Glossary
Glossary
Numerals
10Base2
Thin coax Ethernet cable.
10Base5
Thick coax Ethernet cable.
10BaseFL
2 strands of multimode optical fiber Ethernet cable.
10BaseT
2-pair UTP Ethernet cable.
100BaseFX
2 strands of multimode optical-fiber Fast Ethernet cable.
100BaseT2
2-pair Cat 3 UTP Fast Ethernet cable.
100BaseT4
4-pair Cat 3 UTP Fast Ethernet cable.
100BaseTX
2-pair Cat 5 UTP Fast Ethernet cable.
1000BaseCX
Coax patch cable Gigabit Ethernet cable.
1000BaseLX
Long-wavelength single-mode optical-fiber Gigabit Ethernet cable.
1000BaseSX
Short-wavelength multimode optical-fiber Gigabit Ethernet cable.
1000BaseT
4-pair Cat 5 or Cat 5e UTP Gigabit Ethernet cable.
1000BaseTX
2-pair Cat 6 (currently a TIA draft proposal) Gigabit Ethernet cable.
23B+D
The North American and Japanese infrastructure for PRI, which provides twenty-three 64Kbps B-channels for information and one 64Kbps D-channel for signaling and additional packet data.
2B1Q
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.
3DES (Triple DES)
168-bit encryption 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 will include digital transmission; however, 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.
30B+D
The ITU infrastructure for PRI, which provides thirty 64Kbps channels and one 64Kbps D-channel.
6bone
An experimental network that is being used as an environment for IPv6 research. So far more than 400 networks in more than 40 countries are connected to the 6bone network.
A
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.
ABR (available bit rate)
One of the ATM service classes. ABR supports VBR data traffic with average and peak traffic parameters (for example, 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
The connection from the customer to the local telephone company for access to the public switched telephone network, 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 public switched network 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.
address
(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-1 supports a downstream channel of 1.5Mbps (North America) or 2Mbps (Europe), with an upstream channel of 64Kbps. ADSL-2 supports a downstream channel of 6Mbps (North America) or 8Mbps (Europe) and an upstream channel of 640Kbps.
AES (Advanced Encryption Standard)
An encryption algorithm for securing sensitive but unclassified material by U.S. government agencies. It may eventually become the de facto encryption standard for commercial transactions in the private sector. Uses the Rijndael algorithm to specify three key lengths 128 bits, 192 bits, and 256 bits.
agent
Software that processes queries and sends responses on behalf of an application.
agnostic device
A device that supports multiple data protocols (for example, 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). 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 the applications run.
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. Employs nonuniform quantization to obtain the desired compression characteristic.
alerting signals
The ringing tones, the busy tones, and any specific busy alerts that are used to indicate network congestion or unavailability.
alternate mark inversion
A digital signaling method in which the signal carrying the binary value alternates between positive and negative polarities; zero and one values are represented by the signal amplitude at either polarity; no-value "spaces" are at zero amplitude. Also called bipolar.
alternate routing
The 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 or a 0 bit.
ambient noise
Communications interference that is present in a signal path at all times.
amplifier
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.
amplitude
A measure of the height of the wave, which indicates the strength of the signal.
AMPS (Advanced Mobile Phone System)
A standard for analog telephony that is deployed widely in the U.S.
analog
A signal that varies continuously (e.g., sound waves), along two parameters: amplitude (strength) and frequency (tone). The unit of measurement is the Hertz, 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, where 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.
ANSI X12
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.
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.
AppleTalk
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.
application
(1) Software with which the user interacts. (2) The use to which a system is put; for example, e-mail, videoconferencing, high-speed data access, and network management.
application layer
Layer 7 of the OSI model, which enables users to transfer files, send mail, and perform other functions that involve interaction with the network components and services.
application-layer multicasting
A technique which 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.
architecture
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.
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 that are associated with human logic such as reasoning, learning, and self-improvement.
AS (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.
ASCII (American Standard Code for Information Interchange)
The code developed by ANSI for information interchange among data processing systems, data communication 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.
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 new DTV standards include a 16:9 aspect ratio, which more closely resembles human vision.
asynchronous
(1) Occurring without a regular or predictable time relationship to a specified event. (2) In data communication, 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 start-stop 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's a high-capacity, low-latency switching fabric that's adaptable for multiservice and multirate connections and offers an architected approach to Quality of Service.
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 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 oriented data services. ATM Adaptation Layer 5 supports connectionless variable bit rate data (e.g., IP or signaling) over ATM.
ATM layer
The 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 cell is delivered to or from the adaptation layer; and flow control.
ATM physical layer
A layer 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.
attenuation
A decrease in the power of a received signal due to loss through lines, equipment, or other transmission devices. 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.
authentication
Any technique that enables the receiver to automatically identify and reject messages that have been altered deliberately or by channel errors. Also can be used to provide positive identification of the sender of a message.
B
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.
backbone
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 optic or microwave.
backplane
The board that contains a bus.
backup
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 unintentionally destroyed.
bandpass filter
A circuit that is designed to allow a single band of frequencies to pass, neither of the cut-off frequencies being zero or infinity.
bandwidth
The range of frequencies, expressed in Hertz, that can pass over a given transmission channel. Also, the difference between the lowest and highest frequency carried. The bandwidth determines the rate at which information can be transmitted through a circuit. The greater the bandwidth, the more bits per second that can be carried.
bandwidth exchange
An organization or a facility that functions as an exchange where bandwidth is the commodity. Some exchanges act to 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.
bandwidth-on-demand
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 one-bits are encoded as a particular sequence of 1s and 0s, and the zero-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 or analog signal at its original frequencies (that is, a signal in its original form, not changed by modulation).
batch processing
A processing method in which a program or programs process data with little or no operator interaction.
baud
A unit of signaling speed.
Baudot code
A data code that uses a 5-bit structure that was used on vintage teleprinters (e.g., Telex).
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 in the Frame Relay header that the network uses to inform the transmitter of network congestion.
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. A measure of the quality of a data transmission, usually expressed as number referred to 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 that is as good as possible under the circumstances.
beta test
The stage at which a new product is tested under actual usage conditions.
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.
binary
A base-two system of numbers; the binary digits are 0 and 1.
bipolar
The predominant signaling method used for digital transmission services, such as DDS and T1, in which the signal carrying the binary values successfully alternates between positive and negative polarities. One values are represented by the signal amplitude at either polarity, and no-value "spaces" are at zero amplitude.
bit
The smallest unit of information in a digital device. In binary notation, either the characters 0 or 1.
bit duration
The time it takes one 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 or synchronization problems or complete loss of the picture.
bitmap
A pixel-by-pixel description of an image. 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 bps.
blocking
A network is said to be blocking if there are connection sets that will prevent some additional desired connections from being set up between unused ports, even with rearrangement of existing connections.
Blowfish
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 algorithms.
Bluetooth
A very low-cost chip that gives a device a short-range wireless capability. Personal digital assistants, laptops, cell phones, and any other intelligent appliance embedded with a Bluetooth chip are capable of communicating and linking with each other wirelessly.
BNC connector
A locking type of connector. Thinnet, a form of Ethernet, uses BNC connectors.
board
The circuit card on which integrated circuits are mounted.
bps (bits per second)
A measure of the amount of transmission capacity available. 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. Abbreviated as Kbps for thousands of bits per second; Mbps for millions of bits per second; Gbps for billions of bits per second; Tbps for trillions of bits per second; Pbps for 1,000Tbps; and Ebps for 1 billion Gbps.
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).
bridge
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.
broadband
(1) A multichannel, high-bandwidth transmission line. (2) According to the ITU-T, any transmission rate over 2Mbps. (3) Typically, the technology of CATV transmission, as applied to data communications; employs coaxial cable as the transmission medium and radio frequency carrier signals in the 5MHz to 1,000MHz range.
Broadband ISDN
A standard that was envisioned for use with advanced applications. SDH/SONET and ATM were both born out of the Broadband ISDN standard and a desire to be able to deliver advanced applications.
broadcast
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.
brouter
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 not lost but reduced. 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 can be furnished. A UPS can be installed to ensure continued service during prolonged outages and to regulate power.
BSC (binary synchronous communications)
A half-duplex, character-oriented data communications protocol originated by IBM in 1964. It includes control character and procedures for controlling the establishment of a valid connection and the transfer of data. Also called bisync.
buffer
A storage device that is used to compensate for a difference in rate of data flow, or time of occurrence events, when transmitting data from device to another.
buffered network
A real-time store-and-forward message-switching network, with computers at the switching points, which act as buffers for the packets.
buffered repeater
A hybrid device halfway between 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).
bundled
A pricing strategy in which a service provider or manufacturer includes all products hardware, software, services, training, and the like in a single price.
burst
In data communications, a sequence of signals counted as one unit in accordance with some specific criterion or measure.
burst switching
A switching method to switch digitized voice and data characters in an integrated fashion.
bus
(1) A physical transmission path or channel. 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 used in Ethernet and the token bus, where all network nodes listen to all transmissions, selecting certain ones based on address identification. 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.
bypass
To establish a communication link without using the facilities of the local exchange carrier (that is, the telephone company).
byte
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 (for example, a 10Gbps backbone).
