3DES (Triple Data Encryption Standard)

A stronger form of the Data Encryption Standard (DES), 3DES follows a pattern of encryption/decryption/encryption. 3DES has many different variations.

AAL1 (ATM Adaptation Layer 1)

One of four AALs recommended by the ITU-T. AAL1 is used for connection-oriented, delay-sensitive services requiring constant bit rates, such as uncompressed video and other isochronous traffic.

ABR (Available Bit Rate)

A QoS class defined by the ATM Forum for ATM networks. ABR is used for connections that do not require timing relationships between source and destination. ABR provides no guarantees in terms of cell loss or delay, providing only best-effort service. Traffic sources adjust their transmission rate in response to information they receive describing the status of the network and its capability to successfully deliver data.

ad-hoc conference

A conference call feature where a conference is started by an initiator and only the initiator of the conference can add people into the conference.

ADPCM (Adaptive Differential Pulse Code Modulation)

A waveform process by which analog voice samples are encoded into digital signals.

AF (Assured Forwarding)

A means of providing different levels of forwarding assurances for IP packets. This method is used by providers who offer differentiated services to their customers.

AIM (Advanced Integration Module)

A module in some Cisco routers that provides enhanced processing capabilities to the routers.

AMI (Alternate Mark Inversion)

A line-code modulation type used on T1 and E1 circuits. In AMI, marks (or 1s) cause a pulse in alternating positive and negative directions, while 0s never pulse. Two pulses of the same polarity are not allowed. AMI requires that the sending device maintain ones density. Ones density is not maintained independently of the data stream. Sometimes called binary coded alternate mark inversion.

ANI (Automatic Number Identification)

An SS7 feature in which a series of digits, either analog or digital, are included in the call, identifying the telephone number of the calling device. In other words, ANI identifies the number of the calling party. See also CLID.

ANSI (American National Standards Institute)

A voluntary organization composed of corporate, government, and other members that coordinates standards-related activities, approves U.S. national standards, and develops positions for the United States in international standards organizations. ANSI helps develop international and U.S. standards relating to, among other things, communications and networking. ANSI is a member of the International Electrotechnical Commission (IEC) and the International Organization for Standardization (ISO).

APC (Adaptive Predictive Coding)

A narrowband analog-to-digital conversion technique employing a one-level or multilevel sampling system in which the value of the signal at each sample time is adaptively predicted to be a linear function of the past values of the quantized signals. APC is related to LPC in that both use adaptive predictors. However, APC uses fewer prediction coefficients, thus requiring a higher bit rate than LPC.

API (Application Programming Interface)

The means by which an application program talks to communications software. Standardized APIs allow application programs to be developed independently of the underlying method of communication. An API is a set of standard software interrupts, calls, and data formats that computer application programs use to initiate contact with other devices (for example, network services, mainframe communications programs, or other program-to-program communications devices). Typically, APIs make it easier for software developers to create the links that an application needs to communicate with the operating system or with the network.

AR (Access Rate)

A Frame Relay term that addresses the maximum transmission rate supported by the access link into the network and the port speed of the device (switch or router) at the edge of the carrier network. The AR defines the maximum rate for data transmission or receipt. See also CIR.

ARPA (Advanced Research Projects Agency)

A research and development organization that is part of the Department of Defense (DoD). ARPA is responsible for numerous technological advances in communications and networking. It evolved into the Defense Advanced Research Projects Agency (DARPA), and then back into ARPA again (in 1994).

ARQ (Admission Request)

An RAS (Registration, Admission, and Status) admission message defined as an attempt by an endpoint to initiate a call.


A series of Cisco gateways that provide reliable, scalable, and feature-rich data and voice gateway functionality. The Cisco AS5300 Series Universal Gateways include the Cisco AS5300 Access Server/Voice Gateway and the Cisco AS5350 Universal Gateway.

ATM (Asynchronous Transfer Mode)

The international standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media, such as E3, SONET, and T3.

B8ZS (Binary 8-Zero Substitution)

A line-code modulation type used on T1 circuits. In B8ZS, marks (that is, binary ones) cause a pulse in alternating positive and negative directions, while zeros never pulse. Two pulses of the same polarity are not allowed, except when inserting a code to represent eight zeros. B8ZS maintains ones density by inserting a special code in place of eight consecutive zeros. The special code contains intentional violations of the bipolar pattern.

BC (Basic Call)

A call between two users that does not require Advanced Intelligent Network Release 1 features (for example, a POTS call).

Bc (Committed Burst)

A negotiated tariff metric in Frame Relay internetworks. The maximum amount of data (in bits) that a Frame Relay internetwork is committed to transmit in a timing interval. See also Be and CIR.

Be (Excess Burst)

A negotiated tariff metric in Frame Relay internetworks. The number of bits that a Frame Relay internetwork attempts to transmit after Bc is accommodated. Be data, in general, is delivered with a lower probability than Bc data because Be data can be marked as discard eligible (DE) by the network. See also Bc.

BHCA (Busy Hour Call Attempts)

A traffic engineering term that refers to the number of call attempts made during the busiest hour of the day.

BLF (Busy Lampfield)

A visual display of the status of all or some of your phones. Your BLF tells you if a phone is busy or on hold. A BLF is typically attached to or part of an operator phone.

BOC (Bell Operating Company)

BOC is a term for any of the 22 original companies (or their successors) that were created when AT&T was broken up in 1983 and given the right to provide local telephone service in a given geographic area. The companies previously existed as subsidiaries of AT&T and were called the "Bell System."

BRQ (Bandwidth Change Request)

A RAS bandwidth control message sent from endpoint to gatekeeper requesting an increase or decrease in call bandwidth.

