VideoConferencing Glossary

Analog Transmission

The way information is transmitted over a continuously changing electrical wave that is similar to, or analogous with the original signal. All telephone calls used to be transmitted in an analog format. Today they are translated to digital pulses for both local and long-distance transmission. Your television at home receives analog signals. (But soon, read 5 years, the new sets will also be able to receive digital).

Application Sharing

Application sharing is a feature that allows two people to work together when one of the individuals doesn't have the same application, or same version of the application. In application sharing, one user launches the application and it runs simultaneously. Both users can input information and otherwise control the application using the keyboard and mouse. Although it appears that the application is running on both PC's, it actually is running on only one, yet each user operates as though the application were running on both PC's. Files associated with the application can be easily transferred, so the results of the collaboration are available to both users immediately. The person who launched the application can lock out the other person from making changes, so the locked-out person sees the application running but cannot control it.


Typical transmission method of dial up modems. Data is transmitted using a start bit at the beginning of a character and a stop bit at the end. The time interval between characters may be of varying lengths. Synchronous data uses an external reference clock to unify both ends of the data circuit.


Asynchronous Transfer Mode. High speed (up to 155 Mbps), high bandwidth, low-delay, transport technology, integrating multiple data types (voice, video, and data). ITU has selected ATM as the basis for the future broadband network because of its flexibility and suitability for both transmission and switching. May be used in the phone and computer networks of the future.

Audio Bridge
Equipment that mixes multiple audio inputs and feeds back composite audio to each station after removing the individual station's input. This equipment may also be called a mix-minus audio system.
Autonomous System

Internet (TCP/IP) terminology for a collection of gateways (routers) that fall under one administrative entity and cooperate using a common Interior Gateway Protocol (IGP).


Bearer Channel. The fundamental component of an ISDN circuit, the Bearer Channel carries either voice or data at 64,000 bits per second (64 KBPS) in either direction. A ISDN line has two B channels.


A measure of spectrum (frequency) use or capacity. For instance, a voice transmission by telephone requires a bandwidth of about 3000 cycles per second (3KHz). A TV channel occupies a bandwidth of 6 million cycles per second (6 MHz) in terrestrial Systems. In videoconference based systems a larger bandwidth of 17.5 to 72 MHz is used to spread or "dither" the television signal in order to prevent interference.


The basic direct output signal in an intermediate frequency based obtained directly from a television camera, videoconference television receiver, or video tape recorder. Baseband signals can be viewed only on studio monitors. To display the baseband signal on a conventional television set a "modulator" is required to convert the baseband signal to one of the VHF or UHF television channels which the television set can be tuned to receive.

Bit Interleaving/Multiplexing

A process used in time division multiplexing where individual bits from different lower speed sources are combined (one bit, from one channel at a time) into one continuous higher speed bit stream.

BRI (Basic Rate Interface)

The Basic Rate Interface is one of the two services provided by ISDN. BRI is comprised of two B-channels and one D-channel (2B+D). The B-channels each operate at 64 Kbps and the D-channel operates at 16 Kbps. It is used by single line business customers for typical desk-top type applications.


In videoconferencing vernacular, a bridge connects three or more conference sites so that they can simultaneously communicate. Bridges are often called MCU's - multipoint conferencing units. In IEEE 802 parlance, a bridge is a device that interconnects LAN's or LAN segments at the data-link layer of the OSI model to extend the LAN environment physically. They work with frames (as opposed to packets) of data, forwarding them between networks. They learn station addresses and they resolve problems with loops in the topology by participating in the spanning tree algorithm. Finally, the term bridge can be used in audio conferencing to refer to a device that connects multiple (more than two) voice calls so that all participants can hear and be heard.


The term applied to networks having bandwidths significantly greater than that found in telephony networks. Broadband systems are capable of carrying a large number of moving images or a vast quantity of data simultaneously. Broadband techniques usually depend on coaxial or optical cable for transmissions. They utilize multiplexing to permit the simultaneous operation of multiple channels or services on a single cable. Frequency division multiplexing or cell relay techniques can both be used in broadband transmission.


