Support Node)

A device that provides the packet-switched link to mobile stations in the mobile IMS architecture. The SGSN provides packet routing to and from the SGSN service area for all users in that service area.


SHA-1 (Secure Hash Algorithm-1)

A message authentication mechanism that hashes a file of arbitrary length into a 160-bit value. It is more processor intensive than other mechanisms (such as MD5) but renders greater security.


SHDSL (Symmetric High-Speed DSL)



SHF (Superhigh Frequency)

Frequencies from 3GHz to 30GHz.


shielded cable

A cable in which the signal-carrying wire is enclosed by an outer sheath to reduce the effects of EMI on the signals. The shielding also reduces the effect of these signals on nearby electrical components and helps to prevent electronic eavesdropping.


ships-in-the-night routing

An approach to routing multiple protocols by which each protocol is routed based on a separate routing mechanism. For example, IP routed via OSPF and Connectionless Network Protocol (CLNP) routed via IS-IS might be used to handle the routing for both IP and CLNP.



The frequency band on either the upper or lower side of the carrier frequency band within which the frequencies produced by the process of modulation fall. Various modulation techniques make use of one or both of the sidebands, and some of them also suppress the carrier frequency.



A physical, time-dependent energy value used for conveying information through a transmission line.



The process by which a caller or equipment on the transmitting end of a line informs a particular party or equipment at the receiving end that a message is to be communicated.


SIM (subscriber identity module) card

A smart card that defines the accounting and personal details of a service and can be used in any GSM handset to activate service.


SIMPLE (SIP for Instant Messaging and Presence Leveraging Extensions)

A set of SIP extensions for IM and presence capabilities.



Pertaining to the capability to transmit in one direction only.


simplex circuit

A circuit that permits the transmission of signals in one specified direction only.


single-attached station

A station connected to an FDDI network over a single fiber pair using a concentrator as the dual-attached station. Single-attached stations can be attached to only one ring. These stations are less expensive than dual-attached stations but are less reliable because the optics, electronics, and physical link must all be operational for the single-attached station to connect to the network.


single-carrier modulation

A modulation scheme in which a single channel occupies the entire bandwidth.


SIP (Session Initiation Protocol)

An application-layer control or signaling protocol for creating, modifying, and terminating sessions with one or more participants. SIP can establish sessions for features such as audio- or videoconferencing, interactive gaming, and call forwarding to be deployed over IP networks, thus enabling service providers to integrate basic IP telephony services with Web, e-mail, and chat services.


SIP telephony

Voice telecommunications that occur over a data network, using the SIP protocol. SIP telephony enables capabilities such as presence management across both voice and instant messaging, instant messaging archiving, click-to-talk voice functionality, speech-to-text functionality, personal mobility, and time-of-day routing.


skin effect

An occurrence in which as electricity migrates to the medium's skin, resistance increases because less of the wire is used.


SLA (Service-Level Agreement)

A contract between an ASP and an end user that stipulates and commits the ASP to a required level of service. An SLA should contain a specified level of service, support options, enforcement or penalty provisions for services not provided, a guaranteed level of system performance as related to downtime or uptime, a specified level of customer support, and what software or hardware will be provided and for what fee.



The third contacting part of a telephone plug, preceded in the location by the tip and ring.


SLIP (Serial Line Internet Protocol)

A minimal character-oriented protocol that provides basic IP encapsulation over dedicated or dialup asynchronous lines. It has largely been replaced by PPP.


smart card

A credit-card-sized device with embedded processors that provide a means of secure electronic storage. A smart card can be programmed to decrypt messages, to verify messages and digital signatures, and to create digital signatures for outgoing messages.


smart house

A home in which broadband services connect various intelligent devices, such as smart appliances.


SMDS (Switched Multimegabit Data Service)

A high-speed, packet-switched, datagram-based WAN networking technology that is offered by some telephone companies. It operates from 1.544Mbps to 44.736Mbps.


SMP (Simple Management Protocol)

An SNMP extension that includes security features, bulk retrieval, manager-to-manager communication, better definition of managed objects, improved error handling, and configurable exception reporting while running over protocols other than UDP, and requiring less memory for implementation.


SMR (Specialized Mobile Radio)

Any two-way radio system in which two or more mobile/portable wireless transceivers are linked by a single repeater. The repeater is elevated above average terrain to maximize the area of coverage. An SMR system is similar to but simpler than a cellular telephone network.


SMS (Service Management System)

A system that is used to build and maintain a PSTN-based VPN database and that allows customers to program specific functions to accommodate their particular business applications.


SMS (Short Messaging Service)

A system that enables text messages up to 160 alphanumeric characters in length to be sent to and from a GSM phone and to an external system such as e-mail, paging, and voicemail systems.


SMTP (Simple Mail Transfer Protocol)

The message transport protocol used by TCP/IP networks, such as Internet and other UNIX-based network systems, for the exchange of electronic messages.


SNA (Systems Network Architecture)

(1) IBM's standardized relationship between its Virtual Telecommunications Access Method (VTAM) and the NCP. (2) An architecture that specifies how products connect and communicate with one another in a network. SNA is a design for a total data communications system, encompassing every part of the communication network, from the user's application program at the central site to the terminal at a remote location.


SNMP (Simple Network Management Protocol)

A standard low-level protocol used to monitor and manage nodes on a network. SNMP consists of agent software, which runs on the managed device, and manager software, which resides on a central system that polls the agents. SNMP is an alternative to CMIP.


SNMPv2 (Simple Network Management Protocol version 2)

A revision of SNMP that includes additional security features, the ability to transfer a large chunk of data at once, and the ability to communicate between SNMP management stations.


SNR (Signal-to-Noise Ratio)

The relative power of a signal compared to the power of noise on a line, expressed in decibels. As the ratio decreases, it becomes more difficult to distinguish between information and interference.


SOA (Semiconductor Optical Amplifier)

An optical amplifier that is built on a single chip. SOAs can be integrated into multifunction optical chips and can be less expensive than EDFAs. The disadvantage of SOAs is a higher signal-to-noise ratio.



An interface to the transport layer of the OSI model that consists of a group of functions that can be called from a program written in C. The calls that make up sockets are system calls (i.e., they are direct calls to an operating system). Sockets permit an application program to access the services provided by TCP and UDP.