BVM (BRI Voice Module)

An optional device for Cisco modular routers providing ISDN BRI ports for connection to ISDN PBXs or PINXs.

call agent

The central repository of call routing information in an MGCP-based centralized call processing environment.

CAS (Channel Associated Signaling)

The transmission of signaling information in association with the voice channel. In T1 networks, CAS signaling is often referred to as robbed-bit signaling because the network is robbing framing bits for signaling purposes.

CBR (Constant Bit Rate)

QoS class defined by the ATM Forum for ATM networks. CBR is used for connections that depend on precise clocking to ensure undistorted delivery.

CBWFQ (Class-Based Weighted Fair Queuing)

A congestion management mechanism that extends the standard WFQ functionality to provide support for user-defined traffic classes.

CCIS (Common Channel Interoffice Signaling)

A technology that uses a common link to carry signaling information for a number of trunks. CCIS is similar to the ITU-T SS6 protocol that operated at low bit rates (that is, 2.4, 4.8, and 9.6 kbps) and transmitted messages that were only 28 bits in length.

CCITT (Consultative Committee for International Telegraph and Telephone)

The former name for the international organization responsible for the development of communications standards. It is now called the ITU-T. See also ITU-T.

CCS (Common Channel Signaling)

A signaling system used in telephone networks that utilizes a statistical multiplexing protocol for signaling. A specified channel is exclusively designated to carry signaling information for all channels in the system. An example is ISDN or SS7. See also SS7.

CDVT (Cell Delay Variation Tolerance)

In ATM, a QoS parameter for managing traffic that is specified when a connection is set up. In CBR transmissions, CDVT determines the level of jitter that is tolerable for the data samples taken by the peak cell rate (PCR). See also CBR.

CELP (Code Excited Linear Prediction)

A compression algorithm used in low bit-rate voice encoding. CELP is used in ITU-T Recommendations G.728, G.729, G.723.1.

centum call seconds

Units used to measure traffic load. A centum call second is 1/36th of an Erlang and its formula is the number of calls per hour multiplied by their average duration in seconds, all divided by 100.

CES (Circuit Emulation Service)

A service that enables users to multiplex or to concentrate multiple circuit emulation streams for voice and video with packet data on a single high-speed ATM link without a separate ATM access multiplexer.

CID (Channel ID)

A value that designates the Frame Relay subchannel ID for Voice over Frame Relay.

CIR (Committed Information Rate)

The rate at which a Frame Relay network agrees to transfer information under normal conditions, averaged over a minimum increment of time. CIR, measured in bits per second, is one of the key negotiated tariff metrics. See also Bc.

Cisco AVVID (Cisco Architecture for Voice, Video and Integrated Data)

Cisco AVVID includes three components: infrastructure, such as switches and routers; clients, such as IP phones, H.323 videoconferencing equipment, and PCs; and applications, such as call control, that use a common IP network.

Cisco ICM software (Cisco Intelligent Call Management software)

Software that delivers an integrated suite of contact center capabilities. Cisco ICM software provides intelligent queue management in a contact center environment. It enables improved queue management across a variety of ACD (automatic call Distribution) systems from different vendors and integrates IVR (interactive voice response) systems, database and desktop applications, and CTI (computer telephony integration) solutions.

Cisco IOS software

Cisco software that provides common functionality, scalability, and security for many Cisco platforms. Cisco IOS software allows centralized, integrated, and automated installation and management of internetworks while ensuring support for a wide variety of protocols, media, services, and platforms.

Cisco IP phone

The Cisco family of IP phones provides a complete range of intelligent communication systems that use the data network while providing the convenience and ease of use of a business telephone.

Cisco IP SoftPhone

A Windows-based application for the PC. Used as a stand-alone end station or in conjunction with a Cisco IP phone, Cisco IP SoftPhone provides mobility, directory integration, user interface, and a virtual conference room.

Cisco IPCC (Cisco IP Contact Center)

An integrated suite of products that enables contact center agents using Cisco IP phones to receive both TDM and VoIP calls. IPCC provides ACD (automatic call distribution) and IVR (interactive voice response) capabilities in a single-vendor IP suite. The IPCC can be implemented in a single-site environment or integrated into an enterprise-wide multisite contact center.

Cisco Unified CallManager

A software-based call-processing agent. It is a component of the Cisco IP telephony solution, part of Cisco AVVID. The software extends enterprise telephony features and functions to IP telephony network devices such as IP phones, media processing devices, VoIP gateways, and multimedia applications.

CLEC (Competitive Local Exchange Carrier)

A company that builds and operates communication networks in metropolitan areas and provides its customers with an alternative to the local telephone company.

CLI (Command-Line Interface)

An interface that allows the user to interact with the operating system by entering commands and optional arguments.

CLID (Calling Line ID)

Information about the billing telephone number from which a call originated. The CLID value might be the entire telephone number, the area code, or the area code plus the local exchange. CLID is also known as Caller ID.

CNG (Comfort Noise Generation)

While using VAD, the emulation of background noise from the source side by the DSP at the destination to prevent the perception that a call is disconnected.

CO (Central Office)

The local telephone company office to which all local loops in a given area connect and in which circuit switching of subscriber lines occurs.

Committed Burst

See Bc.

CPE (Customer Premises Equipment)

Terminating equipment, such as terminals, telephones, and modems, installed at customer sites and connected to the telephone company's network.

CRC (Cyclic Redundancy Check)

An error-checking technique in which the frame recipient calculates a remainder by dividing frame contents by a prime binary divisor and compares the calculated remainder to a value stored in the frame by the sending node.