A packet delivery system where a copy of given packet is given to all hosts attached to the network. Example: Ethernet.


Consultative Committee International for Telegraphy and Telephony, (now called the International Telecommunications Union's Telecommunications Standardization Sector or TSS). A European-based, international advisory committee recommending worldwide standards for transmission.  They are responsible for establishing interoperability standards for communications systems. . The world's leading telecommunications standards organization. This committee is now known as ITU.


Common Intermediate Format, an international standard for video display formats developed by TSS. The QCIF format, which employs half the CIF spatial resolution in both horizontal and vertical directions, is the mandatory H.261 format. QCIF is used for most desktop videoconferencing applications where head and shoulder pictures are sent from desk to desk. QCIF displays 176 pixels grouped in 144 non-interlaced luminance lines.


A path for electrical transmission between two or more points. Also called a link, line, circuit, or facility.


An acronym for Coder/Decoder. This device compresses (for transmission) and decompresses (once received) digital video and analog audio signals so that they occupy less bandwidth during transmission.


The process of reducing the information content of a signal so that it occupies less space on a transmission channel or storage device and a fundamental concept of video communications. An uncompressed NTSC signal requires about 90 Mbps of throughput, greatly exceeding the speed of all but the fastest and shortest of today's networks. Squeezing the video information can be accomplished by reducing the quality (sending fewer frames in a second or displaying the information in a smaller window) or by eliminating redundancy.

Continuous Presence

The transmission of two or more simultaneous images.


Customer Premises Equipment - refers to all ISDN compatible equipment connected at the user sight. Examples of devices are telephone, PC, Telex, Facsimile, etc. The exception is the FCC definition of NT1. The FCC views the NT1 as a CPE because it is on the customer sight, but the CCITT views NT1 as part of the network. Consequently the network reference point of the network boundary is dependent on the variant in use.


Channel Service Unit is a device that provides an interface between Codecs and transmission facilities. It is usually equipped with line-conditioning and switched control capabilities.


In an ISDN network the D-channel is a signaling channel over which packet-switched information is passed by the carrier. The D-channel can also support the transmission of low-speed data or telemetry sent by the subscriber.


Information represented in digital form, including voice, text, facsimile, and video.

Digital Speech Interpolation

DSI - A means of transmitting telephony. Two and One half to three times more efficiently based on the principle that people are talking only about 40% of the time.

Distance learning

The incorporation of video and audio technologies into the educational process so that students can attend classes and training sessions in a location distant from that where the course is being presented. Distance learning systems are usually interactive and are becoming a highly-valuable tool in the delivery of training and education to widely-dispersed students in remote locations or in instances where the instructor cannot travel to the student's site.

Document sharing

See Whiteboarding


A 64 kbps channel.


The Level 1 standard for digital systems operating at 1.536 mbps (24 DS-0 channels). Also known as T1.


Digital Signal Level 3. This term is used to refer to the 45 mbps digital signal carried on a T3 facility.


Digital Service Unit. A user device for interfacing to a digital circuit such as DDS or T1 when combined with a CSU. The DSU converts the user's data stream to bipolar format for transmission.

Dedicated Access

A private connection between a customer's equipment and a company providing transmission services. The connection bypasses the local switched telephone network.

Dedicated Network

Sometimes referred to as a private or leased line. This transmission circuit is used exclusively by a single customer.

Digital Transmission

A way of sending coded information via a series of electric or light pulses through the air, over wires, or through glass fibers.

Echo Cancellation

A technique used in high-speed modems and voice circuits to isolate and filter out unwanted signal energy caused by echoes from the main transmitted signal. Internal to Codecs and may be added externally for large conference rooms or multiple microphone installations.

Echo Effect

A time-delayed electronic reflection of a speaker's voice. This is largely eliminated by modern digital echo cancellers.