C
C-band
A portion of the electromagnetic spectrum, approximately 4GHz to 6GHz, that is used primarily for satellite and microwave transmission.
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 public-private 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.
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 local area networks, while they may span several miles, they do not include wide area network 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.
Category 1 UTP
An EIA/TIA 586 standard for commercial building telecommunications wiring. This old-style UTP telephone cable is unsuitable for data transmission.
Category 2 UTP
An EIA/TIA 586 standard of cable that can be used for data rates up to 4Mbps.
Category 3 UTP
An EIA/TIA 586 standard of cable that can be used for data rates up to 10Mbps. This is the minimum cable requirement for 10BaseT.
Category 4 UTP
An EIA/TIA 586 standard of cable that can be used as the lowest grade UTP, acceptable for data rates up to 16Mbps (Token Ring).
Category 5 UTP
An EIA/TIA 586 standard which specifies of cable that can be used for data rates up to 100Mbps.
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.
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 it is associated with minimum latencies and losses.
CC7 (Common-Channel Signaling System 7)
An ITU-T specified signaling protocol used in high-speed digital networks to provide communication between intelligent network nodes.
CCIS (Common-Channel Interoffice Signaling)
An electronic means of signaling between any two switching systems independent of the voice path. The use of CCIS makes possible new customer services, versatile network features, more flexible call routing, and faster connections.
CCITT (Comit Consultatif International de T l phonie et de T l graphie)
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.
CDDI (Copper-Distributed Data Interface)
A version of FDDI that runs on UTP cabling rather than on fiber-optic cable.
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.
CDPD (Cellular Digital Packet Data)
A North American standard for transferring packet data over cellular phone channels.
CD-ROM (compact disc-read only memory)
A storage device that is used in computer systems and typically contains multimedia information.
cell
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.
cellular
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.
centrex
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 (Commission of European Post and Telecommunications)
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.
channel
The logical conversation path. It 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 can be handled by the channel.
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.
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.
cladding
In fiber-optic cable, a colored, 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 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.
client
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.
clocking
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.
cluster
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. It is not widely implemented.
CMISE (Common Management Information Services)
A service interface created and standardized by ISO for managing heterogeneous networks.
CMTS (cable modem termination system)
A device on which coax trunks terminate. CMTSs are linked together by accessing 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 which 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 consisting 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, depending on the degree of EMI shielding afforded, voltages, and frequencies accommodated.
code
The conventions that specify how data may be presented in a particular system.
code character
A set of conventional elements established by the 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 the channel.
coding scheme
A pattern of bits that are 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.
collision
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.
command
A signal or group of signals that cause a computer to execute an operation or series of operations.
command-driven
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 accomplished the control function.
communication adapter
A hardware feature that permits telecommunication lines to be attached to the processor.
communication line
A physical link (wire, telephone circuit, microwave, satellite) that is used to transmit data between computers and/or remote devices.
communications controller
(1) A hardware device that lets you attach either communication lines, ASCII devices, or a local area network 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 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 kilometers) above the equator so that it appears from earth to be stationary.
compander
The combination of 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. Designed to improve the ratio of the signal to the interference entering in the path between the compressor and expandor.
compression
The application of any several techniques that reduce the number of bits required to represent information in data transmission or storage, thereby conserving bandwidth and/or memory.
compressor
An electronic device that compresses the volume range of a signal.
concatenation
The linking of transmission channels (phone lines, coaxial cable, optical fiber) end-to-end. Also refers to the linking of SONET STS-1 frames in order to carry a broadband information stream.
concentrator
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 it refers to a multiport repeater or hub that 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.
conditioning
A procedure for making transmission impairments of a circuit lie within certain specified limits and 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.
configuration
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.
configure
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 in this type of network after the connection is set up.
connectivity
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.
content delivery network
A network with delivery services that are structured specifically for a client that are focused on streaming audio, video, and media, as well as the supporting e-commerce applications.
contention
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 will have 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 that is 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 station
The station in a point-to-point or multipoint network that controls the sending and receiving of data. A station that can poll or address tributary stations.
control unit
Circuitry or a device that is 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.
conversion
The process of changing from one method to another; may refer to changing processing methods, data, or systems.
COPS (Common Open Policy Services)
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.
core
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 premise that is owned and managed by the customer.
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.
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.
crosstalk
Interference or an unwanted signal from one transmission circuit detected on another, usually parallel, circuit.
CSMA (Carrier Sense Multiple Access)
A LAN access technique in which multiple stations connected to the same channel are able to sense transmission activity on that channel and to defer the initiation of transmission while the channel is active. 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 are able to detect the interference caused by simultaneous transmissions by two or more stations (collisions) and to retransmit colliding messages in an orderly manner.
CSU (channel service unit)
A component of customer premises equipment (CPE) used to terminate a digital circuit, such as a leased line or T-1/E-1 facility, at the customer site. Performs certain line-conditioning functions and responds to loopback commands from the local exchange. Also, ensures proper ones density in a transmitted bitstream, and performs bipolar violation correction.
CTS (clear to send)
A control circuit that indicates to the data terminal equipment that data can or cannot be transmitted.
customization
The process of designing or configuring a device, installation or network, to meet the requirements of particular users.
cutover
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.
CXR (carrier)
A signal of known characteristics (for example, frequency) that is altered (modulated) to transmit information.
cycle
One complete repetition of a regularly repeating electronic function. The number of cycles per second, measured in Hertz (Hz), is called the frequency.
D
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 that is, there are 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 communication link. 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
Critical system extension lines, such devices as facsimile machines and computer terminals, are very sensitive to extraneous noise. Data privacy prohibits activities that would insert tones on the station line while the line is in use. Data lines can then be connected through the PBX without danger of losing data through interference.
data link
(1) The equipment and rules (protocol) used for sending and receiving data. (2) Any serial data communication 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: medium access control and logical link control.
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 data transmission, and not voice, is supported.
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 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.
datagram
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 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 ratio of the power of one signal to a reference level.
DBS (Direct Broadcast Satellite)
A satellite system that can transmit digital TV signals directly to individual homes.
DCE (data communications equipment)
Equipment that provides an interface between the DTE and the transmission channel (that is, 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 (for example, providers that offer DSL services to end users).
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.
dedicated line
An end-to-end communications line used exclusively by one organization. Also called a dedicated circuit.
delay distortion
The change in a signal from the transmitting end to the receiving end resulting 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 wave form (or other analog signal) in which successive bits represent increments of the wave form. The increment size is not necessarily constant. Produces digitized voice at 56Kbps.
demodulation
The process of recovering data from a modulated carrier wave. 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 designed by the National Bureau of Standards (now the National Institute of Standards and Technology) to encipher and decipher data using a 56-bit key. As a secret-key, symmetric system, it requires the exchange of secret encryption keys between users.
diagnostics
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 user as an audible indication that the switch is ready to receive dialing digits.
dialup
The process of, or the equipment or facilities involved in, establishing a temporary connection via the switched telephone network.
dialup line
A circuit that is established by a switched circuit connection; generally refers to the public telephone network.
DID (direct inward dialing)
Incoming calls from the exchange network can be completed to specific station lines without attendant assistance. Also called direct dialing in (DDI).
Diffie-Hellman
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.
digital
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, wave forms.
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 will echo 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 and an output port. Generally, a virtual circuit is derived from a series of time slots (time-division multiplexing), which is more efficient than requiring dedicated physical circuits for the period of time that 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 pages-like 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 only aware of routers that are 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.
distortion
The modification of the wave form 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 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 data processing
Data processing in which some or all of the processing, storage and control functions, in addition to input/output functions, are situated in different places and connected by transmission facilities.
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 connector block, distribution block, MDF, or 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 new 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.
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 TCP/IP based internet.
downlink
The portion of a satellite circuit extending from the satellite to the earth station.
download
To load software into the nodes of a network from one node over the network media.
downstream
The direction of transmission flow from the source toward the user.
downtime
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.
DQDB (distributed queue dual bus)
The media access method of the IEEE 802.6 standard for metropolitan area networks.
drop
A connection point between a communicating device and a communications network.
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; DS-4 is a 274.176Mbps digital signal carried over 4032 DS-0 channels on a T-4 facility.
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.
DSL bonding
The process of linking together several DSL lines to configure bandwidth in 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.
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 (digital or 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 as modem replacements in digital networks.
DTE (data-terminating equipment)
Equipment (including any type of computer terminal, including PCs, 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 digital TV signals directly to individual homes.