A connection scheme between cabling runs, subsystems, and equipment, using patch cords or jumpers that attach to connecting hardware on each end. Cross-connection is the attachment of one wire to another, usually by anchoring each wire to a connecting block and then placing a third wire between them so that an electrical connection is made.

cRTP (Compressed Real-Time Transport Protocol)

A type of header compression designed to reduce the IP/UDP/RTP headers to two bytes for most packets in the case where no UDP checksums are being sent, or four bytes with UDP checksums.

CS-ACELP (Conjugate Structure Algebraic Code Excited Linear Prediction)

A CELP voice compression algorithm providing 8 kbps, or 8:1 compression, standardized in ITU-T Recommendations G.729 and G.729A.

CTI (Computer Telephony Integration)

The name given to the merger of traditional telecommunications (PBX) equipment with computers and computer applications. The use of caller ID to retrieve customer information automatically from a database is an example of a CTI application.

DACS (Digital Access and Crossconnect System)

A digital cross-connect system that provides switching and aggregation.

dB (decibel)

A unit for measuring relative power ratios in terms of gain or loss. The rule of thumb to remember is that 10 dB indicates an increase (or a loss) by a factor of 10; 20 dB indicates an increase (or a loss) by a factor of 100; 30 dB indicates an increase (or a loss) by a factor of 1000.

DCD (Data Carrier Detect)

A signal from the DCE (for example, a modem or printer) to the DTE (typically a PC), indicating that the modem is receiving a carrier signal from the DCE (modem) at the other end of the telephone circuit.

DCE (Data Communications Equipment - EIA expansion) (data circuit-terminating equipment - ITU-T expansion)

Devices and connections of a communications network that comprise the network end of the user-to-network interface. The DCE provides a physical connection to the network, forwards traffic, and typically provides a clocking signal used to synchronize data transmission between DCE and DTE devices. Modems and interface cards are examples of DCE.

DDS (Digital Data Service)

A class of service offered by telecommunications companies to transport data rather than voice. DDS was originally called Dataphone Digital Service by AT&T in the late 1970s.

DE bits (Discard Eligible bits)

Bits that are used to tag Frame Relay frames eligible to be discarded if the network becomes congested.

delay budget

The maximum amount of delay in data, voice, and video applications. The total end-to-end delay when engineering a VoIP implementation should not exceed the 150 ms to 200 ms delay budget.

delay dial

A signaling method in which the terminating side remains off hook until it is ready to receive address information. The off-hook interval is the delay dial signal.

DHCP (Dynamic Host Configuration Protocol)

A protocol that provides a mechanism for allocating IP addresses dynamically so that they can be reused when hosts no longer need them.

dial plan mapper

Technology that provides the mapping of IP addresses to telephone numbers. After enough digits are accumulated to match a configured destination pattern, the dial plan mapper maps the IP host to a telephone number.


Modem access to a data network. The use of a dial or push-button telephone to create a telephone or data call. Dial-up calls are usually billed by time of day, duration of call, and distance traveled. Dial-up is a connection to the Internet, or any network, where a modem and a standard telephone are used to make a connection between computers.

dial-up remote access server

Computer hardware that resides on a corporate LAN and into which employees dial on the PSTN to get access to their e-mail, software, and data on the corporate LAN (for example, status on customer orders). Remote access servers are also used by commercial service providers, such as ISPs, to allow their customers access into their networks. Remote access servers are typically measured by how many simultaneous dial-in users (on analog or digital lines) they can handle and whether they can work with cheaper digital circuits, such as T1 and E1 connections.

Digital T1/E1 Packet Voice Trunk Network Module

A flexible and scalable T1/E1 voice solution for Cisco 2600 and 3600 Series Modular Access routers that supports up to 60 voice channels in a single network module.

Digital T1/E1 Voice Port Adapter

A single-width port adapter that incorporates one or two universal ports configurable for either T1 or E1 connection with high-performance DSP support for up to 24 to 120 channels of compressed voice.

DLCI (Data-Link Connection Identifier)

A value that specifies a PVC (permanent virtual circuit) or an SVC (switched virtual circuit) in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant. (Connected devices might use different values to specify the same connection at different ends of the network.)

DNIS (Dialed Number Identification Service)

A feature of trunk lines where the called number is identified. This called number information is used to route the call to the appropriate service. DNIS is a service used with toll-free dedicated services whereby calls placed to specific toll-free numbers are routed to the appropriate area within the company.

DP (Dial Pulse)

A means of signaling that consists of regular momentary interruptions of a current at the sending end in which the number of interruptions corresponds to the value of the digit or characterin short, the old style of rotary dialing. For example, dial the number 5, and you will hear five clicks.

DPNSS (Digital Private Network Signaling System)

A common-channel, message-oriented signaling protocol commonly used by PBXs.

drop and insert

A function that allows DS-0 channels from one T1 or E1 facility to be cross-connected digitally to DS-0 channels on another T1 or E1. By using this method, channel traffic is sent between a PBX and a CO PSTN switch or other telephony device so that some PBX channels are directed for long-distance service through the PSTN while the router compresses others for interoffice VoIP calls. In addition, drop and insert can cross-connect a telephony switch (from the CO or PSTN) to a channel bank for external analog connectivity. Also called TDM cross-connect. See also DACS.

DRQ (Disengage Request)

An RAS message sent by the gateway to the gatekeeper during the process of a call. The gateway waits for the DCF message before it sends the setup message to the new destination gatekeeper.

DS0 (Digital Service level zero)

A single timeslot on a DS1 (also known as T1) digital-interface (that is, 64 kbps), synchronous, full-duplex data channel, typically used for a single voice connection on a PBX. It can also be a single timeslot on an E1.