SOFDMA (Scalable Orthogonal Frequency Division Multiple Access)

A multicarrier modulation technique in which the number of tones, or subcarriers, used to transport the data and signaling traffic scales with the channel bandwidth. Mobile WiMax uses SOFDMA.



A device that implements service logic to control external trunking gateways, access gateways, and remote access servers. Softswitches run on commercial computers and operating systems, and they provide open applications programming interfaces. Also called a call agent or a media gateway controller.



Computer instructions that perform a common function for all users as well as specific applications for particular user needs.


solid-state device

Electronic pathways made of solid materials (e.g., chips, bubble memories).


SONET (Synchronous Optical Network)

An ANSI standard, incorporated into the SDH standard, that defines a line rate hierarchy and frame formats for use with high-speed optical fiber transmission systems (50.84Mbps to 2.488Gbps).


source route bridging

A bridging protocol supported by the 802.5 Token Ring standard. With source routing, the sending station is responsible for providing routing information for messages that cross multiple network segments. The sending station acquires routing information by first issuing a command to find the location of a particular destination. When the message is sent, the source station specifies the route to use by using the routing information field of the frame.


source routing transparent

The combination of source routing and transparent bridging in the same device.


SP (Service Provider)

A generic term for providers of different types of services.


space division

The carving up of a cellular service area into smaller coverage areas.


Space Division Multiplexing

The use of multiple antennas to simultaneously transmit data to the receivers in small pieces. The receivers then process the data flows and put them back together. This process proportionally boosts the data transmission speed by a factor equal to the number of transmitting antennas (i.e., using four antennas increases the data rate by a factor of four). In addition, because all data is transmitted both in the same frequency band and with separate spatial signatures, Space Division Multiplexing uses spectrum very efficiently. Also referred to as spatial multiplexing.


space division switching

A method for switching circuits in which each connection through the switch takes a physically separate path.


spanning tree

A loop-free subset of a network's topology.


spatial capacity

A measure of the number of bits per second per square meter that can be supported over a given technology. UWB has a much greater spatial capacity than the 802.11a and 802.11b standards and than Bluetooth.


S-PCS (Satellite Personal Communications System)

A system that uses satellites to provide ubiquitous mobile communications.


SPE (Synchronous Payload Envelope)

The payload portion of an STS or STM frame.


spectral compatibility

The capability of two DSL systems to operate inside a single cable bundle. The spectral compatibility of the DSL systems depends on the effect of crosstalk that one DSL system has on the other inside the cable bundle.


spectral efficiency

A measure of the number of digital bits that can be encoded in a single cycle of a waveform.


spectrum utilization

The efficient use of the available bandwidth by many competing devices. Spectrum utilization techniques include space division, multiple-access techniques, spread spectrum, duplexing, and compression techniques.



A filter used at each end of a copper pair to split the frequency bands.



Temporarily storing input and output data streams on disk or tape files until the processor is ready.


spread spectrum technique

A technique by which a signal is transmitted in a bandwidth considerably greater than the frequency content of the original information. FHSS, DSSS, CDMA, and OFDM are examples of spread spectrum techniques.


SPX (Sequenced Packet Exchange)

The NetWare communications protocol used for interprocess communications. SPX guarantees that an entire message arrives intact and uses the NetWare IPX protocols as its delivery mechanism.


SRTP (Secure Real-Time Transport Protocol)

A security profile for RTP that adds confidentiality, message authentication, and replay protection to RTP. It provides a framework for encryption and message authentication of RTP and RTCP streams.


S/S (Start/Stop) signaling

A form of asynchronous communication line control that uses start elements and stop elements to control the transfer of data over a communication line. Each group of signals representing a character is preceded by a start signal and followed by a stop element.


SS7 (Signaling System 7)

A group of telephony signaling protocols used in the PSTN to set up calls and provide services. SS7 is characterized by high-speed circuit switching and out-of-band signaling using SS7 nodes. The SS7 network sets up and tears down a call, handles all the routing decisions, and supports all telephony services, such as 800 numbers, call forwarding, caller ID, and local number portability. Because the signaling network is separate, a voice circuit is not tied up until a connection is made between two parties.


SS7 gateway

A device that allows an interface between circuit-switched networks (with their powerful SS7 infrastructure) and packet-switched networks for support of traditional telephony applications.


SSL (Secure Sockets Layer)

A commonly used protocol for managing the security of a message transmission on the Internet.


SSP (Service-Switching Point)

A switch that originates or terminates a call.


ST connector

A type of connector used on optical fiber cable, using a twist and lock coupling similar to the BNC connector that is used with thinnet.



A network topology in which nodes are connected to a single central hub rather than to each other, forming a star-shaped configuration. If the hub is an intelligent device that controls the nodes' access to the network, the star topology is called active; when the hub is only a wiring connector, it is called passive.


static routing

Routing in which the routes are manually entered into the routing table.



A computer or device that can send and receive data over a communication line.


statistical multiplexer

A multiplexer that uses the idle time of connected devices to carry data traffic from active devices.


STDM (Statistical Time Division Multiplexing)

A method of TDM in which time slots on a shared medium are allocated on demand.


STM (Synchronous Transport Module)

The basic building block of SDH. STM-1 consists of 9 rows of 270 bytes each. The first 9 columns contain the section and line overhead for all STS-3s. The remaining 261 columns contain the combined envelope capacity of the component STS-1s. Common STM levels include STM-0, STM-1, STM-4, STM-16, STM-64, and STM-256.


stop bit

In asynchronous transmission, the quiescent state following the transmission of a character; usually required to be at least 1-, 1.5-, or 2-bit times long.


stop element

The last bit of a character in asynchronous serial transmission, used to ensure recognition of the next start element.



A technique that involves accepting a message or packet on a communications pathway, retaining it in memory, and retransmitting it to the next station. Synonymous with message switching.


STP (Shielded Twisted-Pair)

Two insulated wires in a cable that is wrapped with metallic braid or foil to prevent interference and to provide noise-free transmission.


STP (Signal Transfer Point)

A switch that is responsible for translating SS7 messages and then routing those messages between the appropriate network nodes and databases.