DTMF (dual-tone multifrequency) signaling
The basis for operation of pushbutton telephone sets. 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) 1209Hz, 1336Hz, 1477Hz, and 1633Hz.
DTV (digital TV)
Television sent over a digital network. It is nearly immune to interference and degradation, and it can display a much better range of colors than can analog television.
duplex
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. 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 the incoming and outgoing conversations.
DVB (Digital Video Broadcasting Group)
A European organization that has authored many specifications for satellite and cable broadcasting of digital signals.
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.
E
E.164
The ITU-T's international public telecommunication numbering plan for the PSTN.
E-carrier
A time-division multiplexed, digital transmission facility, operating 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).
e-commerce (electronic commerce)
The secure exchange of funds, executed over a network, for goods and services exchanged between parties.
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.
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)
One billion Gbps.
echo
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.
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.
edge
The network boundary between the customer and the core or central network.
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.
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, then, are satisfied from the nearby machine. This improves the speed and reliability of access because it avoids the Internet backbone and its peering points.
EDI (electronic data interchange)
The asynchronous exchange from computer to computer of intercompany business documents (such as purchase orders, bills of lading, and invoices) and information. EDI can be accomplished through OSI standards or through proprietary products.
EGP (Exterior Gateway Protocol)
A routing protocol that is 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 BGP-4.
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 signals characteristics (time duration, voltage, and current) specified by the Electronic Industries Association.
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, even 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 that are created when electrons move. It ranges from extremely low-frequency radio waves of 30 Hz with a wavelength of nearly the earth's diameter to high-frequency cosmic rays of more than 10 million trillion Hz with 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
Operating 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)
The noise on data transmission lines that reduces data integrity. Motors, machines, and other generators of electromagnetic radiation cause it. Shielding can reduce EMI.
emulate
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.
encapsulation
The process of encasing one protocol into another protocol's format. Also called tunneling.
encryption
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 Class 5 office, local exchange, central office, and serving office.
end-to-end optical architecture
A network in which the optical signal never needs to be converted to an electronic signal.
ENUM
A protocol that is the result of work of the IETF's Telephone Number Mapping working group. The charter of this working group 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.
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 not only connects the PCs on a LAN, it also provides the flexibility to perform a number of network functions that can benefit network administrators and network users in general.
equalization
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.
error
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 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.
Ethernet
A baseband LAN specification invented by Xerox Corp. and jointly developed by Xerox, Intel, and DEC. Ethernet networks operate at 10/100/1000Mbps by using CSMA/CD running over thick or thin coaxial, twisted-pair, or fiber-optic cable. Standards are being developed for 10Gbps Ethernet as well. Ethernet is defined in IEEE 802.3.
ETSI (European Telecommunication Standards Institute)
A telecommunications standardization organization.
even parity check (odd parity check)
A test of whether the number of digits in a group of binary digits is even (even parity check) or odd (odd parity check).
exchange
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 co-sited to perform different functions, for example, local exchange/central office, tandem exchange, toll/trunk/transit exchange, and so on.
extranet
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
F (fully associated) link
A link that directly connects to signaling endpoints, generally SSPs.
facility
(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.
fading
A phenomenon, generally of microwave or radio transmission, where atmospheric, electromagnetic, or gravitational influences cause a signal to be deflected or diverted away from the target receiver. The reduction in intensity of the power of a received signal.
Fast Ethernet
A standard for high-speed Ethernet that has a rate of 100Mbps.
fast packet switching
A packet processing technology that has streamlined protocol handling, including Frame Relay and ATM.
fault
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, charged with regulating all electrical and radio communications in the United States.
FDD (Frequency Division Duplexing)
A full-duplex technique that it is 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. It 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.
FECN (forward explicit congestion notification)
A bit in the Frame Relay header by which the network can inform the receiver of network congestion.
femtosecond
0.000000000000001 (that is, 10 15) 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.
FHHS (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 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 kilometers) 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.
filter
To selectively forward data, based on criteria specified by the network manager.
firewall
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.
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's relatively easy to engineer.
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 of modifying 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.
forward channel
The communications path that carries voice or data from the call initiator to the network.
forward error correction
A system that uses redundant information in received data to permit the receiver to correct transmission errors.
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.
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.
fragmentation
The process of splitting a packet into pieces when it is larger than the MTU it must transmit.
frame
(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-switch 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 standard addresses data communications speeds up to 45Mbps.
framing
A control procedure used with multiplexed digital channels, such as T1 carriers, in which bits are inserted so that the receiver can identify the time slots that are allocated to such subchannel; framing bits may also carry alarm signals indicating specific alarm conditions.
Free Space Optics
An optical wireless networking option that uses low-powered infrared lasers. There are two options in Free Space Optics: 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.
frequency
An expression of how frequently a periodic (repetitious) wave form or signal regenerates itself at a given amplitude. It can be expressed in hertz (Hz), kilohertz (KHz), megahertz (MHz), and so on.
FSK (frequency shift keying)
A method of modulation that uses two different frequencies to distinguish between a mark (digital 1) and a space (digital 0) when transmitting on an analog line. Used in modems operating at 1,200bps or slower.
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 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 (fiber-to-the-home)
A system in which fiber cable extends from a switching office to the subscriber's house.
full-duplex
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 in motion pictures, 25 frames per second in the PAL system, and 30 frames per second in the NTSC system.
FX (foreign exchange) line
A line that makes a toll call appear to be a local call.
G
gateway
A device or program (that is, 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.
GEO (geosynchronous orbit)
A circular orbit with a 24-hour orbital period approximately 22,300 miles (36,000 kilometers) 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 transmitting to fixed earth stations.
GHz (gigahertz)
1 billion cycles per second.
Gigabit Ethernet
An Ethernet standard, introduced in 1997, that supports 1,000Mbps.
global information infrastructure
A vision of individual national information infrastructures joined together to form an international network.
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.
grooming
The process of selectively removing channels from a digital facility for routing to a designated remote location via another digital facility. Grooming basically allows you to drop and add payload flexibly.
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 earth at one or more points.
ground start
A signaling method whereby one station detects that a circuit is grounded at the other end.
ground 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 earth station.
Group 3 fax
An ITU-T standard for encoding an image and transmitting it over dial-up lines.
Group 4 fax
An ITU-T standard for encoding an image and transmitting it over ISDN or other wideband digitized services.
GSM (Global System for Mobile Communications)
A European standard for 2G wireless digital communications, it is globally implemented and supports both voice and data communications. It operates in three frequency bands: 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.
H
H.323
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.
H-channel
A class of high-speed ISDN channels. H-0 is 384Kbps, H-11 is 1.536Mbps, and H-12 is 1.920Mbps.
half-duplex
Communications in which data can be transmitted between two stations in both directions, but only one direction at a time.
HAN (home area network)
A broadband network in a smart house that connects the various smart devices.
handoff
The transfer of duplex signaling as a mobile terminal passes to an adjacent cell in a cellular radio network.
handshake
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 unalterable circuit layout, rather than under computer or stored-program control.
hardware
The physical equipment, as opposed to programs or procedures of a computer system.
harmonic distortion
A wave form distortion, usually caused by the nonlinear frequency response of a transmission.
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.
HCSD (high-speed circuit-switched data)
A standard for transferring high-speed data over aggregated GSM channels; provides data rates up to 64Kbps.
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. 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 kilometers). HDSL is deployed over two twisted-pair cables, and it affords equal bandwidth in both directions (i.e., it is symmetrical). 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 require a screen aspect ratio of 16:9 (versus 4:3 with current TVs) and which is capable of reproducing at least four times more detail than is the existing broadcasting system.
headend
The control center of a cable TV network.
header
The initial portion of a message or file, which contains statistic and control information.
HELLO
The protocol used by a group of cooperative, trusting packet switches to allow them to discover minimal delay routes.
heuristic
An exploratory method of problem solving in which solutions are arrived at by an interactive, self-learning method.
hexadecimal
A system of numbers in base 16; hexadecimal digits range from 0 (zero) through 9 (nine) 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, typically used in shortwave radio applications; frequencies approximately in the 3MHz to 30MHz range.
HIPPI (High Performance Parallel Interface)
A gigabit-per-second OSI Layer 1 and 2 interface standardized by ANSI. HIPP5 supports 800Mbps up to 82 feet (25 meters) 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.
holding time
The length of time a communication channel is in use for each transmission. Includes both message tone and operating time. Also called connect time.
HomeRF
An open standard for short-range transmission of digital voice and data between mobile devices.
hop
A unit of network distance. In particular, 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 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.
host
An end user computer system that connects to a network. Hosts range in size from personal computers 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.
howler tone
The tone that alerts a subscriber when the 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 (but the norm is generally around 28.8Kbps) by enabling the concurrent usage of up to four traffic channels of a GSM network.
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.
HTML (Hypertext Markup Language)
A document standard that defines a simple logical structure including titles, heading, 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.
hub
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 particular 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 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. 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.