DSI (Digital Speech Interpolation)

An algorithm that analyzes voice channels for silence. It suppresses the voice bits to conserve packet-line bandwidth and inserts a code to indicate to the far end that these bits have been removed. Also referred to as VAD (voice activity detection).

DSL (Digital Subscriber Line)

A public network technology that delivers high bandwidth over conventional copper wiring at limited distances. Because most DSL technologies do not use the whole bandwidth of the twisted pair, there is room remaining for a voice channel.

DSP (Digital Signal Processor)

An electronic circuit that compresses voice signals, generates tones, and decodes received compressions. DSPs can also emulate modems for purposes of fax relay.

DTE (Data Terminal Equipment)

A device at the user end of a user-network interface that serves as a data source, a destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by the DCE. DTE includes such devices as computers, protocol translators, and multiplexers.

DTMF (Dual-Tone Multifrequency)

Tones generated when a button is pressed on a telephone to convey address signaling.

DTR (Data Terminal Ready)

An EIA/TIA-232 circuit that is activated to let the DCE know when the DTE is powered up and not in test mode.


A wide-area digital transmission scheme used throughout the world that carries data at a rate of 2.048 Mbps. E1 lines can be leased for private use from common carriers.

E&M (Ear and Mouth, Earth and Magneto, recEive and transMit)

A trunking arrangement generally used for two-way switch-to-switch or switch-to-network connections. Cisco's analog E&M interface is an 8-pin modular connector that allows connections to PBX trunk lines (tie-lines). E&M also is emulated on E1 and T1 digital interfaces.

ECMA (European Computer Manufacturers Association)

A group of European computer vendors that have done substantial OSI standardization work.


The wiring arrangement on an E&M circuit in which the signal side sends its signaling information.

ESF (Extended Superframe)

A framing type used on T1 circuits that consists of 24 frames, 193 bits each, with the 193rd bit providing framing information. ESF is an enhanced version of SF. See also SF.

ETSI (European Telecommunication Standards Institute)

A nonprofit organization that produces voluntary telecommunications standards used throughout Europe.

excess burst

See Be.

FDM (Frequency-Division Multiplexing)

A technique whereby information from multiple channels can be allocated bandwidth on a single wire based on frequency. An example is DSL.

FIFO (First-In/First-Out)

A buffering scheme where the first byte of data entering the buffer is the first byte retrieved by the CPU. In telephony, FIFO refers to a queuing scheme where the first calls received are the first calls processed.

flash memory

A special type of EEPROM (electrically erasable programmable read-only memory) that can be erased and reprogrammed in blocks instead of one byte at a time. Many modern PCs have their BIOS (basic input/output system) stored on a flash memory chip so that it can be updated easily if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in modems because it enables the modem manufacturer to support new protocols as they become standardized.


One of two distinct types of audio interfaces (two-wire and four-wire). The four-wire implementation provides separate paths for receiving and sending audio signals, consisting of T, R, T1, and R1 leads.

frame forwarding

A mechanism by which frame-based traffic, such as HDLC and SDLC, traverses an ATM network.

FRTS (Frame Relay Traffic Shaping)

A queuing method that uses queues on a Frame Relay network to limit surges that can cause congestion. Data is buffered and sent into the network in regulated amounts to ensure that the traffic can fit within the promised traffic envelope for the particular connection.

FXO (Foreign Exchange Office)

An interface that connects to the PSTN central office. Cisco's FXO interface is an RJ-11 connector that allows an analog connection at the PSTN's central office or to a station interface on a PBX.

FXS (Foreign Exchange Station)

An FXS interface connects directly to a standard telephone and supplies ring, voltage, and dial tone. Cisco's FXS interface is an RJ-11 connector that allows connections to basic telephone service equipment, key sets, and PBXs.


In telecommunications, an H.323 entity on a LAN that provides address translation and control access to the LAN for H.323 terminals and gateways. The gatekeeper can provide other services to the H.323 terminals and gateways, such as bandwidth management and locating gateways. It maintains a registry of devices in the multimedia network. The devices register with the gatekeeper at startup and request admission for a call from the gatekeeper.


An H.323 term that describes the component of a H.323 telephony network that translates between one technology and another, typically between a traditional telephony network and an IP network.

generic traffic shaping

A way of shaping traffic by reducing outbound traffic flow to avoid congestion by constraining traffic to a particular bit rate using the token bucket mechanism.

GRQ (Gatekeeper Discovery Request)

An RAS gatekeeper discovery message sent from endpoint to gatekeeper.

HDB3 (High-Density Binary 3)

A line coding method used to maintain synchronization by ensuring a sufficient number of binary ones. HDB3 is used on E1 circuits.

HDLC (High-Level Data Link Control)

A bit-oriented synchronous data-link-layer protocol developed by the International Organization for Standardization (ISO). See also SDLC.

hoot and holler

A broadcast audio network used extensively by the brokerage industry for market updates and trading. Similar networks are used in publishing, transportation, power plants, and manufacturing.

HSRP (Hot Standby Router Protocol)

A protocol that provides high network availability and transparent network topology changes. HSRP creates a hot standby router group with a lead router that services all packets sent to the hot standby address. Other routers in the group monitor the active router, and if it fails, one of the standby routers inherits the lead position and the hot standby group address.

HTTP (Hypertext Transfer Protocol)

The protocol used by web browsers and web servers to transfer files, such as text and graphic files.

HyperTerminal software

Terminal emulation software.

IETF (Internet Engineering Task Force)

A task force consisting of over 80 working groups responsible for developing Internet standards.

