The following is a list of terms related to the VideoServer product family and the telecommunications industry. To help ensure accuracy, many of the definitions were obtained from The Information Age Dictionary. Definitions from this source are preceded with a symbol.

Symbols| A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | X



See mu-law (m-law).


accounting management

A reporting of costs for network resources requested by users and/or groups. Accounting management is one of five categories of network management defined by the ISO.


See Audio Conferencing module.

address register (I/O address)

A register used to store the label for a memory location of data being fetched or stored, a sequence of instructions to be executed, or the location to which control will be transferred.

address space

The number of storage locations in computer memory that can be located by the addressing technique used by the CPU. The addressing range is determined by the number of bits that can be held in the address register (2x combinations).


The owner of an MCS account that is authorized to perform administrative tasks, such as setting up accounts and groups and performing diagnostic tests.


See Audio Interface unit.


A companding method for encoding-decoding audio signals in 30channel pulse-code-modulated (PCM) systems. A-law is the primary method used in Europe. See also companding; -law (m-law).

alternate mark inversion (AMI)

The binary modulation code used by telephone companies for data and digital voice transmission. AMI uses RZ coding in an alternate bipolar scheme, with logical zeros corresponding to 0 V, and logical ones alternating between + 3 V and - 3 V. Self synchronization is possible with this approach, but the number of continuous zeros must be limited. See also bipolar with eight zero substitution (B8ZS).


See alternate mark inversion.


See Audio Processing unit.

asynchronous transmission

The transmission at irregular intervals of single characters, each preceded by a start bit and followed by a stop bit.

attachment unit interface (AUI)

The connector used with thick Ethernet. It connects the Ethernet module to an Ethernet hub.

Attended conference

A conference setup so that sites receive audio and video messages and assistance from an operator.

Audio Conferencing module (ACM)

An optional module that processes audio-only (G.711 A-law and m-law) signals and transmits that data to all conference participants. The ACM supports as many as 24 POTS analog calls being added to a conference without using BPU resources. On the hardware module, the label reads BPU.

Audio Interface unit (AIU)

An optional module that provides audio input to support music-on-hold for an attended conference.


See attachment unit interface.

Audio Processing unit (APU)

A module or board within an MCS that supplies the processing power required when using the ITU-T G.728 audio standard.


The attributes of an MCS account that enable its owner to perform varying levels of tasks.



See bipolar with eight zero substitution.


An indication of the information-carrying capacity of a communications channel.


See bit-rate allocation signal.

Basic Rate ISDN (BRI)

An optional module-called Quad BRI-that provides an ISDN access rate of 144 kbps subdivided into two B channels of 64 kbps each and one D channel of 16 kbps. See also Network Side BRI; User Side BRI

B channel

A channel in an ISDN network that provides a 64-kbps connection to a switching system or to the nonswitched portion of a network. This channel is used for transferring audio, video, and data information between customer premises equipment and an end-office switching system. See also D channel.

Bearer Service

In the ISDN, a service provided over the first three layers of the OSI standard. Examples include circuit-switched voice or voice-band data, unrestricted data at 64 kbps on the B channel, and 7-kHz audio services.

bipolar with eight zero substitution (B8ZS)

A line code in which eight consecutive zeros are replaced with a unique signature. The substitution ensures a sufficiently high density of ones on a transmission line. Replacement with the original string of zeros at a receiving end restores the signal. See also alternate mark inversion (AMI).


A binary digit that represents a signal, wave, or state as either a zero or a one. It is the minimum unit of information in a binary pulse stream. In transmission, it is a time slot that may or may not contain a pulse.

bit error

The receipt, by a receiver, of a code that differs from that sent by the transmitter.

bit rate

The speed at which digital signals are transmitted; that is, the number of bits per unit of time.

bit-rate allocation signal (BAS)

As specified in the ITU-T H.320 series of recommendations, bits in a frame that enable the transmission of code words to describing the capability of a terminal to structure the capacity of the channel or synchronized multiple channels in various ways and commanding a receiver to demultiplex and make use of the constituent signals in such structures. The BAS signal is also used for controls and indications.


See Bridge Processing unit. See also Audio Conferencing module.


See Bridge Processing unit. See also Audio Conferencing module.

BPU bus

A ribbon cable that connects BPUs. The 32-Mbps bus connects BPU Is; the 64-Mbps bus that connects BPU IIs. See also dual-network bus.