STP (Spanning Tree Protocol)

A protocol for complex bridge configurations defined by IEEE 802.1D. This protocol ensures that a complex bridge configuration has no loops (i.e., there is one and only one possible path from any particular endpoint to any other).


streaming media

Data that is transferred so that it can be processed as a steady and continuous stream. Streaming technologies are increasingly important with the growth of the Internet because many users do not have fast enough access to download large multimedia files quickly. With streaming media, the client browser or plug-in can start displaying the data before the entire file has been transmitted.


STS (Synchronous Transport Signal)

The basic building block of SONET. The basic building block signaling rate for a SONET transmission medium is STS-1, which is 51.8Mbps. The signal is composed of 8,000fps, with 810 8-bit bytes per frame (8 bits/byte x 810 bytes/frame x 8,000fps = 51.840Mbps). A SONET STS-1 frame contains more than 30 times the data of a T-1 frame. Other STS rates are STS-3, STS-12, STS-48, STS-192, and STS-768.


subnet mask

A configuration feature of a router that is used to select the portion of a 32-bit TCP/IP node address that refers to the LAN and to select the portion that refers to the node on the LAN.



A technique used to impose a hierarchy on IP addresses that supports a form of area routing.



Transmission speeds below DS-0/64Kbps.



To split the frequencies in a cable so that the lower frequencies are used for one purpose and the higher for another. Subsplitting is used when two cables are not used but there is a need to simulate a dual-cable system.



A part of a system that performs defined functions.


subvoice grade channel

A channel with bandwidth narrower than that of voice-grade channels. Such channels are usually subchannels of a voice-grade line.



A feature-rich new-generation mobile phone that offers applications such as music, graphics, games, and videos.



The process of detecting a change of state between idle and busy conditions on a circuit.


supervisory signal

A signal that handles the on-hook/off-hook condition.


SVC (Switched Virtual Circuit)

A virtual connection that is set up on a call-by-call basis.


switched digital access

A dialup option in which facilities are allocated based on demand rather than being associated with a specific customer all the time.


switched line

A temporary connection between computers or devices that is established by dialing.



Establishing a transmission path from a particular inlet to a particular outlet of a group of such inlets and outlets.


switching center

A location that terminates multiple circuits and can interconnect circuits or transfer traffic between circuits.


switching system

A device that connects two transmission lines together.



A switch on a telephone set that is associated with the structure supporting the receiver or handset and is often used to signal the switching equipment or an attendant during a call.



A switch to an alternative component that happens when a failure occurs in the equipment.


symmetric encryption

Encryption in which the sender and the receiver use the same key or machine setup.



The process of adjusting a receiving terminal's clock to match the clock of the transmitting terminal.



(1) Having a constant time interval between successive bits, characters, or events. Synchronous transmission uses no extra information (such as the start and stop bits in asynchronous transmission) to identify the beginning and end of characters and thus is faster and more efficient than asynchronous transmission. The timing is achieved by transmitting sync characters prior to data. Usually, synchronization can be achieved in two- or three-character times. (2) Occurring with a regular or predictable time relationship. In data transmission, the time of occurrence of each signal representing a bit is related to a fixed time frame.


synchronous communications

High-speed transmission of contiguous groups of characters in which the stream of monitored and read bits uses a clock rate to transfer the characters over a communication line.


synchronous network

A network in which all the communications links are synchronized to a common clock.


Synchronous TDM (Synchronous Time Division Multiplexing)

A method of TDM in which time slots on a shared transmission line are assigned on a fixed, predetermined basis.


synchronous transmission

In data communications, a method of transmission in which the sending and receiving of characters is controlled by timing signals. The sending and receiving terminals operate continuously in step with each other.



A computer and its associated devices and programs.


system test

A complete simulation of an actual running configuration for purposes of ensuring the adequacy of the configuration.



TA (Terminal Adapter)

A protocol converter that connects a non-ISDN device to the ISDN network.


TACS (Total Access Communication System)

A standard for analog telephony that was deployed widely in Europe before GSM became the most popular approach.



(1) The connection of networks or circuits in series (i.e., the connection of the output of one circuit to the input of another). (2) An intermediate switch used in a tandem network, which connects only to other switches instead of to customers.


tandem data circuit

A data circuit that contains two or more pieces of DCE in series.


tandem switch

A switch used to connect local exchanges throughout the metropolitan area. Also called a junction exchange or a tandem office.



An open point on a cable bundle that is left so that technicians can easily splice off a pair to bring new or additional service to a home.


TAPI (Telephony Applications Programming Interface)

An API that enables PCs to use telephone services. TAPI is a call-processing protocol that resides between the signaling and application layers.



(1) The published rate for the use of a specific unit of equipment, facility, or type of service provided by a communications common carrier. (2) The vehicle by which regulating agencies approve or disapprove such facilities or services.


TASI (Time-Assignment Speech Interpolation)

Specialized switching equipment that connects a party to an idle circuit while speech is taking place and disconnects the party when speech stops so that a different party can use the same circuit. During the periods of heavy traffic, TASI can improve line efficiency by 45% to 80%.


Tbps (Terabits per second)

Trillions of bits per second.


TBRPF (Topology Broadcast Based on Reverse-Path Forwarding)

A proactive link-state routing protocol designed for mobile ad hoc networks. It provides hop-by-hop routing along minimum-hop paths to each destination.


TCAP (Transaction Capabilities Applications Part)

A system that supports intelligent network service by enabling the exchange of non-circuit-related information between different signaling points (i.e., network nodes).



The North American standard of the PDH, a TDM digital transmission system. T-carrier is a PCM system that uses 64Kbps per channel as the basis of the hierarchy. Higher levels reflect aggregation of the 64Kbps channels. T-1 is a 1.544Mbps 24-channel communications circuit; T-2 is a 6.312Mbps 96-channel communications circuit; T-3 is a 44.736Mbps 672-channel communications circuit; and T-4 is a 274.176Mbps 4,032-channel communications circuit.


TCP (Transmission Control Protocol)

One of the core protocols of the Internet Protocol suite. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange data. TCP supports many of the Internet's most popular application protocols and applications.