ILEC (Incumbent Local Exchange Carrier)

A telephone company in the United States that was providing local service when the Telecommunications Act of 1996 was enacted. ILECs include the former Bell operating companies (BOCs), which were grouped into holding companies known collectively as the regional Bell operating companies (RBOCs) when the Bell System was broken up by a 1983 consent decree.

IMAP (Internet Message Access Protocol)

A method of accessing e-mail or bulletin board messages kept on a mail server that can be shared. IMAP permits client e-mail applications to access remote message stores as if they were local without actually transferring the message.

IMT (Inter-Machine Trunk)

A means for giving service providers access to more favorable tariffs and rates. In SS7 environments, IMTs terminate bearer traffic on the voice gateways.

IN (Intelligent Network)

A network that provides IP routing, QoS, network access and control, and network management services.

IP (Internet Protocol)

A network layer protocol in the TCP/IP stack offering a connectionless internetwork service. IP provides features for addressing, type-of-service specification, fragmentation and reassembly, and security. Defined in RFC 791.

IP cloud

The area in which data travels through an IP network. It is illustrated in diagrams as a cloud.

IP precedence

A 3-bit value in an IP version 4 type of service (ToS) byte used to assign priority to IP packets.

ISDN (Integrated Services Digital Network)

Communication architecture offered by telephone companies that permits customers to access digital networks to carry data, voice, and other source traffic.

ISUP (ISDN User Part)

An SS7 protocol layer that defines the protocol used to prepare, manage, and release trunks that carry voice and data between calling and called parties under the auspice of ISDN.

ITU (International Telecommunication Union)

An organization established by the United Nations to set international telecommunications standards and to allocate frequencies for specific uses.

ITU-T (International Telecommunication Union Telecommunication Standardization Sector)

An international body that develops worldwide standards for telecommunications technologies. The ITU-T carries out the functions of the former CCITT. See also CCITT.

IVR (Interactive Voice Response)

A term used to describe systems that provide information in the form of recorded messages over telephone lines in response to user input in the form of spoken words, or, more commonly, DTMF signaling. Examples include banks that allow you to check your balance from any telephone and automated stock quote systems.

IXC (Inter-Exchange Carrier)

A common carrier providing long-distance connectivity between local access and transport areas (LATAs). The three major IXCs are AT&T, MCI, and Sprint, but several hundred IXCs offer long-distance service in the United States.

JTAPI (Java Telephony Application Programming Interface)

A Java API for call control developed by Sun Microsystems.

LDAP (Lightweight Directory Access Protocol)

A protocol that provides read/write interactive access to X.500 directories for uniform application security and access levels.

LDCELP (Low-celay CELP)

A CELP voice compression algorithm requiring 16 kbps of bandwidth (that is, 4:1 compression). Standardized in ITU-T Recommendation G.728.

LEC (Local Exchange Carrier)

A public telephone company in the United States that provides local service. Some of the largest LECs are the Bell operating companies (BOCs), which were grouped into holding companies known collectively as the regional Bell operating companies (RBOCs) when the Bell System was broken up by a 1983 consent decree. In addition to the Bell companies, there are a number of independent LECs, such as Alltel. LEC companies are also sometimes referred to as telcos. A local exchange is the local "central office" of a LEC. Lines from homes and businesses terminate at a local exchange. Local exchanges connect to other local exchanges within a local access and transport area (LATA) or to inter-exchange carriers (IXCs) such as long-distance carriers AT&T, MCI, and Sprint.

LFI (Link Fragmentation and Interleaving)

A Cisco IOS feature that reduces delay on slower-speed links by breaking up large datagrams and interleaving delay-sensitive traffic packets, with the smaller packets resulting from the fragmented datagram.

line code

An electrical modulation scheme used by digital carrier systems. In North America, T1 uses AMI or B8ZS line coding. In other countries, E1 uses AMI or HDB3 line coding.

LLQ (Low Latency Queuing)

A queuing scheme that enables use of a single priority queue in conjunction with CBWFQ. The priority queue typically carries VoIP traffic, while other traffic is carried in the user-defined queues of CBWFQ.

LPC (Linear Predictive Coding)

Voice coding that uses a special algorithm that models the way human speech works. Because LPC can take advantage of an understanding of the speech process, it can be efficient without sacrificing voice quality.

LRQ (Location Request)

An RAS location request message sent from one gatekeeper to another gatekeeper to request contact information for one or more E.164 addresses.

LSB (Least Significant Bit)

The bit of a binary expression having the least value.

MC (Multipoint Controller)

A required part of an MCU. The MC is the conference controller. It handles negotiation between all terminals to determine common capabilities and controls conference resources such as multicasting. The MC does not deal directly with any of the media streams.

MCS (Media Convergence Server)

An integral component of the Cisco IP Communications system. A high availability server platform for Cisco AVVID.

MCU (Multipoint Control Unit)

A component that manages videoconferences of three or more participants.

MDF (Main Distribution Frame)

The point where all network-related external services, IP equipment, and wiring converge within a building.

meet-me conference

A conference feature where everyone who dials the same meet-me number will join the conference.

MEL CAS (Mercury Exchange Limited Channel Associated Signaling)

A voice signaling protocol used primarily in the United Kingdom.

MGCP (Media Gateway Control Protocol)

A protocol that helps bridge the gap between circuit-switched and IP networks. A combination of IPDC (Internet Protocol Device Control) and SGCP (Simple Gateway Control Protocol), MGCP allows external control and management of data communications devices, or "media gateways" at the edge of multiservice IP networks.

MICA (Modem ISDN Channel Aggregation)

A modem module and card used in the Cisco AS5300 universal access servers. A MICA modem provides an interface between an incoming or outgoing digital call and an ISDN telephone line. The call does not have to be converted to analog as it does with a conventional modem and an analog telephone line. Each line can accommodate, or aggregate, up to 24 (T1) or 30 (E1) calls.