See Basic Rate ISDN.


See Multimedia Conference Server.

Bridge Processing unit (BPU)

The main board within an MCS that processes the audio, video, and data signals received from the conferencing systems. Each BPU supports four conferencing connections.

The BPU I is available in versions running at 33 MHz or 40 MHz. The 40MHz version is required to support conferences that use a transfer rate of 768kilobits per second or greater.

The BPU II running at 40 MHz supports a 1,920-kbps conference with eight conferencing systems. The BPU II running at 50 MHz supports G.728 conferences. APUs are not permitted in a BPU II configuration.


One-way transmission of information.


A communication pathway within communication and computing equipment.


CAP set

Data passed between an MCS and a conferencing system that identifies the capabilities of the equipment, such as audio coding and transfer rate capabilities.


A company engaged in carrying signals or messages for hire.


Connecting two or more MCSs together to increase the number of sites included in a multipoint conference.


See International Telecommunication Union Telecommunication Standardization Sector.

central office (CO)

An exchange carrier building housing a switching system and related equipment.

chair-control video switching

A video-switching method using the ITU-T Recommendation H.243 standard in which a participant at a conference site selects the current broadcaster from the controls provided by the conferencing system. The conference must be using voice-activated video switching, and the H.243 Chair Control option must be installed at the site. Contrast with user-selected video switching. See also director-selected video switching; voice-activated video switching.


1. The smallest subdivision of a circuit that provides a single type of communication service. 2. A transmission path between two or more termination points.

channel negotiation

In the ISDN Q.931 protocol, the process by which a stored-program control system enables the equipment at the customer premises to request a channel that is different from the one indicated in the setup message.

channel service unit (CSU)

A device that interfaces between a communication network and data terminal equipment. In T1 applications, referred to as a DSU (data service unit).


In a data stream, a number derived from the sequence of ones and zeros used to detect transmission errors.


See Common Intermediate Format.


A communication path through which a signal can flow between two or more points.


A reference source of timing information for communication and computing equipment and systems.


See central office.


An acronym for coder-decoder. A device that codes analog signals into digital signals for transmission and decodes digital signals into analog signals for receiving. See also videoconferencing.

Common Intermediate Format (CIF)

An ITU-T video format that governs the transmission of video signals over the ISDN. The video format specifies 288 lines of 360 pixels. Contrast with Quarter Common Intermediate Format.


The conveyance of information, such as audio, video, or data, through a transmission channel without altering the content.


The compression and expansion of a signal using nonlinear quantization. See also mu (m) law and A-law.

Continuous Presence

An optional type of conference where all sites view four other sites arranged in a quadrant-screen display. The MCS must use one CPM for each Continuous Presence conference.

Continuous Presence module (CPM)

The hardware module component of the Continuous Presence software option. See Continuous Presence. On the hardware module, the label reads VPU.


See customer premises equipment.


See Continuous Presence module.


See channel service unit.


A proprietary video protocol developed by Compression Labs, Inc. CTX supports data transfer rates less than 384 kbps.


A proprietary video protocol developed by Compression Labs, Inc. CTX+ supports data transfer rates greater than or equal to 384 kbps.

customer premises equipment (CPE)

The network interface hardware not provided by the carrier including, handsets, private branch exchanges (PBXs), and data terminals.



A type of conferencing in an H.320 environment where conferencing systems transmit ITU-T T.120 information-for example, a PC file-for interactive use by conference participants. The MCS must have a TCM installed.

data circuit-terminating equipment (DCE)

A term that refers to the network side of a communication link. Contrast with DTE.

data terminal equipment (DTE)

In a data communication network, the data source, such as a computer, and the data sink, such as an optical storage device.


See data circuit-terminating equipment.

D channel

An ISDN channel that provides a 16-kbps or 64-kbps packet-mode connection between a servicing switch and a customer's premises. The channel carries signal and control information for B-chamel activity and also can carry user data in the form of packets. See also B channel.

dedicated line

A nailed-up or nonswitched connection between two pieces of communication equipment. A dedicated line is also referred to as a direct connection.

dedicated network

See private network.


A device used to separate two or more signals that were previously combined by a compatible multiplexer.