I
ICANN (Internet Corporation for Assigned Names and Numbers)
A nonprofit corporation that was formed in 1998 to take over work previously done by the U.S. government in managing the domain name and root server systems.
ICMP (Internet Control Message Protocol)
An integral part of the Internet Protocol that handles error and control messages. Specifically, routers and hosts use ICMP to send reports of problems about datagrams back to the original source that sent the datagram. ICMP also includes an echo request/reply 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.
IDEA (International Data Encryption Algorithm)
An algorithm developed by ETH Zurich that is free of charge for noncommercial use. It is viewed as a good algorithm and 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.
IDSL (ISDN DSL)
A transmission medium that has a maximum loop length of 3.4 miles (5.5 kilometers), and it is deployed as a single twisted-pair cable that offers 128Kbps in each direction.
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. 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 Membership 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 (that is, a set of hosts interested in traffic for a particular multicast address).
IGP (interior gateway protocol)
Any protocol that is used to propagate network reachability and routing information within an autonomous system. RIP and IGRP are examples.
IGRP (Interior Gateway Routing Protocol)
A proprietary network protocol, developed by Cisco Systems, designed to work on autonomous systems. IGRP is a distance-vector routing protocol, which means that each router sends all or a portion of its routing table in a routing message update at regular intervals to each of its neighboring routers.
IKE (Internet Key Exchange)
The key exchange protocol used by IPSec. It 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.
immersion
In virtual reality, refers to the user's subjective sensation of being inside the virtual world, and not observing it from an outside perspective.
i-mode
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").
IMT-2000 (International Mobile Telecommunications 2000)
An evolving standard for third-generation mobile communications, enabling personal mobility and converging mobile and fixed networks.
IMUN (International Mobile User Number)
A number used to dial a subscriber in third-generation mobile networks.
IN (intelligent network)
An architecture for providing advances 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 of 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 that are 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.
information technology
A broad term that describes the computer hardware, software, and networking industry, including telecommunications and audiovisual equipment.
infrared
The frequency range in the electromagnetic spectrum than is higher than radio frequencies but below the range of visible light.
infrastructure
The underlying structure or framework of the telecommunications system (for example, switching, multiplexing, and transmission systems) that allows for the transmission of voice, video, and data.
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.
interactive
A term that describes the mode of transaction with a particular information service. An interactive service allows for both input and output. It is sometimes referred to as 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 supporting the interconnection.
interconnection
The interworking of two separate networks, including wireline and wireless. Interconnection is used to refer both to the technical interface and to the commercial arrangements between two network operators providing service.
interface
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 the 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.
Internet
Physically, a collection of packet-switching networks interconnected by routers along with protocols that allow them to function logically as a single, large, virtual network. Internet with a capital I refers to 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 for the academic network. Internet 2 acts as a testbed for many of the latest and greatest technologies. The 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 that is comprised 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.
interoperability
The ability to exchange information in a network that contains computers and additional devices that have dissimilar operating systems or protocols.
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. It 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, requiring RSVP. Its key building blocks include resource reservation and admission control.
intranet
A network based on TCP/IP protocols (an internet) that belongs to an organization and is accessible only by the organization's members, employees, or others with authorization.
Inverse ARP
An extension to the ARP protocol 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 twenty-four 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, specifying how and when to generate error messages, and specifying when to discard unreliable data.
IP address
The 32-bit address assigned to hosts that want 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. The partition makes routing efficient.
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 comprised of more than 10,000 service providers.
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 wholesale
VoIP service providers that offer 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 PBX
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 router that provides connection-oriented services at the IP layer.
IP telephony
The use of the Internet or a private IP network for telephony.
IPDC (Internet Protocol Device Control)
A specification that creates flexible management of media gateway devices.
IPNSIG (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.
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.
IPS7 (IP Signaling System 7)
A signaling protocol that works with SS7.
IPv4 (Internet Protocol version 4)
The current 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 all the routers out there, and hence we haven't seen the community jump at the migration from IPv4 to IPv6. Also called IPng (IP Next Generation).
IPX (Internetwork Packet Exchange)
The Novell equivalent of IP. It 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 descendent of BGP that is being considered by ISO as the basis for an inter AD routing protocol standard.
ISC (international switching center)
An exchange used to switch traffic between different countries over international circuits.
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 of 2.048Mbps in Europe.
IS-IS (Intermediate System to Intermediate System)
The OSI's emerging IS-IS protocol 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 of sharing 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.
isochronous
A descriptor signifying enabling network characteristics. This includes 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. A circuit-switched, fixed rate, continuous data stream, such as voice, video, or real-time sensor data.
IS-54
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.
IS-95
TIA's Interim Specification 95. A spread-spectrum wireless network, operating in the 800MHz range, using a CDMA/FDD scheme.
IS-136
TIA's Interim Specification 136. An updated TDMA-based standard that updates IS-54. Provides 3x spectral efficiency over analog AMPS systems.
ISV (independent software vendor)
A vendor that develops the applications that the ASPs then put up for sale or for rent.
ITU (International Telecommunication Union)
A telecommunications agency of the United Nations, established to provide standardized communications procedures and practices including frequency allocation and radio regulations on a worldwide basis. Parent group of the ITU-T (telecommunications), ITU-R (radio), and ITU-D (developing nations).
IXC (interexchange carrier)
A long-distance telephone company that offers circuit-switched, leased-line, or packet-switched service.
J
jack
A device used generally for terminating the permanent wiring of a circuit, access to which is obtained by the insertion of a plug.
J-carrier
The Japanese standard of the PDH, a time-division multiplexed, digital transmission system. J-carrier is a PCM system that uses a 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 501-channel communications circuit; J-4 is a 97.728Mbps 1,527-channel communications circuit; and J-5 is a 397.200Mbps communications circuit.
jitter
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.
jumper
A patch cable or wire used to establish a circuit, often temporarily, for testing or diagnostics.
K
Ka-band
The portion of the electromagnetic spectrum allotted for satellite transmission; 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.
Ku-band
The portion of the electromagnetic spectrum being used increasingly for satellite communications. Frequencies are approximately in the 12GHz to 14GHz range.
L
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, for example 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 on to its destination.
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 Bell operating companies (BOCs) can offer exchange and exchange access services (local calling, private lines, and so on).
latency
The delay associated with the time it takes a packet to travel from entry point to exit point.
layer
In the OSI reference model, a collection of related network-processing functions that comprise one level of a hierarchy of functions.
L-band
The portion of the electromagnetic spectrum commonly used in satellite and microwave applications. L-band operates in the 390MHz to 1550MHz 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 communication 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-optics communications, particularly for short-haul links. Also used in alphanumeric displays in electronic telephones and calculators.
LEO (low earth-orbit) satellite
A satellite that orbits at about 400 to 1,000 miles (640 to 1,600 kilometers) 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 vise versa) for transfers over limited distances; some operate at higher speeds than modems that are designed for use over analog telephone facilities.
line
(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 communication, a circuit connecting two or more devices. (3) The transmission path from 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 (such as microwave, infrared, and 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.
link
(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 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 upon 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 link-level transmission control. The upper sublayer of the IEEE Layer 2 (OSI) 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's generally located somewhere in the range of 28GHz to 45GHz, depending on the country. It 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 or router) and the network.
loading
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 kilometers) in length, that compensates for the wire capacitance and serves to boost voice grade frequencies. They 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.
local
Pertains to a device that is connected directly to the computer without using a WAN communication line.
local exchange
The switching center in which subscribers' lines terminate. The exchange has access to the other exchanges and to national trunk networks. Also called central office, end office, serving office, and Class 5 office.
local exchange trunk
A trunk between the CPE and the local exchange. Also referred to as central office trunk.
local loop
A line connecting a customers' 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 you to keep your own telephone number when you move to a new location.
local service area
The area within which the telephone operating company uses local rates for calling charge.
LocalTalk
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 by global positioning systems (GPS) 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 that is used to identify the communications program by "name" to the protocol stack with which you are working. No matter where your program is put in the network, your logical address will remain the same, even though your physical address may change.
long-haul
Long-distance, describing (primarily) telephone circuits that cross out of the local exchange.
loop
(1) A local circuit between an exchange and subscriber CPE, either residential (single line telephone) or business (PBX). Also called subscriber loop, local line, and local loop. (2) In programming, a sequence of computer instructions that repeats itself until a predetermined count or other test is satisfied.
loop back
A diagnostic procedure used for transmission devices; 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. Loop-back testing may be performed within a locally attached device or conducted remotely over a communications circuit.
loop circuit
The circuit connecting the subscriber's equipment with the local exchange switch. Also called metallic circuit and 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 (a) opening and closing the DC path around the loop, (b) reversing the voltage polarity, or (c) 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.
loss
A decrease in energy of 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 the 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 or Huffman encoding.
lossy compression
In data compression, the case 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 indicates frequencies between 30KHz and 300KHz.
low-level language
A programming language in which instructions have a 1-to-1 relationship with machine code.