Echo suppression

Used to reduce annoying echoes in the audio portion of a videoconference. An echo suppressor is a voice-activated "on/off" switch that is connected to the four-wire side of a circuit. It silences all sound when it is on by temporarily deadening the communication link in one direction. Unfortunately, not only the echo is stopped but also the remote end's new speech, which results in clipping.


Full Common Intermediate Format describes the type of video format transmitted using TSS standard coding methods.


Frames Per Second.

Fractional T1

FT-1 or fractional T-1 refers to any data transmission rate between 56 Kbps and 1.544 Mbps. It is typically provided by a carrier in lieu of a full T-1 connection and is a point-to-point arrangement. A specialized Multiplexer is used by the customer to channelize the carrier's signals.

Full Duplex

Two-way, simultaneous transmission of data; a communication protocol in which the communications channel can send and receive data at the same time. Compare to half-duplex, where information can only be sent in one direction at a time.

Full-motion video

Video reproduction at 30 frames per second (fps) for NTSC signals or 25 fps for PAL signals. Also known as continuous-motion video. In the videoconferencing world, the term "full-motion video" is often used, and often misunderstood. Videoconferencing systems cannot provide 30 fps for all resolutions at all times nor is that rate always needed for a high-quality, satisfying video image. Picture quality must sometimes be sacrificed to achieve interactive visual communication across the telephone network economically. Videoconferencing vendors often use "full-motion video" to refer to any system that isn't still-frame. Most videoconferencing systems today run 10 to 15 frames per second at 112 Kbps.


The original Internet term for what is now called router or more precisely, IP router. Refers to systems that translate from one native format to another.

Gigahertz (GHz)

One billion cycles per second. Signals operating above 3 Gigahertz are known as microwaves. above 30 GHz they are know as millimeter waves. As one moves above the millimeter waves signals begin to take on the characteristics of Iightwaves.


Gigabits per second. 1 Billion bits per second.

G. 723.1

G.723.1 (previously just "G.723") is a standards-based voice codec.  Its was designed for video conferencing / telephony over standard phone lines, and is optimized for real-time encode & decode  G.723.1 is part of the H.323 (IP) and H.324 (POTS) standards for video conferencing.  Has a compression ratio 5.3 and /or 6.3 kbits/sec (0.7-0.8 Kbytes/sec), depending on implantation.


H.225.0 v2 is a standard which covers narrow-band visual telephone services defined in H.200/AV.120-Series Recommendations. It specifically deals with those situations where the transmission path includes one or more packet based networks, each of which is configured and managed to provide a non-guaranteed Quality of Service (QoS) which is not equivalent to that of N-ISDN such that additional protection or recovery mechanisms beyond those mandated by Rec. H.320 is necessary in the terminals. H.225.0 describes how audio, video, data, and control information on a packet based network can be managed to provide conversational services in H.323 equipment.


H.245 is line transmission of non-telephone signals. It includes receiving and transmitting capabilities as well as mode preference from the receiving end, logical channel signaling, and Control and Indication. Acknowledged signaling procedures are specified to ensure reliable audiovisual and data communication.


H.261 describes a video stream for transport using the real-time transport protocol, RTP, with any of the underlying protocols that carry RTP.

An international standard designed that is optimized for low data rates and relatively low motion.  Has a strong temporal compression component, and works best on movies in which there is little change between frames.


An international standard designed that is optimized for low data rates and relatively low motion.  Is an advancement  and is generally better quality than of the H.261 standard.  Has a strong temporal compression component, and works best on movies in which there is little change between frames.


An international standard designed to bring interoperability to videoconferencing.


The H.323 standard provides a foundation for audio, video, and data communications across IP-based networks, including the Internet. H.323 is an umbrella recommendation from the International Telecommunications Union (ITU) that sets standards for multimedia communications over Local Area Networks (LANs) that do not provide a guaranteed Quality of Service (QoS). These networks dominate today’s corporate desktops and include packet-switched TCP/IP and IPX over Ethernet, Fast Ethernet and Token Ring network technologies. Therefore, the H.323 standards are important building blocks for a broad new range of collaborative, LAN-based applications for multimedia communications. It includes parts of H.225.0 - RAS, Q.931, H.245 RTP/RTCP and audio/video Codecs, such as the audio Codecs (G.711, G.723.1, G.728, etc.) and video Codecs (H.261, H.263) that compress and decompress media streams.