TCP stack

A suite of multiple protocols used to exchange information between computers. The Internet and most commercial networks run on the TCP stack.


TCP/IP (Transmission Control Protocol/Internet Protocol)

The most widely used internetworking protocol. It ensures that packets of data are delivered to their destinations in the sequence in which they were transmitted. TCP roughly corresponds to Layer 4 (the transport layer) of the OSI model. It provides reliable transmission of data. IP corresponds to Layer 3 (the network layer) of the OSI model and provides connectionless datagram service.


TD-CDMA (Time Division Code Division Multiple Access)

A 3G combined time division and CDMA scheme used by UMTS TDD. TD-CDMA mobile devices consume significantly less power than other devices, resulting in improved talk times and standby times.


TDD (Time Division Duplex)

A half-duplex technique in which each end of the conversation makes use of the same frequency.


TDM (Time Division Multiplexing)

A means of obtaining a number of channels over a single path by dividing the path into a number of time slots and assigning each channel its own intermittently repeated time slot. At the receiving end, each time-separated channel is reassembled.


TDMA (Time Division Multiple Access)

A multiple-access technique that first divides the available or allocated frequency spectrum into a number of channels. Then, within each channel, TDM is applied to carry multiple users interleaved in time.


TD-SCDMA (Time Division Synchronous Code Division Multiple Access)

A 3G mobile telecommunications standard used primarily in China. It is based on spread spectrum CDMA technology.



A telephone company.


telecommunication lines

Telephone and other communication pathways used to transmit information from one location to another.



Any process that permits the passage of information from a sender to one or more receivers in any usable form (e.g., printed copy, fixed or moving pictures, visible or audible signals) by means of any electromagnetic system (e.g., electrical transmission by wire, radio, optical transmission, waveguides).



The number of main telephone lines per 100 inhabitants.



A technology that combines virtual reality and videoconferencing in order to enable people who are geographically separated from one another to collaborate in real-time in a shared, simulated environment, with access to the same media, computer applications, images, audio, and simulation.


telephone channel

A transmission path designed for the transmission of human speech or other telephone communication (e.g., fax) requiring the same bandwidth. The bandwidth of an analog telephone channel is 4,000Hz.



A generic term for voice telecommunications.



The TCP/IP site protocol that supports a remote login capability.



One trillion.


terabit switch router

A class of backbone platform that supports Tbps capacity. This type of router is agnostic, and it involves short and predictable delay, robust QoS features, multicast support, and carrier-class availability.



(1) A point at which information can enter or leave a communication network. (2) Any device capable of sending and/or receiving information over a communications channel. (3) A workstation.


terminal multiplexer

A device that permits two or more terminals to share one cable as a data transmission path.


terminal server

A device that connects terminals to a network that is typically running Ethernet.


thicknet (thick Ethernet cable)

Coaxial cable with electrical characteristics that meet the 10BASE5 specification for Ethernet networks. Thicknet enables a signal to be carried as far as 1,640 feet (500 m) before requiring a repeater. Also called 10BASE5 cable.


thinnet (thin Ethernet cable)

Coaxial cable with electrical characteristics that meet the 10BASE2 specification for Ethernet networks. Thinnet enables a signal to be carried as far as 607 feet (185 m) before requiring a repeater. Also called 10BASE2 cable or cheapernet.



The useful information processed or communicated during a specific time period, expressed in bits per second or packets per second.


TIA (Telecommunications Industry Association)

An organization that recommends standards for telecommunications.


time division switching

A switching method for a TDM channel that requires the shifting of data from one slot to another in the TDM frame. The slot in question can carry a bit, a byte, or, in principle, any other unit of data.



A set time period for waiting before a terminal system performs some action. Typical uses include a poll release (when a terminal is disconnected if the time-out period elapses before keying resumes) and an access timeout (when a terminal on a LAN using a CSMA/CD access method is prevented from transmitting for a specified time after a collision occurs).


time-slot interchange

The interchange of time slots within a TDM frame.


Time to Live

A field in an IP datagram that is decremented by each router in order to guarantee that datagrams do not circulate on the Internet forever.


time transparency

The absence of delay and delay jitter through an ATM network, thus allowing high-speed transportation of real-time services.



The contacting part at the end of a telephone plug or the top spring of a jack; the conductors associated with these contacts. The other contact is called a ring.


TMN (Telecommunications Management Network)

A protocol model defined by the ITU-T for managing open systems in a communications network. TMN can be used in the management of ISDN, B-ISDN ATM, and GSM networks. It is not as commonly used for purely packet-switched data networks.



An 8-bit word that is circulated in a unidirectional fashion among the devices on a network.


token passing

A network access method that uses a special bit pattern called a token that gives permission to nodes to transmit data, one node at a time. Each node is allowed to transmit a message only when it has the token. If a node has no messages to transmit when it receives the token, it passes the token to the next node.


token ring

A networked ring of devices that passes a special bit pattern called a token from node to node to determine which device can transmit data on the network. Token ring operates at either 4Mbps or 16Mbps.


toll center

The exchange that is responsible for establishing and completing national, long-distance communications. Also called the Class 4 office, transit switch, toll office, or trunk exchange.


tone signaling

Transmission of supervisory, address, and alerting signals over a telephone circuit by means of tones.


tone-to-dial-pulse conversion

A system of converting DTMF signals to dial pulse signals when the trunks associated with outgoing trunk calls are not equipped to receive tone signals. Auxiliary dial pulse conversion equipment is not necessary.



The connectivity among a group of nodes. Physical topology relates to how devices are cabled. Logical topology refers to how nodes actually interact.


trace packet

In packet switching, a packet that functions as a normal packet but causes a report of each stage of its process to be sent to the network control center.



(1) Messages sent and received over a communications channel. (2) A quantitative measurement of the total messages and their length, expressed in 100 call seconds or other units.


traffic flow

A measure of the density of traffic.



An item of business (e.g., the handling of customer orders and billing).


transaction processing

(1) In batch or remote batch processing, the processing of a job or job step. (2) In interactive processing, an exchange between a terminal and another device that does a particular action (e.g., entering a customer's deposit and updating the customer's balance).



A device that connects a host interface to a LAN.