LPC (linear predictive coding)
A vector-quantization-based compression scheme for speech. It can compress speech down to 2.4Kbps.
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.
M
MAC (Media Access Control)
The part of OSI Layer 2, it 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.
MAE (metropolitan area exchange)
An interconnection and exchange point where public Internet backbones meet and exchange traffic. Also called a NAP.
MAN (metropolitan area network)
A network spanning a geographical distance of up to a 62-mile (100-kilometer) diameter; a citywide network.
managed object
A data processing or data communications resource that may 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. It 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.
mapping
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., name-address mapping, internet work-route mapping).
mark
The signal (communications channel state) corresponding to a binary 1.
marker
A wired-logic control circuit that, among other functions, tests, selects, and establishes paths through a switching state(s) in response to external signals.
matrix
(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 one million bytes.
mbone (multicase backbone)
An experiment to upgrade the Internet to handle live multimedia messages. With mbone, a single packet can have multiple destinations and isn't split up until the last possible moment. This means that it can pass through several routers before it needs to be divided 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 one million bits per second.
MBS (mobile broadband services)
Very high bit-rate services (100Mbps+) over wireless channels.
m-commerce (mobile commerce)
Financial transactions that occur on mobile devices.
MCU (multipoint control unit)
A device that supports multiparty video conferencing between several individual circuit videoconferencing systems. The MCU acts as a videoconference hub.
MD5 (Message Digest-5)
An algorithm, created in 1991, that is used to create digital signatures. It is intended for use with 32-bit machines. It is a one-way hash function, meaning that it converts a message into a fixed string of digits called a message digest.
MDF (main distribution frame)
A structure containing all of 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.
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 (that is, IP, ATM, and Frame Relay networks). It interconnects with the SS7 network and enables the handling of IP services.
medium
(1) The material on which data is recorded; for example, magnetic tape or floppy disk. (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, such as optical fiber, cable, wire, dielectric slab, water, air, or free space.
medium frequency
Frequencies in the range between 300KHz and 3MHz.
mega
One million.
MEGACO (Multimedia Gateway Control)
An emerging ITU standard that describes how the media gateway should behave and function. Also called H.248.
memory
Area of computer system that accepts, holds, and provides access to information.
menu
A displayed list of items from which you can make a selection.
menu-driven
A set of instructions that was a list of commands and available options. The user only has to select the desired option; compare to command driven.
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 kilometers) above the earth.
mesh
A topology in which nodes are connected in an unconstrained way which may contain loops. A fully connected mesh has every pair of nodes directly connected.
message
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 a sequential number.
message switching
A technique that transfers messages between points 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.
metric
Information that a routing algorithm uses to determine the best path to the destination. Some examples of the 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 that is used in building citywide rings.
MGCP (Multimedia Gateway Control Protocol)
A combination of the SGCP and IPDC specifications. In this protocol, softswitches provide the external control and management, making MCGP a good way to connect an IAD to a gateway.
MHz (megahertz)
A unit of frequency equal to one 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.
micro
One millionth.
microsecond
One millionth of a second.
microwave
(1) The portion of the electromagnetic spectrum ranging 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 the way of coding all the elements of musical scores, such as sequences of notes, timing conditions, and the instrument that is to play each note.
milli
One thousandth.
millisecond
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.
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 kilometers]). It provides great capacity in that it enables 150 channels. Also called wireless cable.
mobile earth station
A radio transmitter and/or receiver situated on a ship, vehicle, or aircraft, or in a briefcase, and used for satellite communications.
mobile wireless local loop
A last-mile solution that uses cellular telephone or cordless technology, along with satellites. This approach enables subscriber mobility, so you could use it as a replacement for a fixed line when you're in your home, but you could also move outside those boundaries and use it as a mobile line.
mobility network
A wireless network that allows you to roam within the range of your 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 passes it to the destination business machine.
modular
A design technique that permits a design or system to be assembled from interchangeable components; permits the system or device to be expanded or modified simply by adding another module.
modulation
The process of converting voice or data signals for transmission over a network. Also called line coding.
modulation division multiplexing
A mutliplexing methods in which the signals that modulate the optical carriers are mutliplexed.
modulator
A device that converts a signal (voice or other) into a form that can be transmitted.
module
A hardware or software component that is discrete and identifiable.
monitor
(1) A software tool used to supervise, control, or verify the operations of a systems. (2) A device used to display computer-generated information.
MPEG (Motion Picture Experts Group)
The ISO standards body responsible for the MPEG international video compression standards. MPEG-1 addresses VHS-quality images with a 1Mbps to 2Mbps data rate. MPEG-1 can play back from a single-speed CD-ROM player (150Kbps, or 1.2Mbps) at 352x240 (that is, quarter screen) at 30 frames per second (fps). MPEG-2, which at this point is the compression scheme of choice, addresses DTV or computer-quality images with a 6Mbps data rate. MPEG-2 offers resolutions of 720x480 and 1280x720 at 30 fps, 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 variable-bit-rate 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 objects such as 2D or 3D video objects, text, graphics, and sound to be manipulated and made interactive through Web-like hyperlinks and/or multimedia triggers.
MPLS (Multiprotocol Label Switching)
An IETF initiative that integrates Layer 2 information about network links (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.
MPlS (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.
MPOA (Multiprotocol over ATM)
An ATM Forum standard to link a number of local networks across an ATM backbone catering to many different network protocols.
mrouter (multicast router)
A router that enfolds IP packets in special multicast packets and forwards them on toward their destination.
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.
MSP (management service provider)
A provider that takes over the actual management and monitoring of the network.
MSPP (multiservice provisioning platform)
An access-oriented device that can handle all the popular data protocols and interfaces, except that they are not designed to be optical aggregators.
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 the system, or a component of the system, works without fault.
MTS (mobile telephone service)
A telephone service provided between mobile stations and the public switch telephone network; 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, 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 bit streams from the subbands are bonded together at the receiver.
multicasting
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.
multichannel
Pertaining to broadcasting media capable of carrying multiple different television and radio channels.
multidrop
A communications arrangement in which multiple devices share a common transmission channel, although only one may transmit at a time.
multimedia
The presentation of more than one medium, typically images, sound, and text in 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.
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
Indicates that the switching system is capable of being equipped for more than one group of trunk circuits.
multiplex
To interleave or simultaneously transmit two or more messages on a single channel.
multiplexer
A device that enables more than one signal to be sent simultaneously over one physical channel. It combines inputs from two or more terminals, computer ports, or other multiplexers, and transmits the combined datastream over a single high-speed channel. At the receiving end, the high-speed channel is demultiplexed, either by another multiplexer or by software. Sometimes called a mux.
multipoint
Pertaining or referring to a communications line to which three or more stations are connected. It implies that the line physically extends from one station to another until all are connected.
multipoint network
In data communication, a configuration in which more than two terminal installations are connected to a single port.
multiprocessing
The simultaneous execution of two or more computer programs.
mux
See [multiplexer]
N
NACK or NAK (negative acknowledgment)
A message that says there was an error in transmission and the previous block needs to be re-sent before anything else can happen.
NADC (North American Digital Cellular)
A TIA standard that is specified as IS-54 and was adopted in 1992. It has since been updated, and it is now called IS-136. It uses TDMA and TDD schemes, and it offers a total of three time slots. It 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. UWC-136, an advancement of the U.S. TDMA (IS-136) standard, uses EDGE technology.
nailed-up connection
A slang term for a permanent, dedicated path through a switch; often used for lengthy, regular data transmission going through a PBX.
name resolution
The process of mapping a name into a corresponding address. The domain name system provides a mechanism for naming computers in which programs use remote name servers to resolve machine names into IP addresses for those machines.
nanosecond
One billionth of a second.
NAP (network access point)
The point where 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.
narrowband
A service occupying low bandwidth (64Kbps or below).
narrowcast
Transmission of specific programming to predetermined users of a telecommunications network. Only some users of the network are receiving the same information.
NCP (Network Control Program)
A program that resides in a communications controller that controls the operation of the communications controller.
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. 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.
network
(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 network 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 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.
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. It 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 the 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 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 link and nodes of a network, typically either a star, ring, tree, or bus topology, or some hybrid combination thereof.
networking
Communication between stations in a network.
next-generation gateway switch
A switch that is designed to support a wide variety of traffic data, voice, fax, multimedia, and other emerging sensory forms over 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- or cell-based network that's capable of transporting and routing a multitude of services, including voice, data, video, and multimedia. It is a common platform for applications and services that the customer can assess across the entire network, as well as outside the network.
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 introduced in 1983 for an all-digital network. It 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 group that prior to 1988, was called the National Bureau of Standards.
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. May provide traffic analysis, call detail recording, configuration control, fault detection and diagnostics, and maintenance.