Microsoft NetMeeting

A complete H.323 desktop Internet multimedia solution for all Windows users with multipoint data conferencing, text chat, whiteboard, and file transfer, as well as point-to-point audio and video.


The wiring arrangement on an E&M circuit in which the trunking side sends its signaling information.

MLP (Multilink Point-to-Point Protocol)

A method of splitting, recombining, and sequencing datagrams across one or more data links under the PPP protocol.

MOS (Mean Opinion Score)

A common benchmark used to determine the perceived quality of sound produced by specific CODECs.

MP (Multipoint Processor)

The part of an MCU that processes the media streams. It receives audio, video, or data bits from the endpoints for which it does the required mixing, switching, and other processing before distributing the stream to the videoconference participants.

MTP (Media Termination Point)

A device that allows the Cisco Unified CallManager or hardware containing digital signal processors to extend supplementary services, such as hold and transfer, to calls routed through an H.323 endpoint or an H.323 gateway.

Multicast Backbone (MBONE)

The multicast backbone of the Internet. The MBONE is a virtual multicast network composed of multicast LANs and the point-to-point tunnels that interconnect them.

NSAP (Network Service Access Point)

A network address, as specified by ISO. An NSAP is the point at which OSI network service is made available to a transport layer (Layer 4) entity.

ODBC (Open DataBase Connectivity)

A specification that abstracts data using applications from database management systems. It is the standard API for accessing data in both relational and nonrelational database management systems. Using this API, common database applications can be written to access data stored in a variety of database management systems on a variety of computers regardless of the DBMS or programming interface.

off hook

A call condition, also known as busy, in which transmission facilities are already in use.

OMAP (Operations, Maintenance, Administration, and Provisioning)

A set of services that provides telephony operation functions including monitoring and discovery of problems before they negatively impact service. The telephony maintenance function is similar to the data networking processes of fault isolation and correction. Administration deals with billing, department cross-charges, accounting, and capacity management. The final element, provisioning, is used to define services for individual subscribers.

on hook

A condition that exists when a receiver or a handset is resting on the switch hook or is not in use.

OOS (Out-Of-Service)

A state of the call or trunk.

OPX (Off-Premises eXtension)

A telephone line from a telephone system that is terminated in a different building than the one in which the telephone system resides.

OSI (Open System Interconnection)

An international standardization program created by ISO and ITU-T to develop standards for data networking that facilitate multivendor equipment interoperability.

PAM (Pulse Amplitude Modulation)

A modulation scheme where samples of a waveform are represented as amplitudes of a higher-frequency waveform, known as the carrier frequency.

PBX (Private Branch eXchange)

Digital or analog telephone switches located on the customer premises and used to connect private and public telephone networks.

PCI (Protocol Control Information)

Control information added to user data to construct an OSI packet. It is the OSI equivalent of the term header.

PCM (Pulse Code Modulation)

The technique of encoding analog voice into a 64 kbps data stream by sampling with 8-bit resolution at a rate of 8000 samples per second.

PCMCIA (Personal Computer Memory Card Industry Association)

A standard interface that connects a device to a portable computer.

PINX (Private Integrated services Network eXchange)

A PBX or key system, in which a BRI voice application uses QSIG (Q Signaling).

PLAR (Private Line, Automatic Ringdown)

A voice circuit that connects two single endpoints together. When a telephone handset is taken off hook, the remote telephone automatically rings.

PLAR-OPX (Plar Off-Premises eXtension)

A PLAR Off-Premises eXtension connection. Using this option, the local voice port provides a local response before the remote voice port receives an answer. On FXO interfaces, the voice port will not answer until the remote side answers.

PLL (Phase-Lock Loop)

A circuit on a T1 or E1 module that provides clocking information.

POP (Point Of Presence)

In an OSS (Operations Support System), a physical location where an inter-exchange carrier installed equipment to interconnect with a LEC.

POTS (Plain Old Telephone Service)

Basic telephone service supplying standard single-line telephones, telephone lines, and access to the PSTN. See also PSTN.

PQ (Priority Queuing)

A queuing scheme that ensures "important" traffic gets the fastest handling at each point where it is used. Priority queuing was designed to give strict priority to important traffic. PQ is a legacy queuing method, which is typically replaced by LLQ in modern configurations.

PSQM (Perceptual Speech Quality Measurement)

A technique used for measuring voice quality. It compares the received audio with the transmitted audio.

PSTN (Public Switched Telephone Network)

A general term referring to the variety of telephone networks and services in place worldwide.

PTT (Post, Telephone, and Telegraph)

A government agency that provides telephone services. PTTs exist in most areas outside North America and provide both local and long-distance telephone services.

PVC (Permanent Virtual Circuit)

A virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and teardown in situations where certain virtual circuits must exist all the time. In ATM terminology, a PVC is called a permanent virtual connection.

PVDM (Packet Voice Digital signal processor Module)

A product that provides the ability to increase the voice processing capabilities within a single network module.

QoS (Quality of Service)

A set of tools used in networking devices to ensure best-of-class transmission quality and service availability.

QSIG (Q Signaling)

An inter-PBX signaling protocol for networking PBX supplementary services in a multivendor or single-vendor environment.

RAS (Registration, Admission, and Status)

A protocol used between endpoints and the gatekeeper to perform management functions. The RAS signaling function performs registration, admissions, bandwidth changes, status, and disengage procedures between the VoIP gateway and the gatekeeper.