Half Duplex

A system capable of transmitting and receiving signals in one direction at a time.


A state of compatibility between videoconferencing units that may support differing levels of compatibility.


A CCITT standard for integrated transmission of voice, video and data. Bandwidths include: Basic Rate Interface - BR (144 Kbps - 2 B & 1 D channel) and Primary Rate - PRI (1.544 and 2.048 Mbps).

Integrated Services Digital Network. A set of protocol and interface standards that effectively constitute an integrated (voice, video, and data) telephone "network." These standards promote global availability and compatibility of ISDN products and services. The two types of ISDN discussed in this Guide are Basic Rate Interface (BRI) and Primate Rate Interface (PRI). ISDN BRI (ISDN Basic Rate Interface) is the interface to connect the desktop to the digital long distance network. ISDN BRI provides two 64Kbps B ("bearer") channels to carry information content, the voice, video, and data substance of a transmission. A separate 16Kbps D ("data") channel is used for call setup and signaling. ISDN BRI is often called "2B+D" ISDN, for its combination of two B and one D channel. This service is marketed and supported by the LECs. ISDN PRI (Primary Rate Interface) is the ISDN equivalent of a T-1 circuit. It provides 23B+D (in North America) or 30B+D (in Europe) running at 1.544 Mbps and 2.048Mps, respectively. Each channel (time slot) is 64Kbps. One channel is reserved as the D channel; the other 23, as bearer channels (23+D).


International Telecommunication Union. Formerly known as CCITT. Organization that develops and sets communications standards


The deviation of a transmission signal in time or phase. It can introduce errors and loss of synchronization in high-speed synchronous communications.


Kilobits per second. 1 Thousand bits per second.


Local Area Network. A high volume data transmission signal is returned to the sending device after passing through all or part of a communications link or network.


Megabits per second. 1 Million bits per second.

Multiplexer (MUX)

A device allowing two or more signals to pass over and share a common transmission path simultaneously.


Communication configuration in which several terminals or stations are connected. Compare to point-to-point, where communication is between two stations only.

Multipoint Control Unit

(MCU) A device that bridges together multiple inputs so that more than three parties can participate in a video conference. The MCU uses fast switching techniques to patch the presenters or speaker's input to the output ports representing the other participants. Site switching can be accomplished automatically using voice or manually.

  1. An interconnected group of nodes.

  2. A series of points, nodes, or stations connected by communications channels; the collection of equipment through which connections are made between data stations.


Network Termination equipment exists in two forms and is referred to accordingly. The two forms are each responsible for different operations and functions.


Is the termination of the connection between the user sight and the LE. NT1 is responsible for performance, monitoring, power transfer, and multiplexing of the channels. 

NTSC - National Television Standards Committee

(Never Twice the Same Color)
A video standard established by the United States (RCA/NBC} and adopted by numerous other countries. This is a 525-line video with 3.58-MHz chroma subcarrier and 60 cycles per second. Frames are displayed at 30 frames per second.


An ordered group of data and control signals transmitted through a network as a subset of a larger message.


Private Branch Exchange.


The physical interface to a computer Multiplexer, for connection of terminals and modems.


Plain Old Telephone Service. Conventional analog telephone lines using twisted-pair copper wire. This is used to provide residential service.


The Primary Rate Interface is one of the two services provided by ISDN. PRI is standard dependent and thus varies according to country. In North America, PRI has twenty-three B-channels and one D-channel (23B+D). In Europe, PRI has thirty B-channels and one D-channel (30B+D).

The American B- and D-channels operate at an equal rate of 64 Kbps. Consequently, the D-channel is sometimes not activated on certain interfaces, thus allowing the time slot to be used as another B-channel. The 23B+D PRI operates at the CCITT designated rate of 1544 Kbps.