DES encryption

A data encryption algorithm in the public domain. It is the Data Encryption Standard of the United States National Bureau of Standards.

dialed network

See switched network.

digital data

Data represented by discrete values or conditions. Contrast with analog data.

digital signal

A discrete or discontinuous signal, such as a sequence of voltage pulses.

digital transmission

The transmission of digital data or analog data that have been digitized, using either an analog or digital signal, in which the digital content is recovered and repeated at intermediate points to reduce the effects of impairments, such as noise, distortion, and attenuation.

digital hierarchy (DS0, DS1)

A system of standardized transmission rate for digital signals using time-division multiplexing techniques. The rates are:


Conversion of an analog signal to a digital signal.

direct connection

A nailed-up or nonswitched connection between two pieces of communication equipment. A direct connection is also referred to as a dedicated line.


The person who controls the video switching and other activities during a multipoint conference.

director-selected video switching

A video-switching method in which the conference director selects the current broadcaster. The MCS automatically switches the video image seen by all other sites to the new broadcaster site. Contrast with voice-activated video switching. See also chair-control video switching; user-selected video switching.


A listing of information stored in the MCS. See record.


See digital hierarchy.


See digital hierarchy.


See data terminal equipment.

dual-network bus

An optional bus configuration for the Model 2020Model 520 that increases the number of conference sites.



A Europe-based digital system for transmitting voice and data over 31 channels operating at a combined rate of 1.920 Mbps.


See Electronics Industry Association.


An electrical and mechanical standard for the serial transfer of digital information between digital systems, such as computers and communication equipment. It is a joint standard of the Electronic Industry Association and the Telecommunications Industries Association and was formerly known as RS-232.

Electronics Industry Association (EIA)

A standards organization, based in the United States, that specializes in the electrical and functional characteristics of interface equipment.


The process of encoding a message by means of numeric keys. The message is unintelligible without the keys to decryption.


See extended superframe format.

Ethernet module

An interface unit that supports a transmission protocol for packet-switched local area networks. It allows the MCS to connect to workstations over a TCP/IP-compliant local area network. Ethernet is a registered trademark of the Xerox Corporation.

extended superframe format (ESF)

A framing format that extends a DS1 superframe from 12 to 24 frames so that an 8 kbps F-bit pattern can be broken into three separate patterns: framing, cyclic redundancy check, and a data link.



See frequency-division multiplexing.

field replaceable unit (FRU)

Any component or assembly within a Multimedia Conference Server that can be replaced by a field service representative.


A service provided by local exchange carriers that enables automatic and direct control over the configuration of the customer's private-line networks.

fractional T1

A service that enables a customer to use a portion of a full 1.544-Mbps T1 channel at a cost that is lower than either T1 rates or the combined rate for the multiple DS0 channels needed to carry a customer's payload.


A segment of an analog or digital signal that has a repetitive characteristic, in that corresponding elements of successive frames represent the same things.

frequency-division multiplexing (FDM)

Division of a transmission facility into two or more channels by splitting the frequency band transmitted by the facility into narrower bands, each of which is used to constitute a distinct channel.


See field-replaceable unit.



An ITU-T Recommendation that defines an international standard for mlaw and A-law PCM encoding. See also m-law and A-law.


An ITU-T Recommendation for the encoding of wide-band audio signals.


An ITU-T Recommendation for the encoding of 128-kbps companded PCM data to 16-kbps LD-CELP code words.



In the ISDN, a channel that provides a 384-kbps connection, or the equivalent six B channels, via a switched or nonswitched portion of a network.


In the ISDN, a channel that provides a 1.536-Mbps connection via a switched or nonswitched portion of a network. The connection operates at the equivalent 24 B channels.


In the ISDN outside North America, a channel that provides a 1984-Mbps connection via a switched or nonswitched portion of a network. The connection operates at 1920 Mbps, or the equivalent 30 B channels, plus 64 kbps, the equivalent of one D channel.


An ITU-T Recommendation that defines an international standard for using video codecs with audiovisual services at px64kilobits.


The ITU-T family of recommendations detailing the compression techniques for video and audio, the display resolutions for video, and where to place all the digitally compressed information in a data stream so it can be translated into video, audio, and graphics when received at a distant site. H.320 may be known as px64.

hertz (Hz)

The unit of frequency, one cycle per second.


See hertz.



See inverse multiplexer.

industry standard architecture (ISA)

The architecture for personal computer systems based on the IBM PC, XT, and AT machines.

in band

Analog-generated signaling frequencies that use the same band as the message.