A device for converting signals from one form to another, such as a microphone or a receiver.


transfer rate

The speed at which information can be sent across a bus or communications link.


translational bridge

A bridge that may connect two different MAC layers (e.g., 802.3 and FDDI).



Information that is sent in the form of electrical signals over electric wires, waveguides, or radio.


transmission line

A connection over which communications devices communicate. There are five main types of transmission lines: circuits, channels, lines, trunks, and virtual circuits.


transmission medium

A physical pathway that connects computers, other devices, and people on a network. Transmission media can be either wireline (e.g., twisted-pair, fiber, coax) or wireless (e.g., microwave, satellite, infrared).


transmission speed

The rate at which information is passed through communications lines, generally measured in bits per second.



To send information from one location to another.


transparent bridging

A system in which a bridge functions as a node on each of the connected networks, and its presence in the communications path between nodes is transparent. Communication proceeds as if the nodes were on the same network.



The receiver, transmitter, and antenna equipment that forms a single repeater channel on a satellite.


transport layer

Layer 4 of the OSI model, which provides reliable, transparent transfer of data between endpoints.



A type of bus network topology in which the medium branches at certain points along its length to connect stations or clusters of stations; also called a branching bus.


triple-play services

A service provider's bundle of voice, data, and video/TV services, delivered over the same infrastructure.



Monitoring and testing the performance of a network to detect and fix problems.



A circuit that is configured to support the calling loads generated by a group of users; the transmission facility that ties together switching systems.


TTC (Telecommunications Technology Committee)

A body that creates telecommunications standards that are followed in Japan.



The encapsulation of one protocol within another. Tunneling is often used to transport a LAN protocol across a backbone network that does not support the LAN protocol.


turnkey system

A complete communications system, including hardware and software, that is assembled and installed by a vendor and sold as a total package.


twisted-pair cable

A cable that consists of two insulated wires twisted around each other (and sometimes wrapped in additional insulation to help shield the signals from electromagnetic noise). Twisted-pair cable is often used in telephone wires; it also serves as the transmission medium in some LANs.



A very strong encryption algorithm that was one of the five initial candidates for AES.


two-wire circuit

A circuit formed by two insulated electrical conductors. One wire is used for the transmission of information, and the other wire acts as a return to complete the circuit.


type approval

An administrative procedure of technical tests and vetting that is applied to items of telecommunication equipment before they can be sold or interconnected with the public network. Also known as homologation.



UBR (Unspecified Bit Rate)

An ATM service class that provides best-effort service. UBR offers no service guarantee, so it is used for text data, image transfer, messaging, and distribution of information that is noncritical, where it is not necessary to have a set response time or service guarantee.


UDP (User Datagram Protocol)

A minimal-functionality transport-layer protocol in the TCP/IP protocol suite.


UHDV (Ultra High Definition Video)

A digital video format proposed by NHK of Japan. UHDV provides a resolution that is 16 times greater than that of HDTV.


UHF (Ultrahigh Frequency)

The portion of the electromagnetic spectrum that ranges from about 300MHz to about 3GHz and includes television and cellular radio frequencies.


UMTS (Universal Mobile Telecommunications System)

An evolving European standard for 3G mobile communications. UMTS represents the convergence of mobile and fixed networks as well as terrestrial wireless and satellite-based networks.



A 3G mobile standard that uses TDD and is designed to work in a single, unpaired frequency band. UMTS TDD uses the TD-CDMA scheme.


UNI (User-to-Network Interface)

A type of network interface that defines how customers can access their providers' networks. It shields the client from network complexities.



Sending streams from a single origination point directly to a single destination point.



A coding scheme that assigns 16 bits per character (i.e., 216), which translates to more than 65,000 possible characters.


uniform-spectrum random noise

Noise distributed over the spectrum in such a way that the power-per-unit bandwidth is constant. Also called white noise.


UNII (Unlicensed National Information Infrastructure)

Unlicensed radio bands that operate at 5.2GHz.


Universal Service Fund

A U.S. government program that provides subsidies for telephone service in areas that might otherwise have difficulties obtaining economical telephone service, particularly low-income areas and rural areas.



A multiuser, open operating system developed by Bell Laboratories.


unlicensed bands

Radio bands that can be used to provide broadband access, without the need for obtaining a license. Networks that use unlicensed bands have a range of about 35 miles (55 km) and offer throughput from 128Kbps to 10Mbps over shared media.


unshielded cable

A cable in which the signal-carrying wire or circuit is not shielded to keep out electromagnetic noise that can potentially damage the data stream.



To send data from a local system to a remote system.


UPS (Uninterruptible Power Supply)

A device that usually includes an inverter, drawing its power from batteries, which generates an extremely well-behaved AC power signal for a PBX or other equipment. If a particularly heavy demand is anticipated, the system can be coupled with an auxiliary generator that is started when commercial power is interrupted.



The direction of transmission flow away from a user.


UPT (Universal Personal Telephony)

An ITU-R standard for the provision of personal mobility across many different kinds of fixed and mobile networks.


UTP (Unshielded Twisted-Pair)

A twisted-pair cable in which the two insulated wires that twist around each other are not surrounded by additional insulation.


UWB (Ultra-Wideband)

A technique based on transmitting very-short-duration pulses, where the occupied bandwidth is very large, allowing for very high data rates. UWB is used in both wired and wireless PANs to relay data from a host device to other devices up to 30 feet (10 m) away.


UWC (Universal Wireless Communications)

Also called ANSI-136, a wireless standard that uses TDMA and TDD schemes.




An ITU-T standard that describes electrical characteristics for balanced double-current interchange circuits for general use with integrated circuit equipment.



An ITU-T standard that describes 300bps modems for use in the PSTN.



An ITU-T definition for interchange circuits between data terminal equipment and data circuit-terminating equipment.



An ITU-T standard that describes automatic calling and/or answering equipment on the general switched network.



An ITU-T standard that describes 9,600bps modems for use in the PSTN.



An ITU-T standard that describes data transmission at 56Kbps that uses balanced transmission methods through a 34-pin physical interface.


VAD (Voice Activity Detection)

A technique that reduces the amount of information needed to re-create voice at the destination end by removing silent periods and redundant information found in human speech. VAD also helps with compression.