NMT (Nordic Mobile Telephone System)
A Scandinavian wireless system that originally operated at 450MHz, offered around 220 channels, and had a very large coverage area.
node
Any device on a network that can independently send or receive information and that has a network address; also the point at which the device is linked to the network.
noise
Unwanted electrical signals, introduced by circuit components or natural disturbances, that tend to degrade the performance of a communications channel.
nonblocking
Describes a switch where a through traffic path exists for each attached station. Generically, a switch or switching environment designed to never experience a busy condition due to traffic volume.
nonswitched line
In data communication, 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 the duration of time.
NSP (network service provider)
A very large, global backbone carrier that owns its own infrastructures (for example, AT&T, WorldCom, UUnet, Sprint, Verizon, Cable & Wireless, and Qwest).
NTSC (National Television Systems Committee)
A television broadcasting system that uses 525 picture lines and a 60Hz field frequency. It was developed by the committee, and is used primarily in the United States, Canada, Mexico, and Japan.
See also [PAL]
See also [SECAM]
NTU (network terminating unit)
The part of the network equipment that connects directly to the data terminal equipment.
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.
NXX
The current exchange numbering plan, in which N is any digit from 2 to 9 and X is any digit from 0 to 9.
O
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 that are used to convert between light and electricity in a nonoptical multiplexer.
OC-1 to OC-48 (Optical Carrier-1 to Optical Carrier-48)
OC-1 is the base optical carrier transmission speed of 51.840Mbps. To calculate OC-2 to OC-48 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).
octet
8-bit byte. Used instead of byte when talking about packet services.
OFDM (Orthogonal Frequency Division Multiplexing)
A multicarrier modulation scheme that broadcasts on many frequencies, reducing interference from collisions with walls and objects.
off-hook
A state in which a telephone set is in use (that is, the handset is removed from its cradle).
off-line
(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.
offload
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.
one-way trunk
A trunk between a switch (that is, a PBX) and an exchange, or between exchanges, where traffic originates from only one end.
ones density rule
A principle which says that if you transmit more than 15 zeros in a row, the network may lose synchronization, which means transmission errors could occur.
on-hook
A state in which a telephone set is not in use (that is, the handset is resting in the cradle).
online
(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 not owning a host computer; includes time sharing, archival storage, and prepared software programs.
ONP (Open Network Provision)
A pan-European standard ensuring 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 takes 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.
optical carrier
Specifications defining 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 serves to reflect 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 carriers to string together wavelengths to provide end-to-end connections. It links any of several incoming lines to any of several outgoing lines and automatically reroutes traffic when a network path fails. Sometimes referred to as an optical cross-connect or a wavelength router.
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 that is used for the definition of network protocol standards to enable an OSI compatible computer or device to communicate with any other OSI-complaint computer or device for a meaningful exchange of information. Layer 7, the application layer, is responsible for exchanging information between the programs that are 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.
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.
out-of-band management
A system 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.
output
Data that has been processed.
overflow
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 which has its own lines, exchanges, and, often, a separate international gateway.
overnet
A PNAP.
override
To seize a circuit even though the circuit is already occupied.
P
PABX (private automatic branch exchange)
Another term for a PBX.
packet
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. It 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 percent.
packet radio
A data network that uses licensed bandwidth and which is specifically built for two-way data, not for voice communications.
packet-switched network
A network consisting 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, and ATM.
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 on to the next node. The packets are then reassembled into the original message at the receiving end.
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 (e.g., X.25 network).
PAL (Phase Alternate 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]
See also [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 bids making up a character or byte, either over separate channels, or on different carrier frequencies on the same channel.
parity
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 the addition of noninformation bits are added to data to make the number of ones 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 VRC.
password
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 users onto ports on one or more network hubs.
path
(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)
1,000Tbps.
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. Digital transmission technique that involves sampling of an analog information signal at regular time intervals and coding, the measured amplitude into a series of binary values, which are transmitted by modulation of a pulsed, or intermittent, carrier. A common method of speech digitizing by using 8-bit code words, 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, defining the available digital transmission rates and number of channels. It 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 packet-switching network (e.g., X.25, Frame Relay, Internet, IP backbones) that is designed to provide low error-rate data transmission.
PDU (protocol data unit)
The OSI term for a packet.
peer-to-peer
The interaction by which computers and other network devices communicate with each other as equals and on their own initiative (as opposed to a host/terminal environment).
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.
performance
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 stand-alone boxes to which we are tied and put into ordinary things, in everyday objects around us. 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, as it serves to reconstruct other frames in some instances.
PGP (Pretty Good Privacy)
A technique for encrypting messages. 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 that you disseminate to anyone from whom you want to receive a message and a private key that you use to decrypt messages that you receive.
phantom circuit
A third voice circuit that is superimposed on two 2-wire voice circuits.
phase
The angle of a wave form at a given moment.
phased-array antenna
A small, flat antenna that is steered electronically. It provides great agility and fast tracking, as well as the ability to form multiple antenna beams simultaneously. The beam is electrically pointed by adjusting the phases of the individual transmitters. This 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.
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.
PHS (Personal Handyphone System)
A Japanese standard for 2G PCS wireless networks.
physical address
The address of the physical communications device in a system.
physical interface
The definition of the number of pins in the connector, the number of wires in the 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.
physical layer
Layer 1 of the OSI model. Defines the electrical, optical, mechanical, and procedural characteristics of the interface.
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 certificate authority to handle the verification process.
plant
The physical equipment of a telephone network that provides communications services.
plotter
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.
PM (phase modulation)
A way of modifying 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.
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 you can associate information about individual users, groups, organizational units, and entire organizations, as well as events (such as the beginning of the accounting department's month-end closing), with various network services or classes of service.
polling
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.
port
A point of access into a communications switch, a computer, a network, or other electronic device; the physical or electrical interface through which one gains access; 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 more like a regional memory address.
port speed
The maximum signaling rate on a digital access line.
POTS (plain old telephone service)
The standard analog telephone service that most homes use.
PPP (Point-to-Point Protocol)
A successor to SLIP, this protocol provides router-to-router and host-to-network connections over synchronous and asynchronous circuits.
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 its weaknesses are on the security front.
presentation layer
Layer 6 in the OSI model that 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 the 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 controlling communications.
private key
The decryption (reception) or encryption (signature) component of an asymmetric 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 public switched telephone network. Also called leased line.
private network
A network based on leased lines or other facilities that provide telecommunication services, within an organization or within a closed user group, as a complement or as a substitute to the public network.
protocol
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 nods contend for the chance to transmit data over the 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)
The 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, 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 transmitting 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 in places other than the United States.
PT&T (postal, telegraph, and telephone) organization
Usually a governmental department that acts as its nation's common carrier.
public key
A device that is 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.
pulse
A momentary, sharp alteration in the current or voltage produced in a circuit to operate a switch or relay which can be detected by a logic circuit; 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.
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).
Px64
The ITU-T H.320 standard for interoperability in video conferencing over ISDN.
Q
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 QAM nn, where nn indicates the number of states per Hertz. The number of bits per symbol time is k, where 2k = nn. So, 4 bits/Hz is equivalent to QAM 16, 6 bits/Hz is equivalent to QAM 64, and 8 bits/Hz is equivalent to QAM 256.
QoS (Quality of Service)
In networking, a concept by which applications may 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. Explicit QoS means that the network manager controls that decision.
QPSK (Quadrature Phase Shift Keying)
A single-carrier modulation scheme that supports 2 bits per symbol time. Equivalent to QAM 4.
Q.Sig
The ECMA standard for common-channel signaling between PBXs.
quantization noise
Signal errors caused by the process of digitizing a continuously variable slope.
query
A request for information entered while the computer system is processing.
queue
An ordered accumulation of data or transactions stored for later processing.
queuing
The process whereby communications calls, processing requests, processes, and so on are stacked or held so that they can be worked with in sequence.
R
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 that is used for purposes of authenticating whether a user is allowed access into the corporate resources.
RADSL (Rate-Adaptive DSL)
A symmetrical or asymmetrical transmission medium that has a maximum loop length of 18,000 feet (5.5 kilometers) and is deployed as a single twisted-pair cable. It 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; that is, any byte of memory can be accessed without touching the preceding bytes.
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 break-up of AT&T.
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.
redundancy
(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.
register
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.
reliability
The measure of a network's availability. 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.
remote
Pertains to a computer or device that is connected to another computer or device over a communication line.
remote access
Communications with a computer or PBX in one location from a device that is physically removed from the location of the computer.
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 that is 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 can be connected to the system processor to run diagnostics and perform system administration.
remote monitoring MIB
The MIB that enables any SNMP management console to extract information from a protocol analyzer running RMON.
repeater
(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 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.
reverse channel
A simultaneous data path in the reverse direction over a half-duplex facility. Normally it 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; usually, above 20KHz.
ring
(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
A connection on a Token Ring MAU used to tie multiple MAUs into a larger ring.
ring out
A connection on a Token Ring MAU used to tie multiple MAUs into 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.