Integrated Services Digital Network (ISDN)

A network architecture that enables end-to-end digital connections. The network supports diverse services through integrated access arrangements and defines a limited set of standard, multipurpose interfaces for equipment vendors, network providers, and customers. Interworking with a public switched telephone network is retained.


A shared boundary defined by common physical interconnection characteristics, signal characteristics, and meanings of interchanged signals.

International Standards Organization (ISO)

An international body concerned with worldwide standardization for a broad range of products, including communication equipment.

International Telecommunication Union Telecommunication Standardization Sector (ITU-T)

An international standards organization for the telecommunication industry. The ITU-T was formerly known as the Comite Consultatif International Telegraphique et Telephonique (CCITT).

Internet Protocol (IP)

A protocol that provides for transmitting blocks of data between hosts identified by fixed-length addresses. Because the IP has no mechanisms to augment end-to-end data reliability, flow control, sequencing, or other services, these routines are provided by other protocols.

inverse multiplexer (IMUX)

A device using a technology that aggregates multiple switched channels to achieve a desired data rate for a session.

I/O address

See address register.


See Internet Protocol.


See industry standard architecture.


See Integrated Services Digital Network.


See basic rate.


See primary rate.


See International Standards Organization.


See International Telecommunication Union Telecommunication Standardization Sector.



A pair of wires used to establish a cross connection.



See kilobits per second.

kilobits per second (kbps)

A unit of data transmission equal to 1000 bits per second.


See kilohertz.

kilohertz (kHz)

A unit of measure of frequency equal to 1000 hertz.



See local area network.


See local exchange carrier.

Lecturer mode

A choice for a site participating in a Continuous Presence conference definition to be the broadcaster to all other sites. The lecturer sees a mix of four other sites. See Continuous Presence. See also Mixed user.


An acronym for low-delay, code-excited linear prediction. This type of coder uses a parameterized coding technique to encode defining characteristics of an input signal.


A transmission medium and associated equipment required to transport information between two networks.

line code

The representation of digital information as it appears on a transmission medium. The distinguishing characteristics of a line code are its spectral shape and its bandwidth.

local area network (LAN)

A private network offering high-speed digital communications channels for the connection of computers and related equipment in a limited geographic area.

local exchange carrier (LEC)

A common carrier transporting telecommunications signals within a local area.


All facilities required for a telecommunication path from a customer's premises to a central office.


A test of the transmission capability of a system in which a signal is transmitted through a loop that returns the signal to the source. The test verifies the capability of the source to transmit and receive signals.



See mu () law.


See megabits per second.


See Multimedia Conference Server.

megabits per second (Mbps)

A digital transmission speed in millions of bits per second.

Mixed user

A choice for a site participating in a Continuous Presence conference definition to be displayed as one of four sites on the quadrant-screen display. No site sees its own image. See Continuous Presence.

mu-law (-law)

A method of compressing and expanding voice signals into digital signals for transmission. Mu-law (m-law) is the method used in North America and Japan. See also companding; A-law.

Multimedia Conference Server (MCS)

A software-controlled switching device that interconnects H.320-compliant conferencing systems and accepts T.120-compliant endpoints in the H.320 conference.

multiplexer (MUX)

A device that divides the bandwidth of a channel by frequency or time to enable multiple devices to share the channel.


In data transmission, a function that permits two or more data sources to share a common transmission medium with each data source having its own channel.


See multiplexer.



A series of points interconnected by communication channels, often on a switched basis. Networks are either common to all users or privately leased by a customer for a specific application.

Network Side BRI

A protocol choice for a Quad-BRI module that provides four-port access to locally attached BRI terminals or PCs. Thus, local desktop users can participate in H.320-multipoint conferences using the public network. Contrast with User Side BRI. See also basic rate ISDN.



The site that manages the conference queue in an Attended conference. See also Attended conference.



See private branch exchange.


See pulse-coded modulation.


See primary rate interface.

primary rate interface (PRI)

In the ISDN, a channel that provides digital transmission capacity of no more than 1.536 Mbps (1.984 Mbps in Europe) in each direction. The interface supports combinations of one 64 kbps D channel and several 64 kbps B channels or H channel combinations.

private branch exchange (PBX)

A private switching system.

private line

A dedicated circuit leased by a customer for audio, video, or data. A private line does not depend on public connections; however, it can be switched or nonswitched.

private network

A network designed for the exclusive use of one customer.