An attempt to find errors by executing a program in a given environment.


value-added carrier

A company that sells services of a value-added network. It can be a PTT, a subsidiary, or an independent company.


VAN (value-added network) services

Telecommunication services provided over public or private networks that, in some way, add value to the basic carriage, usually through the application of computerized intelligence (e.g., reservation systems, bulletin boards, information services).


VAR (Value-Added Reseller)

A provider that deals with distribution and sales.


VAS (Value-Added Service)

A communications facility that uses communications common carrier networks for transmission and that provides enhanced extra data features with separate equipment. Such extra features, including store-and-forward message switching, terminal interfacing, and host interfacing, are common.


VBR (Variable Bit Rate)

An ATM service class for network traffic that is typically from bursty data transfer applications, such as client/server and LAN-to-LAN interconnection. VBR offers guaranteed service delivery. VBR-RT (real-time) is designed for real-time voice and videoconferencing applications, and VBR-NRT (non-real-time) is for mission-critical data applications.


VC (Virtual Circuit)

A series of logical, rather than physical, connections between sending and receiving devices. With a virtual circuit, two hosts can communicate as though they have a dedicated connection, although the packets may be taking very different routes to arrive at their destination.


VC (Virtual Container)

A data structure designed for the transport and switching of sub-STM-0 network services such as CEPT-1. All network services below E-3 are mapped into VCs, and VCs are multiplexed into the SPE of an STM-1.


VCI (Virtual Channel Identifier)

In ATM, the part of a cell header that identifies the channel associated with the cell.


VDSL (Very-High-Bit-Rate DSL)

A transmission medium that provides a maximum span of about 5,000 feet (1,500 m) over a single twisted-pair cable. Over this distance, VDSL can provide a rate of up to 13Mbps downstream, and with a shorter distance of 1,000 feet (300 m), it can provide up to 52Mbps downstream, which is enough capacity to facilitate DTV. VDSL provides 1.5Mbps to 2.3Mbps upstream.


VDSL2 (VDSL version 2)

A broadband wireline medium whose key applications are the next generation of TVVOD, DTV, HDTV, and forms of interactive multimedia Internet access. VDSL2 has two bandwidth options: LR-VDSL212MHz (the "long reach" alternative) and SR-VDSL230MHz (the "short reach" alternative).


VDT (Video Dial Tone)

A U.S. term that defines the capability of a network access provider to offer video access and carriage directly to or from subscribers.


vertical blanking interval

Unused lines in each field of a TV signal. Some of these lines may be used for captions and specialized signal and cable service.


VF (Virtual Fiber)

A wireless solution to the "first-mile" problem of delivering high-speed access to the end user. The use of VF as a last-mile solution is based on MMW technology to deliver line-of-sight broadband. Also referred to as wireless fiber (WiFiber).


VF (Voice Frequency)

Any frequency within the part of the audio frequency range that is essential for the transmission of speech of commercial quality (i.e., 250Hz to 3,400Hz). Also called telephone frequency.


VHF (Very High Frequency)

The portion of the electromagnetic spectrum with frequencies between about 30MHz and 300MHz. VHF is the operating band for radio and television channels.


video signal

A signal in the frequencies normally required to transmit moving image information.



A two-way communication between two or more parties that involves the exchange of images as well as voice. The images may or may not be in full motion.



Interactive communication between two parties involving the transmission and reception of images as well as voice. The degree to which the images are in full motion depends on the level of data compression used and on the available bandwidth.



A public or private telecommunications service that offers interactive browsing of a menu of textual and graphical information. The most widely used public videotext service is France Telecom's service, offered via Minitel terminals.


virtual channel

In ATM, the specific conversation path over which the cells from a given conversation flow from one ATM switch to another.


virtual router

An abstract representation that has the same mechanisms as a physical router and, therefore, can inherit all existing mechanisms and tools for configuration, deployment, operation, troubleshooting, monitoring, and accounting. Multiple virtual routers can exist in a single physical device, and virtual routers can be deployed in various VPN configurations.


VISP (Virtual Internet Service Provider)

A provider that offers outsourced Internet service, running as a branded ISP. It is a turnkey ISP product aimed at affinity groups and mass marketers that want to add Internet access to their other products and services.


VLAN (Virtual Local Area Network)

A network of computers that behave as if they are connected to the same wire even though they may actually be physically located on different segments of a LAN. VLANs are configured through software rather than hardware, which makes them extremely flexible. A big advantage of VLANs is that when a computer is physically moved to another location, it can stay on the same VLAN without any hardware reconfiguration.


VLF (Very Low Frequency)

Frequencies below 30KHz.


VoATM (Voice over ATM)

A technology for transporting integrated digital voice, video, and data over ATM networks.


vocoders (voice coder/decoder)

A device used for compression of voice traffic. High-bit-rate vocoders are used by PCS, wireless local loops, and wireless office telecommunication systems applications. Low-bit-rate vocoders are used in cellular systems that deal with vehicular traffic, where there are large cells and a need to facilitate a large number of conversations.


VOD (Video-On-Demand)

A service whereby a subscriber can order and almost immediately view films and other entertainment from a television set. In true VOD, the film can be paused, rewound, and fast-forwarded. In "near" VOD, these functions are not possible.


VOD Metadata

A project established by CableLabs that is pertinent to the future of interactive services. The initial efforts of VOD Metadata are focused on creating specifications for VOD and subscription VOD (SVOD) applications.


VoDSL (Voice over DSL)

A technology for delivering voice transmissions over DSL lines. VoDSL enables service providers to deliver high-speed data access and up to 16 telephone lines over 1 DSL line.


VoFR (Voice over Frame Relay)

A technology for transporting integrated digital voice, video, and data over Frame Relay networks.


voice digitization

Conversion of an analog voice into digital symbols for storage or transmission.


voice grade

A telecommunications link with a bandwidth (about 4KHz) appropriate to an audio telephone line.


voice-grade channel

A channel with a frequency range of 4KHz. Also referred to as a telephone channel.


Voice over Cable

A technology for delivering cable-based IP telephony.