RBOCs (Regional Bell Operating Companies)

Seven regional telephone companies formed by the breakup of AT&T. RBOCs differ from regional Bell holding companies (RBHCs) in that RBOCs do not cross boundaries that were set out by the consent decree.

RBS (Robbed-Bit Signaling)

A technique by which a single bit in every DS0 bearer channel is "stolen" from every sixth frame. The stolen bit is then used to carry signaling information.

redirect server

A server that accepts a SIP request, maps the address into zero or more new addresses, and returns these addresses to the client. A redirect server does not initiate its own SIP request nor does it accept calls.

RFC (Request For Comments)

A document series generated by the IETF and used as the primary means for communicating information about the Internet. Some RFCs are designated by the Internet Architecture Board (IAB) as Internet standards. Most RFCs document protocol specifications, such as Telnet and FTP, but some are humorous or historical. RFCs are available online from numerous sources.

RISC (Reduced Instruction Set Computing)

A microprocessor design that provides fewer and simpler instructions burned into the silicon than other processors. Fewer instructions let a processor perform at a higher speed. The difference is made up by requiring more work to be done by compilers and greater memory usage.


An algorithm used to schedule processes in a fixed cyclic order. Simply put, it means to "take turns."

RQNT (Request Notification)

An RAS message that instructs a gateway to watch for specific events.

RRQ (Registration Request)

An RAS message sent as a registration request.

RSVP (Resource Reservation Protocol)

A protocol that supports the reservation of resources across an IP network. Applications running on IP end systems can use RSVP to indicate to other nodes the nature (for example, bandwidth, jitter, and maximum burst) of the packet streams they want to receive. It is also known as the Resource Reservation Setup Protocol.

RTCP (RTP Control Protocol)

A protocol that monitors the QoS of an IP RTP connection and conveys information about the ongoing session.

RTP (Real-Time Transport Protocol)

A protocol commonly used with IP networks. RTP is designed to provide end-to-end network transport functions for applications transmitting real-time data (for example, audio, video, or simulation data over multicast or unicast network services). RTP provides such services as payload type identification, sequence numbering, time stamping, and delivery monitoring to real-time applications.

RTSP (Real Time Streaming Protocol)

A protocol that enables the controlled delivery of real-time data, such as audio and video. Sources of data can include both live data feeds (such as live audio and video) and stored content (such as prerecorded events). RTSP is designed to work with established protocols, such as RTP and HTTP.

SAP (Session Announcement Protocol)

A protocol used to assist in the advertisement of multicast multimedia conferences and other multicast sessions, and to communicate relevant session setup information to prospective participants.

SCCP (Skinny Client Control Protocol)

The Cisco standard for real-time calls and conferencing over IP.

SCP (Service Control Point)

An element of an SS7-based intelligent network that performs various service functions, such as number translation and call setup and teardown.

SDLC (Synchronous Data Link Control)

An IBM Systems Network Architecture (SNA) data-link-layer communications protocol. SDLC is a bit-oriented, full-duplex serial protocol that has spawned numerous similar protocols, including HDLC. See also HDLC.

SDP (Session Description Protocol)

A protocol used to describe multimedia sessions in order to enable session announcement, session invitation, and other forms of multimedia session initiation.

SF (Super Frame)

A framing type used on T1 circuits. SF consists of 12 frames of 193 bits each, with the 193rd bit providing frame synchronization. SF is superseded by ESF but is still widely used. It is also called D4 framing. See also ESF.

signal ground

The common electrical reference point of a circuit.

SIMM (Single In-line Memory Module)

A small circuit board that holds a number of memory chips.

SIP (Session Initiation Protocol)

A protocol developed by the IETF MMUSIC Working Group as an alternative to H.323. SIP features are compliant with IETF RFC 2543, published in March 1999. SIP equips platforms to signal the setup of voice and multimedia calls over IP networks.

SLA (Service-Level Agreement)

An agreement between the ISP and the client that guarantees a certain level of data transmission over the network.

SMDS (Switched Multimegabit Data Service)

A high-speed, packet-switched, datagram-based WAN networking technology offered by telephone companies.

SMTP (Simple Mail Transfer Protocol)

The standard Internet protocol providing e-mail services.

SNR (Signal-to-Noise Ratio)

A measure of transmission quality. The ratio of good or usable data (signal) to bad or undesired data (noise) on a line, expressed in decibels (dB).

SOHO (Small Office/Home Office)

Networking solutions and access technologies for offices that are not directly connected to large corporate networks.


A loop-free subset of a network topology.

SQL (Structured Query Language)

An international standard language for defining and accessing relational databases.

SRST (Survivable Remote Site Telephony)

A feature on some Cisco routers that uses the Skinny protocol (SCCP) to provide call-handling support for the local IP phones if the WAN connection to the Cisco Unified Communications CallManager fails.

SS7 (Signaling System 7)

A standard CCS system used with BISDN and ISDN. Developed by Bellcore. See also CCS.

SSP (Service Switching Point)

An element of an SS7-based intelligent network that performs call origination, termination, and tandem switching.

STP (Signal Transfer Point)

An element of an SS7-based intelligent network that performs routing of SS7 signaling.

STUN (Serial Tunnel)

A router feature allowing two SDLC-compliant or HDLC-compliant devices to connect to one another through an arbitrary multiprotocol topology (using Cisco routers) rather than through a direct serial link.

SVC (Switched Virtual Circuit)

A virtual circuit that is dynamically established on demand and is torn down when transmission is complete. SVCs are used in situations where data transmission is sporadic.


The standard digital multiplexed 24-channel voice/data digital span line. T1 is used predominantly in North America. It operates at a data rate of 1.544 Mbps. T1 is a digital WAN carrier facility. T1 transmits DS-1 formatted data through the telephone-switching network using AMI or B8ZS coding. See also AMI and B8ZS.

tabletop phone

A conference telephone used on a conference room table.