RJ-11
A standard four-wire modular connector used with telephones.
RJ-45
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 capable of receiving, but not of transmitting, a signal.
robot
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.
router
A device that connects two LAN segments, which use similar or different architectures, at the OSI network layer, Layer 3. The router determines the most efficient route for passing data through an internet. Those packets that contain a network address different from the originating PC's address are forwarded onto an adjoining network. Multiprotocol routers can handle this job for two or more protocols simultaneously.
routing algorithm
A formula that uses metrics (such as path length, destination, next-hop associations, reliability, delay, bandwidth, load, and 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 the 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 with 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.
RS-232-C
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.
RS-366-A
An EIA standard for interfaces between DTE and automatic calling equipment for data communication.
RS-422-A
An EIA specification for electrical characteristics of balanced voltage digital interface circuits.
RS-423-A
An EIA specification for electrical characteristics of unbalanced-voltage digital interface circuits.
RS-449
An EIA specification for general-purpose, 37-position and 9-position interface 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 (Reservation Resource Protocol)
A protocol that enables an internet to support specified levels of QoS. By using RSVP, an application is capable of reserving resources along a route from source to destination. RSVP-enabled routers then schedule and prioritize packets to fulfill the QoS.
RTCP (Real-Time Control Protocol)
An ITU protocol that provides status feedback from senders to receivers.
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.
S
sampling
A statistical procedure whereby generalizations are drawn from a relatively small number of observations.
satellite communications
The use of orbiting satellites to relay transmissions from one earth station to another, or to multiple earth stations.
scattering
Causing of lightwave signal loss in optical fiber transmission; diffusion of a light beam 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 the call.
scrambler
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).
SDH (Synchronous Digital Hierarchy)
An ITU-T standard for digital broadband communications.
SDLC (Synchronous Data Link Control)
An IBM data-link layer protocol associated with SNA. In contrast to BSC, SDLC provides for full-duplex transmission and is more efficient.
SDSL (Symmetrical or Single-Line DSL)
A symmetrical service that has a maximum loop length of 18,000 feet (5.5 kilometers) 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. SDTV and HDTV are the two categories of display formats for DTV transmissions, which are becoming the television standard.
SECAM (Systeme Electronique Couleur Avec Memoire)
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 used a modified form of SECAM. There are two versions of SECAM: SECAM vertical and SECAM horizontal.
See also [PAL]
See also [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.
security
The 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 turned to one of five possible frequencies are used to achieve this effect.
server
A processor that provides a specific service to the network. A routing server connects nodes and network 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 (that is, the server) and the other computers request information from and store information on the server.
session
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.
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, but renders higher security.
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 electromagnetic interference 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 CLNP routed via IS-IS might be used to handle the routing for both IP and CLNP.
sideband
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, some of which also suppress the carrier frequency.
signal
A physical, time-dependent energy value used for the purpose of conveying information through a transmission line.
signaling
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.
simplex
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 DAS. Single-attached stations can only be attached to one ring. These stations are less expensive than dual-attached stations, but are less reliable because the optics, electronics, and the physical link must all be operational for the SAS 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.
skin effect
An occurence where 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 the 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 relates to downtime or uptime, a specified level of customer support and what software or hardware will be provided and for what fee.
sleeve
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. Largely replaced by PPP.
smart card
A credit-card sized device with imbedded 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.
SMS (short message 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 VTAM and the NCP. (2) SNA specifies how products connect and communicate with one another in a network. SNA is a design for a total data communication 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 possible hundreds of miles away. SNA itself is not a system; it is an architecture. The purpose of SNA is to define uniform formats and protocols for data communication networks.
SNMP (Simple Network Management Protocol)
A standard low-level protocol that is 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 capability to transfer a large chunk of data at once, and the capability 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 (dB). As the ratio decreases, it becomes more difficult to distinguish between information and interference.
socket
An interface to the transport layer 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, that is, they are direct calls to an operating system. Sockets permit an application program to access the services provided by TCP and UDP.
software
Computer instructions that perform common function for all users as well as specific applications for particular user needs.
softswitch
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.
solid-state device
Electronic pathways made of solid materials (e.g., chips and bubble memories).
SONET (Synchronous Optical Network)
The ANSI standard, incorporated into the SDH standard, 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 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.
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; that is, there is one and only one possible patch from any particular end point to any other.
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 efficiency
A measure of the number of digital bits that can be encoded in a single cycle of a wave form.
splitter
A filter used at each end of a copper pair to split the frequency bands.
spooling
Temporarily storing input and output data streams on disk or tapes 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. FHHS and DSSS are examples of spread-spectrum techniques.
SPX (Sequenced Packet Exchange)
The NetWare communications protocol used for interprocess communications. It guarantees that an entire message arrives intact and uses the NetWare IPX protocols as its delivery mechanism.
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 telecommunications protocol defined by the ITU as a way to offload PSTN data traffic congestion onto a wireless or wireline digital broadband network. SS7 architecture is set up in a way so that any node could exchange signaling with any other SS7-capable node, not just signaling between switches that are directly connected.
SS7 gateway
A device that allows an interface between circuit-switched networks (with their powerful SS7 infrastructure) and the emerging packet-switched networks that need to be able to handle the more traditional type of voice communications on a more cost-effective basis.
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.
star
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's called passive.
static routing
Routing in which the routes are manually entered into the routing table.
station
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.
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.
store-and-forward
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.
STM (Synchronous Transmission 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. STM levels include STM-0, STM-1, STM-4, STM-16, STM-64, and STM-256.
STP (service transfer point)
A switch that is responsible for translating the SS7 messages and then routing those messages between the appropriate network nodes and databases.
STP (shielded twisted-pair)
Two insulated wires in a cable wrapped with metallic braid or foil to prevent interference and to provide noise-free transmission.
streaming media
Data that is transferred so that it can be processed as a steady and continuous stream. Streaming technologies are becoming increasingly important with the growth of the Internet, because most 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 51.8 million bits per second. The signal is composed of 8000 frames per second, with 810 8-bit bytes per frame (8 bits/bytex810 bytes/framex8000 frames/sec = 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.
subnetting
A technique used to impose a hierarchy on IP addresses that supports a form of area routing.
subrate
Transmission speeds below DS-0/64Kbps.
subsystem
A part of a system that does 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.
supervision
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.
switching
Establishing of 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 is capable of interconnecting circuits or transferring traffic between circuits.
switching system
A device that connects two transmission lines together.
switchook
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.
switchover
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.
synchronization
The process of adjusting a receiving terminal's clock to match the clock of the transmitting terminal.
synchronous
(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 (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 communication, a method of transmission in which the sending and receiving of characters is controlled by timing signals. The sending and receiving terminals are operating continuously in step with each other.
synchronous transport module level n
One of the elements that comprise the SDH signal hierarchy. STM-1 defines the basic building block corresponding to a data rate of 155.52Mbps and a frame structure of 2,430 bytes every 125 microseconds.
synchronous transport signal level n
One of the elements that comprises the SONET signal hierarchy. STS-1 defines the basic building block corresponding to a data rate of 51.84Mbps and a frame structure of 810 bytes every 125 microseconds.
system
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.
T
TA (terminal adapter)
A protocol converter that connects a non-ISDN device to the ISDN network.
TACS (Total Access Communications System)
A standard for analog telephony that was deployed widely in Europe before GSM became the most popular approach.
T-carrier
The North American standard of the PDH, a time-division multiplexed, 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; T-4 is a 274.176Mbps 4,032-channel communications circuit.
tandem
(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 customers.
tandem data circuit
A data circuit that contains two or more pieces of DCE in series.
tandem switch
Also called a junction exchange or a tandem office, a switch that is used to connect local exchanges throughout the metropolitan area.
tap
An open point on a cable bundle that is left so that technicians can easily splice off a pair to bring additional service to a home or to bring service to a new home.
tariff
The published rate for the use of a specific unit of equipment, facility, or type of service provided by a communications common carrier; also, the vehicle by which the 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, but 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.
TCAP (Transaction Capabilities Applications Part)
A system that supports intelligent network service by enabling the exchange of noncircuit-related information between different signaling points (that is, network nodes).
TCP/IP (Transmission Control Protocol/Internet Protocol)
The most widely used internetworking protocol. Ensures that packets of data are delivered to their destinations in sequence in which they were transmitted. TCP roughly corresponds to Layer 4 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.
TDD (Time Division Duplexing)
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 in which you first divide the available or allocated frequency spectrum into a number of channels. Then, within each channel you apply TDM to carry multiple users interleaved in time.
telco
A telephone company.
telecommunication lines
Telephone and other communication pathways that are used to transmit information from one location to another.
telecommunications
Any process that permits the passage of information from a sender to one or more receivers in any usable form (printed copy, fixed or moving pictures, visible or audible signals, and so on) by means of any electromagnetic system (electrical transmission by wire, radio, optical transmission, waveguides, and so on).