A technology for transporting real-time voice traffic over packet networks. VoIP and VoFR are examples of voice-over-packet solutions.


voice recognition

A technology that allows spoken words in the form of human voice to provide input to a computer.


voice synthesis

Computer-generated sounds that simulate the human voice.


VoIP (Voice over IP)

A technology for transporting integrated digital voice, video, and data over IP networks.


VoIP gateway

A device that bridges the traditional circuit-switched PSTN and the packet-switched Internet.


volatile storage

Memory that loses its contents when electrical power is removed.


VoWLAN (Voice over Wireless Local Area Network)

A local voice solution that uses VoIP and WLANs to marry the convenience of mobility with the cost-effectiveness of an IP PBX.


VP (Virtual Path)

A generic term for a collection of virtual channels that have the same endpoint.


VPDN (Virtual Private Dial Network)

A type of network that was developed for telecommuters and road warriors who needed secure and convenient access to their enterprise servers. VPDNs have included the use of PPTP, L2TP, or IPsec.


VPI (Virtual Path Identifier)

In ATM, the portion of a cell header that identifies the virtual path to which the cell belongs. Virtual paths are defined to permit groups of virtual channels to be manipulated as if they were a single channel.


VPLS (Virtual Private LAN Services)

A VPN technology that provides Ethernet-based multipoint-to-multipoint communication over IP-only networks. VPLS allows geographically dispersed sites to share an Ethernet broadcast domain by connecting sites through pseudo-wires.


VPN (Virtual Private Network)

A software-defined network offered by telephone carriers for voice and data communications among multiple sites. A VPN provides the appearance of a private-line network, except that it makes use of the PSTN rather than physically dedicated leased lines. In customer-based VPNs, carriers install gateways, routers, and other VPN equipment on the customer premises. This is preferred when customers want to have control over all aspects of security. In network-based VPNs, the carrier houses all the necessary equipment at a POP near the customer's location. Customers that want to take advantage of the carrier's VPN economies of scale prefer this type of VPN.


VPN gateway

A device that enables VPNs to set up and maintain secure tunnels through the Internet.


VPWS (Virtual Private Wire Service)

A type of VPN that emulates a point-to-point link and provides a single service that is perceived by its user as an unshared link or circuit of the chosen service. VPWS provides a mesh of point-to-point customer edgetocustomer edge Layer 2 connections over a packet-switched network.


VR (Virtual Reality)

A computer-based application that provides a humancomputer interface such that the computer and its devices create a sensory environment called a virtual world. The sensory environment is dynamically controlled by actions of the individual so that the environment appears real.


VRC (Vertical Redundancy Check)

See parity check.


VSAT (Very-Small-Aperture Terminal)

An earth station with a small antenna, usually 2 feet (0.6 m) or less.


VSF-OFCDM (Variable-Spreading-Factor Orthogonal Frequency Code Division Multiplexing)

A form of multiplexing used in 4G wireless. It increases downlink speeds by using multiple radio frequencies to send the same data stream.


VT (Virtual Tributary)

A data structure designed for the transport and switching of sub-STS-1 network services such as DS-1, DS-1C, and DS-2. All network services below DS-3 are mapped into VTs, and VTs are multiplexed into the SPE of an STS-1.


Walsh Code

An algorithm that generates statistically unique sets of numbers for use in encryption and cellular communications. It is an orthogonal code used to uniquely define individual communications channels. Walsh Code is used in DSSS systems such as CDMA as well as in FHSS systems.


WAN (Wide Area Network)

A group of computer networks connected over long distances, often by telephone lines and satellite transmission.


WAP (Wireless Access Point)

A device that connects wireless communication devices to form a wireless network. A WAP usually connects to a wired network and can relay data between wireless devices and wired devices.


WAP (Wireless Application Protocol)

A protocol for transforming Internet information so that it can be displayed on the small screen of a mobile telephone or other portable device.


war dialing

A method hackers use to locate nonsecure computers by dialing through phone numbers.


war driver

An individual who drives through an area, scanning for wireless networks (using programs such as NetStumbler), and publishing his or her findings on the Web.


WARC (World Radiocommunications Conference)

A regular meeting of global authorities to decide on spectrum allocation. Originally called the World Administrative Radio Conference.



A curve that shows the shape of a wave at any given time.



A transmission path in which a system of boundaries guides electromagnetic energy. The most common of these are hollow metallic conducting tubes (microwave communications) or rods of dielectric material.



The distance between two consecutive maxima or minima of a waveform.


wavelength changer

A device that converts an optical signal to an electronic signal and then sends the signal to a laser that produces an optical signal at a different wavelength than the original.


wavelength division switching

Switching in which input information is used to modulate a light source that has a unique wavelength for each input. All the optical energy is combined and then split, so it can be distributed to all the output channels.


W-CDMA (Wideband CDMA)

A multiple-access technique that operates over allocations of either 5MHz, 10MHz, or 15MHz. W-CDMA can support higher data rates than first-generation CDMA.


WDM (Wavelength Division Multiplexing)

The multiplexing of signals by transmitting them at different wavelengths through the same fiber.



Smart devices that can be networked and are small enough to be worn on the human body.


WEP (Wired Equivalent Privacy)

An encryption standard used to secure wireless networks. WEP has some serious weaknesses and has therefore been superseded by other standards, such as WPA and WPA2.


WFQ (Weighted Fair Queuing)

A packet-scheduling technique that allows guaranteed bandwidth services and lets several sessions share the same link.


WiBro (Wireless Broadband)

A Korean WMAN standard for wireless broadband Internet access that operates in licensed bands.



Refers to the range of speeds between narrowband and broadband, typically ranging between n x 64Kbps up to 45Mbps.


wideband channel

A channel that is wider in bandwidth than a voice-grade channel.


Wi-Fi (Wireless Fidelity)

A WLAN technology that allows users to plug a single high-speed Internet connection such as a cable modem into an inexpensive WAP and share it with scores of people in a building, park, or small neighborhood. Wi-Fi is used to create hotspots in locations such as hotels, airports, shopping centers, restaurants/cafes, and educational environments.


WiMax (Worldwide Interoperability for Microwave Access)

The global IEEE 802.16 standard, also known as the IEEE WirelessMAN air interface standard, which was designed to provide wireless last-mile broadband access in MANs. WiMax represents an evolution to a standards-based, interoperable, carrier-class solution.