TAPI (Telephony Application Programming Interface)

A call control model developed by Microsoft and Intel.

TCAP (Transaction Capabilities Application Part)

An SS7 protocol layer that helps exchange noncircuit-related data between applications.

T-CCS (Transparent Common Channel Signaling)

A feature that allows the connection of two PBXs with digital interfaces that use a proprietary or unsupported CCS protocol without the need for interpretation of CCS signaling for call processing. T1/E1 traffic is transported transparently through the data network, and the feature preserves proprietary signaling. From the PBX standpoint, this is accomplished through a point-to-point connection. Calls from the PBXs are not routed but follow a preconfigured route to the destination.

TDM (Time-Division Multiplexing)

A technique in which information from multiple channels can be allocated bandwidth on a single wire based on preassigned timeslots. Bandwidth is allocated to each channel regardless of whether the station has data to transmit.


A field in certain FastPacket formats that indicates the amount of time the packet has spent waiting in queues during the transmission between its source and destination nodes. It is used to control the delay experienced by the packet.


One of two distinct types of audio interfaces (two-wire and four-wire). With the two-wire implementation, full-duplex audio signals are transmitted over a single pair, which consists of tip (T) and ring (R) leads.

U interface

The ISDN interface between the telco and the user, also known as the local loop.

UAC (User Agent Client)

A client application that initiates the SIP request.

UAS (User Agent Server)

A server application that contacts the user when a SIP request is received and then returns a response on behalf of the user. The response accepts, rejects, or redirects the request.

UDP (User Datagram Protocol)

A connectionless transport layer protocol in the TCP/IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. UDP is defined in RFC 768.

V card

An electronic business card. V cards carry information such as names, telephone numbers, mail addresses, e-mail addresses, and URLs.

VAD (Voice Activity Detection)

A feature used to statistically save bandwidth by not sending packets in the absence of speech. When enabled on a voice port or a dial peer, silence is not transmitted over the network, only audible speech. When VAD is enabled, the sound quality is slightly degraded but the connection uses much less bandwidth.

VBR (Variable Bit Rate)

A QoS class defined by the ATM Forum for ATM networks. VBR is subdivided into an RT class and an NRT class.

VBR-NRT (Variable Bit Rate-Nonreal Time)

A subclass of VBR used for connections in which there is no fixed timing relationship between samples but which still need a guaranteed QoS.

VBR-RT (Variable Bit Rate-Real Time)

A subclass of VBR. Used for connections in which there is a fixed timing relationship between samples.

VIC (Voice Interface Card)

A Cisco interface card used to connect the system to either the PSTN or to a PBX. See also PBX and PSTN.


A meeting between people in different locations, using audio and video. The simplest type of videoconference can involve transmission of static images between two locations. The most complex videoconferences can use full-motion video and high-quality audio between multiple locations.

VoATM (Voice over ATM)

A technology that enables a router to carry voice traffic (for example, telephone calls and faxes) over an ATM network. When sending voice traffic over ATM, the voice traffic is encapsulated using a special AAL5 encapsulation for multiplexed voice. VoATM dial peers point to local PVCs, as compared to remote IP addresses used by VoIP dial peers.

VoD (Video on Demand)

A system using video compression to supply video programs to viewers when requested via ISDN or cable.

VoFR (Voice over Frame Relay)

A technology that enables a router to carry voice traffic (for example, telephone calls and faxes) over a Frame Relay network. When sending voice traffic over Frame Relay, the voice traffic is segmented and encapsulated for transit across the Frame Relay network. VoFR dial peers point to local DLCIs, as compared to remote IP addresses used by VoIP dial peers.

VoIP (Voice over IP)

The capability to carry normal telephony-style voice over an IP-based internetwork with POTS-like functionality, reliability, and voice quality. VoIP enables a router to carry voice traffic (for example, telephone calls and faxes) over an IP network. In VoIP, DSPs segment the voice signal into frames, which are then coupled in groups of two and stored in voice packets. These voice packets are transported using a variety of signaling protocols.

VoIPovFR (VoIP over Frame Relay)

The capability to provide VoIP application interworking over an existing Frame Relay network. VoIPovFR can be used over point-to-point leased lines or over a Frame Relay circuit. It does not require a full-fledged Frame Relay network or service.

VPN (Virtual Private Network)

Technology that enables IP traffic to travel securely over a public TCP/IP network by encrypting all traffic from one network to another. A VPN can use a "tunneling" protocol to encrypt all information at the IP level.

WFQ (Weighted Fair Queuing)

A congestion management algorithm that identifies conversations (in the form of traffic streams), separates packets that belong to each conversation, and ensures that capacity is shared fairly between these individual conversations. WFQ is an automatic way of stabilizing network behavior during congestion and results in increased performance and reduced retransmission.

WIC (WAN Interface Card)

A Cisco interface card that connects a system to a WAN link service provider.


A method of E&M signaling. When the signaling leads indicate a change to an off-hook state, the other side must send a momentary wink (on-hook to off-hook to on-hook transition) on the correct signaling lead before the call signaling information can be sent by the sending side. After the call signaling information is received, the side that sent the wink goes off hook again when the subscriber answers and stays that way for the duration of the call.

Cisco Voice over IP Cvoice (c) Authorized Self-study Guide
Cisco Voice over IP (CVoice) (Authorized Self-Study Guide) (2nd Edition)
ISBN: 1587052628
EAN: 2147483647
Year: 2006
Pages: 111
Authors: Kevin Wallace

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