The European PRI is comprised of thirty B-channels and one D-channel (30B+D). As in the American PRI all the channels operate at 64 Kbps. However, the 30B+D PRI operates at the CCITT designated rate of 2048 Kbps.


A formal set of conventions governing the formatting and relative timing of message exchange between two communicating systems.


Public Switched Telephone Network. The telecommunications network commonly accessed by ordinary telephones, key systems, PBX trunks, and data equipment.


The Real-time Transport (RTP) Protocol provides end-to-end network transport functions suitable for applications transmitting real-time data such as audio, video or simulation data, over multicast or unicast network services. RTP does not address resource reservation and does not guarantee quality-of-service for real-time services. The data transport is augmented by a control protocol (RTCP) to allow monitoring of the data delivery in a manner scalable to large multicast networks, and to provide minimal control and identification functionality. RTP and RTCP are designed to be independent of the underlying transport and network layers. The protocol supports the use of RTP-level translators and mixers.




Synchronous Optical Network. A standard for using optical media as the physical transport for high-speed, long-haul networks. SONET basic speeds start at 51.84 mbps and go as high as 2.5 Gbps.


SPID stands for "Service Profile ID." It is a multidigit number assigned to each local directory number (LDN). When the telephone company installs your ISDN

Statistical Multiplexer (STM or STDM)

A device connecting multiple channels to a single link by dynamically allocating time slots to the channels based on their transmission activity.

Switched 56

Switched 56 service allows customers to dial up and transmit digital information up to 56,000 bits per second in much the same way that they dial up an analog telephone call. The service is billed like a voice line-a monthly charge plus a cost for each minute of usage. Nearly all LECs and IXCs offer switched 56 service and any switched 56 offering can connect with any other offering, regardless of which carrier offers the service.


The transmission bit rate of 1.544 millions bits per second. This is also equivalent to the ISDN Primary Rate Interface for the U.S. The European T1 or E1 transmission rate is 2.048 million bits per second. Also known as DS-1.


Standard Ethernet. A variant of IEEE 802.3 which allows stations to be attached via twisted pair cable.


A standard for audiographics exchange. While H.320 does provide a basic means of graphics transfer, T. 120 will support higher resolutions, pointing and annotation. Users can share and manipulate information much as they would employ if they were in the same room though they are working over distance and using a PC platform. T. 120 will allow audio bridge manufacturers to add graphics to their products in support of a wide range of applications. talking head The portion of a person that can be seen in the typical business-meeting style videoconference; the head and shoulders. This type of image is fairly easy to capture with compressed video because there is very little motion in a talking head image and most occurs in facial expression and torso movement.

T3 Channel (DS-3)

In North America, a digital channel which communicates at 45.304 Mbps.


The process of commuting to work electronically rather than physically. Telecommuting will find much greater acceptance as the public switched telephone network becomes more robust and digital and as videoconferencing and multimedia technologies arrive at the desktop.


The practice of using videoconferencing technologies to diagnose illness and provide medical treatment over a distance. Used in rural areas where health care is not readily available and to provide medical services to prisoners, among other applications.


Telephony System Specification.

Voice Compression

The conversion of an analog voice signal into a digital signal using minimum bandwidth (16 kbps or less).


A term used to describe the placement of shared documents on an on-screen "shared notebook" or "whiteboard." Desktop videoconferencing software includes "snapshot" tools that enable you to capture entire windows or portions of windows and place them on the whiteboard. You can also use familiar Windows operations (cut and paste) to put snapshots on the whiteboard. You work with familiar tools to mark up the electronic whiteboard much like you do with a traditional wall mounted board.


In component video, the "Y" or luminance signal is kept separate from the "C" (hue and color saturation signal) to allow greater control and to enable enhanced quality images. The luminance is recorded at a higher frequency and therefore more resolution lines are available. Super-VHS and Hi8 systems use V/C video.