A UWB common radio platform that enables high speeds (480Mbps and beyond), low power consumption, and multimedia data transfers in a WPAN. WiMedia is optimized for several key market segments: PC, consumer electronic, mobile, and automotive applications.



A generic term for mobile communications services such as cellular, radiopaging, or PCS that do not use wireline networks for direct access to the subscriber.


wireless local loop

A technology that uses low-power radio transmission, cellular radio, and/or cordless telephony as an alternative to local loops for accessing the PSTN.


wireless mesh network

A wireless LAN that routes voice, data, and instructions between nodes, creating a resilient network in which connections are continuous, reconfiguring around blocked paths by hopping from node to node until a connection is established.


wireless micromesh network

A wireless network that is designed for short ranges, up to 300 feet (100 m) between any two nodes. It is characterized by long battery life, relatively low data rates, and good tolerance for latency. Wireless micromeshes eliminate the need to wire every node and thus provide the greatest degree of flexibility possible in sensor/control networks.


wiring closet

A location in a building where the building's wiring terminates and where equipment (e.g., hub electronics) is placed.


WISP (Wireless ISP)

A company that gives users wireless access to the Internet and related services.


WLAN (Wireless Local Area Network)

A network that covers a single room to an entire campus and uses wireless media. The last link with the users is wireless, and the backbone network usually uses cables, with one or more WAPs connecting the wireless users to the wired network. WLANs typically operate on unlicensed frequency bands, and they do not require line of sight.


WLL (Wireless Local Loop)

The use of a wireless communications link as the last-mile connection for delivering POTS and broadband Internet to customers. Also referred to as radio in the loop, fixed-radio access, or fixed-wireless access.


WMAN (Wireless Metropolitan Area Network)

A network that provides broadband wireless network access over a metro area. The WMAN standards most commonly used today are BFWA, IEEE 802.16 (WiMax), WiBro, IEEE 802.20 (Mobile-Fi), HiperAccess, iBurst, Flash-OFDM, DMB, and VF.


WML (Wireless Markup Language)

A markup language used in the WAP environment.



A device (e.g., display station, printer) that can transmit information to or receive information from a computer, or both, as needed to perform a job.


world numbering plan

An ITU-T numbering plan, E.164, that divides the world into nine zones. Each zone is allocated a number that forms the first digit of the country code for every country in that zone. The zones are as follows: (1) North America (including Hawaii and Caribbean islands, except Cuba), (2) Africa, (3 and 4) Europe, (5) South America and Cuba, (6) South Pacific (Australasia), (7) Russia, (8) North Pacific (Eastern Asia), and (9) Asia and the Middle East. A spare code (0) is available for future use.


WPA (Wi-Fi Protected Access)

A wireless encryption standard that the Wi-Fi Alliance created in response to serious weaknesses in WEP. A second version of WPA is called WPA2.


WPAN (Wireless Personal Area Network)

A wireless network that serves a single person or small workgroup and is characterized by limited distance, limited throughput, and low volume. The WPAN standards in use today include IEEE 802.15.1 (Bluetooth), IEEE 802.15.3 (WPAN-HR and WPAN-AHR), UWB, IEEE 802.15.4 (ZigBee), RFID, and NFC.


WRAN (Wireless Regional Area Network)

A regional wireless network being created by the IEEE 802.22 working group that is to use the UHF/VHF TV bands that the FCC is opening up for experimentation.


WRR (Weighted Round Robin)

A queuing mechanism that assigns a weight to each queue and then services each nonempty queue in proportion to its weight, in round-robin fashion. WRR is optimal when using uniform packet sizes, a small number of flows, and long connections.


WWAN (Wireless Wide Area Network)

A network that is global, national, or regional in scope and that operates without wires. Traditional WWAN solutions include cellular radio and PCS networks as well as early wireless data networks. Newer generations are focused on supporting high-speed data as well as video and multimedia, with increased emphasis on mobile content delivery.



CWDM operating at 1310 nm, compared to traditional CWDM, which operates at 850 nm.


WWW (World Wide Web)

An Internet application that uses hypertext links between remote network servers for accessing and displaying multimedia information.




An ITU-T standard that defines the basic functionality of an asynchronous PAD, usually used in conjunction with ITU-T standards X.28 and X.29.



An ITU-T recommendation that defines a general-purpose physical interface between a DTE and DCE for full-duplex synchronous operation on circuit-switched or packet-switched data networks.



A standard that defines the interface between a DTE and DCE for equipment operating in the packet mode on public data networks. It also defines a link control protocol.



A standard for connecting X.25 networks, developed by the ITU-T



The ITU-T standard for the addressing plan used with X.25 PDNs.



An ITU-T standard for electronic mail exchange.



The family of protocols that define the operation of the ITU-T/ISO directory service.



The ITU-T/OSI recommendation for a directory authentication framework.


xDSL (Digital Subscriber Line)

The DSL family of broadband technologies that use sophisticated modulation schemes to pack data onto copper wires. They include ADSL, HDSL, and VDSL.


XMPP (Extensible Messaging and Presence Protocol)

An XML-based extensible open-source protocol that drives the Jabber IM client.



The IEEE 802.15.4 standard, a wireless communication protocol designed for WPANs. ZigBee operates, internationally, in the unlicensed frequency bands, and it supports a low data rate, a very long battery life (months to even years), and very low complexity.

Part I: Communications Fundamentals

Telecommunications Technology Fundamentals

Traditional Transmission Media

Establishing Communications Channels


Part II: Data Networking and the Internet

Data Communications Basics

Local Area Networking

Wide Area Networking

The Internet and IP Infrastructures

Part III: The New Generation of Networks

IP Services

Next-Generation Networks

Optical Networking

Broadband Access Alternatives

Part IV: Wireless Communications

Wireless Communications Basics

Wireless WANs


Emerging Wireless Applications


Telecommunications Essentials(c) The Complete Global Source
Telecommunications Essentials, Second Edition: The Complete Global Source (2nd Edition)
ISBN: 0321427610
EAN: 2147483647
Year: 2007
Pages: 160 © 2008-2020.
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