A precise set of rules and a syntax that govern the accurate transfer of information.


The PictureTel extension to the H.320 standard for optimizing audio bandwidth. It provides 7 kHz of audio using only 24 kbps of bandwidth.

pulse code modulation

A process in which a signal is sampled, and the magnitude of each sample with respect to a fixed reference is quantized and converted by coding to a digital signal.


A synonym for the ITU-T H.320 series of videoconferencing recommendations that specify the transmission of video and audio signals over digital communication networks.



See Quarter Common Intermediate Format.

Quarter Common Intermediate Format (QCIF)

A ITU-T video format that governs the transmission of video signals over the ISDN. The video format specifies 144 lines of 176 pixels. Contrast with Common Intermediate Format.



A collection of specific data treated as a unit. A record is an entry in an MCS database or directory.


A 4-wire modular connector commonly used to interconnect end-user telephone equipment.


An 8-wire modular connector used to connect telephone equipment through a T1 or similar network.


See EIA/TIA 232-E.


An EIA interface standard for a high-speed, synchronous serial port.



The Software Generation 4. PictureTel algorithm for video and audio compression.


Twelve consecutive DS1 frames arranged in a specific format. The format repeats every 12 frames. See also ESF.

switched line

A line that may be temporarily established at the request of one or more of the connected stations. Synonymous with dialed-up line.

switched network (dialed network)

A series of points interconnected by public lines, voice-grade lines, or digital-grade lines with switching facilities or with exchanges operated by public telephone companies.

synchronous network

A network in which all the communication links are synchronized to a common clock that permits end-office digital switching equipment to operate with essentially zero slip rate.


T1 access

Access to switched 56-kbps services and fractional T1 services over T1 lines available in the United States.

T1 access lines

Lines that use in-band signaling to set up calls using switched 56-kbps services.

T1 carrier

A digital system for transmitting voice and data over 24 channels operating at a combined rate of 1.544 Mbps.


An ITU-T Recommendation providing an overview of audiographic conferencing. See dataconferencing.

T.120 Conferencing module (TCM)

An optional module that provides T.120 communication and dataconference functions. It supports H.320 audio-video endpoints that are T.120-compliant and Databeam GCC R2-enabled. See dataconferencing. On the hardware module, the label reads DPU.


See T.120 Conferencing module.


See Transmission Control Protocol/Internet Protocol.


See time-division multiplexing.

time-division multiplexing (TDM)

The division of a transmission facility into multiple channels by allotting the facility to different channels, one at a time.

Transmission Control Protocol/Internet Protocol (TCP/IP)

A standard defined by the U.S. Department of Defense that allows transmission and processing of data between dissimilar computers across networks.


See International Telecommunication Union Telecommunication Standardization Sector.


user-selected video switching

A video-switching method using the ITU-T Recommendation H.243 standard in which a conference site chooses itself as the current broadcaster. The MCS must be set to voice-activated video switching. Contrast with director-selected video switching. See also chair-control video switching; voice-activated video switching.

User Side BRI

A protocol choice for a Quad-BRI module that provides four-port access to PSTN BRI network lines. It supports dial-in and dial-out connections for BRI-equipped endpoints that use an external NT1 device. Contrast with Network Side BRI. See also basic rate ISDN.



An ITU-T standard that governs synchronous data transmission at 48kbps. In practice, V.35 is used for synchronous transmission to 2048 Mbps.


Full-motion, full-color, electronic communication that permits two or more persons or groups of people in different locations to engage in face-to-face visual and audio communication.

video switching

The mechanism the MCS uses to determine the video image that sites see in a multipoint conference. See also voice-activated video switching; director-selected video switching; chair-controlled video switching; user-selected video switching.

voice-activated video switching

A video-switching method in which the MCS automatically selects the current broadcaster based on the audio (voice) activity levels from each site in the conference-the loudest audio signal is designated as the dominant audio. In voice-activated video switching, a conference participant can select the ITU-T Recommendation H.243-Chair Control option or user-selected video switching. Contrast with director-selected video switching. See also chair-control video switching; user-selected video switching.



An ITU-T physical interface standard between data terminal equipment and data circuit-terminating equipment for synchronous V-series modems.