Telecommunications Technology Committee
A body that creates telecommunications standards that are followed in Japan.
teledensity
The number of main telephone lines per 100 inhabitants.
telephone channel
A transmission path designed for the transmission of representing human speech or other telephone communication (e.g., fax) requiring the same bandwidth. The bandwidth of an analog telephone channel is 4,000Hz.
telephony
A generic term describing voice telecommunications.
Telnet
The TCP/IP site protocol that supports a remote login capability.
tera
One trillion.
Terabit switch router
An emerging 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.
terminal
(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 communication channel. (3) Same as work station.
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. It enables a signal to be carried as far as 1,640 feet (500 meters) 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. It enables a signal to be carried as far as 607 feet (185 meters) before requiring a repeater. Also called 10Base2 cable or Cheapernet.
throughput
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 telecommunication.
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 question can carry a bit, byte, or, in principle, any other unit of data.
timeout
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), or in access timeout (when a terminal on a local area network using a CSMA/CD access method is prevented from transmitting for a specified time period 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.
tip
The contacting part at the end of a telephone plug or the top spring of a jack. The conductors associated with these contracts. The other contact is called a ring.
token
A small, 8-bit word that is circulated in a unidirectional fashion among the devices on the 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. 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.
topology
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.
traffic
(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.
transaction
An item of business. The handling of customer orders and billing are examples of transactions.
transaction processing
In batch or remote batch processing, the processing of a job or job step-in interactive processing, an exchange between a terminal and another device that does a particular action; for example, the entry of a customer's deposit and the updating of the customer's balance.
transceiver
A device that connects a host interface to a local area network.
transducer
A device for converting signals from one form to another, such as a microphone or a receiver.
transmission line
A connection over which data 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) or wireless (e.g., satellite, microwave).
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).
transmission
Information that is sent in the form of electrical signals over electric wires, waveguides, or radio.
transmission speed
The rate at which information is passed through communications lines, generally measured in bits per second (bps).
transmit
To send information from one location to another.
transparent bridging
A system in which the bridge functions as a node on each of the connected networks, and its presence in the communications path between nodes is transparent. Communications proceeds as if the nodes were on the same network.
transponder
The receiver, transmitter, and antenna equipment that together forms a single repeater channel on a satellite.
transport layer
Layer 4 of the OSI model, providing reliable, transparent transfer of data between endpoints.
tree
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.
troubleshooting
Monitoring and testing the performance of a network to detect and fix problems.
trunk
A circuit that is configured to support the calling loads generated by a group of users; the transmission facility that ties together switching systems.
tunneling
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, 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 their signals from electromagnetic noise). Twisted-pair cable is often used in telephone wires; it also serves as the transmission medium in some LANs.
two-wire circuit
A circuit formed by two conductors insulated from each other that can be used as either a one-way transmission path, a half-duplex path, or a duplex path.
Twofish
A very strong encryption algorithm that was one of the five initial candidates for the AES.
type approval
An administrative procedure of technical tests and vetting applied to items of telecommunication equipment before they can be sold or interconnected with the public network. Also known as homologation.
U
UBR (unspecified bit rate)
An ATM service class that provides best-effort service. UBR offers no service guarantee, so you would use it for text data, image transfer, messaging, and distributing information that's noncritical, where you don't have 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.
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. The convergence of mobile and fixed networks, as well as terrestrial wireless and satellite-based networks.
unicasting
Sending streams from a single origination point directly to a single destination point.
Unicode
A coding scheme that assigns 16 bits per character (that is, 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.
UNIX
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 kilometers) 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.
UPS (uninterruptible power supply)
A device that usually includes in 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.
upstream
The direction of transmission flow away from the user.
UPT (Universal Personal Telephony)
An evolving ITU-R standard for the provision of personal mobility across many different kinds of fixed and mobile networks.
UTP (unshielded twisted-pair cable)
A twisted-pair cable in which the two insulated wires that twist around each other are not surrounded by additional insulation.
UWC (Universal Wireless Communications)
Also called NADC, a wireless standard specified as IS-54 that was adopted in 1992. It has since been updated, and it is now called IS-136. It uses TDMA and TDD schemes.
V
V.11
An ITU-T standard that describes electrical characteristics for balanced double-current interchange circuits for general use with integrated circuit equipment.
V.21
An ITU-T standard that describes 300bps modems for use in the PSTN.
V.24
ITU-T definitions for interchange circuits between data terminal equipment and data circuit-terminating equipment.
V.25
An ITU-T standard that describes automatic calling and/or answering equipment on the general switched network.
V.29
An ITU-T standard that describes 9,600bps modems for use in the PSTN.
V.35
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 the voice at the destination end by removing silent periods and redundant information found in human speech; this also helps with compression.
validation
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, or a subsidiary, or an independent company.
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.
VAN (value-added network) services
Telecommunication services provided over public or private networks which, in some way, add value to the basic carriage, usually through the applications of computerized intelligence, for instance, reservation systems, bulletin boards, information services.
VAR (value-added reseller)
A provider that deals with distribution and sales.
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 (nonreal-time) is for mission-critical data applications.
VC (virtual circuit)
A logical connection established through a packet network, over which packets are routed, mimicking to some extent the behavior of a dedicated physical connection.
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 meters) over a single twisted-pair cable. Over this distance, you can get up to a rate of 13Mbps downstream, and if you shorten the distance to 1,000 feet (300 meters), you can get up to 52Mbps downstream, which would be enough capacity to facilitate tomorrow's digital TVs. VDSL gives you 1.5Mbps to 2.3Mbps upstream.
VDT (video dial tone)
A U.S. term defining 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 (voice frequency)
Any frequency within the part of the audio frequency range that is essential for the transmission of speech of commercial quality; that is, 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. Operating band for radio and television channels.
video signal
A signal comprising frequencies normally required to transmit moving image information.
videoconferencing
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.
videotelephony
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 upon the level of data compression used, as well as the available bandwidth.
videotext
A public or private telecommunication service that offers interactive browsing of a menu of textual and graphical information. The most widely used public videotex 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 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.
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" video-on-demand, the film can be paused, rewound, or fast-forwarded. In "near" video-on-demand these functions are not possible.
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 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.
VP (virtual path)
A generic term for a collection of virtual channels that have the same endpoint.
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.
VPN (virtual private network)
A software-defined network offered by telephone carriers for voice and data communications among multiple sites. The VPN provides the appearance of a private line network, except that it makes use of the public switched network 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 point-of-presence (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.
VR (virtual reality)
A computer-based application that provides a human-computer interface such that the computer and its devices create a sensory environment called the virtual world. The sensory environment is dynamically controlled by actions of the individual so that the environment appears real.
VRC (vertical redundancy check)
An error-checking method that uses a parity bit for each character.
VSAT (very-small-aperture terminal)
An earth station with a small antenna, usually 0.6 meters or less.
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.
W
WAN (wide area network)
A group of computer networks connected over long distances, often by telephone lines and satellite transmission.
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.
WARC (World Administrative Radio Conference)
A regular meeting of global authorities to decide on spectrum allocation.
wave form
A curve that shows the shape of a wave at any given time.
waveguide
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.
wavelength
The distance between two consecutive maxima or minima of the wave form.
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.
WCDMA (Wideband CDMA)
A multiple-access technique that operates over allocations of either 5MHz, 10MHz, or 15MHz. WCDMA 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.
wearables
Smart devices that are small enough to be worn on the human body and are able to be networked.
wideband
Refers to the range of speeds between narrowband and broadband, typically ranging between nx64Kbps up to 45Mbps.
wideband channel
A channel that is wider in bandwidth than a voice-grade channel.
wireless
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 switched telephone network.
wiring closet
A location in a building where building wiring terminates and where equipment (e.g., hub electronics) is placed.
WML (Wireless Markup Language )
A markup language that is used in the WAP environment.
workstation
A device that lets you transmit information to or receive information from a computer, or both, as needed to perform a job; for example, a display station or a printer.
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 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. There is also a spare code (0), which is available for future use.
WWW (World Wide Web)
An Internet application that uses hypertext links between remote network servers for accessing and displaying multimedia information.
X
X.3
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.
X.21
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.
X.25
A standard that defines the interface between DTE and DCE for equipment operating in the packet mode on public data networks. It also defines a link control protocol.
X.75
A standard for connecting X.25 networks, developed by the ITU-T
X.121
The ITU-T standard for the addressing plan used with X.25 PDNs.
X.400
A standard for electronic mail exchange; developed by the ITU-T.
X.500
The family of protocols that define the operation of the ITU-T/ISO directory service.
X.509
The ITU-T/OSI recommendation for a directory authentication framework.
EAN: 2147483647
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