Glossary of Terms

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(Keyboard shortcuts are available for Windows users: Press "Alt" and any letter, then "Enter/Return" to go to that letter's section in the videoconferencing glossary. Sorry, Netscape and Mac users this feature isn't supported by them yet.)

A
Algorithm
Any step-by-step problem-solving procedure. Transmission of compressed video over a communications network requires sophisticated compression algorithms. Some videoconferencing systems offer both proprietary and standard compression algorithms

Analog Signal
A type of signal that encodes voice, video, or data transmitted over wire or through the air, and is commonly represented as an oscillating wave. An analog signal can take any value in a range and changes smoothly between values, as opposed to digital signals, which is characterized by discrete bits of information in numerical steps. An analog signal can transmit analog or digital data.

ADC
Analog-to-Digital Conversion
Process of converting analog signals to a digital representation. DAC represents the reverse translation.

ATM
Asynchronous Transfer Mode
A high bandwidth, High speed (up to 155 Mbps), controlled-delay fixed-size packet switching and transmission system integrating multiple data types (voice, video, and data). Uses fixed-size packets also known as "cells" (ATM is often referred to as "cell relay").

Asynchronous Transmission
A mode in which the sending and receiving serial hosts know where a character begins and ends because each byte is framed with additional bits, called a start bit and a stop bit. A start bit indicates the beginning of a new character; it is always 0 (zero). A stop bit marks the end of the character. 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.

Audio
In video communications, electrical signals that carry sounds. The term is also used to describe systems concerned with sound with recording and transmission; speech pickup systems, transmission links that carry sounds, amplifiers and the like.

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).

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B

B Channel
A 56Kbps or 64-kbps channel that carries user data on a line using ISDN D-channel signaling.

B-Mac
A method of transmitting and scrambling television signals. In such transmissions MAC (Multiplexed Analog Component) signals are time-multiplexed with a digital burst containing digitized sound, video synchronizing, authorization, and information.

Bandwidth
Bandwidth is the data capacity of a service, measured in thousands of bits per second (kbps) or millions of bits per second (Mbps). In videoconferencing systems a larger bandwidth is used to spread or "dither" the signal in order to prevent interference.

Baseband
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 Error Rate
The percentage of received bits in error compared to the total number of bits received. A bit error rate of 10-6 means that there is an average of one error per million bits.
bps

Bits per second
A unit of measurement of the speed of data transmission and thus of bandwidth. Actually a nested acronym, meaning binary digits per second. (lower case is significant)
Bps (or BPS). (8-bit) bytes per second (upper case is significant)

BRI
Basic Rate Interface
One of two ISDN subscriber "interfaces" in ISDN. 2 voice (B) channels at 64 kbps channels and 1 data signal (D) channel at 16 kbps. The B-channels are for voice, video, and data. The D-channel is for signaling between telephone company switches and for carrying ISDN user-network messages.

Bridge
In videoconferencing vernacular, a bridge connects three or more conference sites so that they can simultaneously pass data, voice, or video. Videoconferencing bridges are often called MCU's - multipoint conferencing units. (See router).

Broadband
A way of transmitting large amounts of data, voice, and video that is greater than telephony networks. In ISDN, broadband channels support rates above the primary rate (1.544 Mbps or 2.048 Mbps). (See wideband and narrowband)

Business Television
Corporate communications tool involving video transmissions of information via videoconference. Common uses of business television are for meetings, product introductions and training.

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C

Camera presets
Allows pre-defined camera angles to be programmed into a videoconferencing system.

CCITT
Consultative Committee for international Telegraphy and Telephony
(Now called the International Telecommunications Union's Telecommunications Standardization Sector or TSS.) The world's leading telecommunications standards organization responsible for establishing interoperability standards for communications systems.

CIF
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.

CODEC
COder-DECoder
In the videoconferencing world, a video codec converts analog video signals from a video camera to digital signals for transmission over digital circuits, and then converts the digital signals back to analog signals for display.

Compressed video
When the vast amount of information in a normal TV transmission is squeezed into a fraction of its former bandwidth by a codec, the resulting compressed video can be transmitted more economically over a smaller carrier. Some information is sacrificed in the process, which may result in diminished picture and sound quality.

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.

Compression
Compression is a technique that reduces the quantity of bandwidth or bits required to encode a block of information so that it occupies less space on a transmission channel or storage device and a fundamental concept of video communications.

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D

DBS
Direct broadcast videoconference
Refers to a service that uses videoconferences to broadcast multiple channels of television programming directly to home mounted small-dish antennas.

D Channel
A channel that carries WAN synchronization information on a line using ISDN D-channel signaling.

Delay
The time it takes for a signal to go from the sending station through the videoconference to the receiving station. This transmission delay for a single hop videoconference connection is very close to one-quarter of a second.

Demodulator
A videoconference receiver circuit which extracts or "demodulates" the "wanted" signals from the received carrier.

Desktop videoconferencing
Videoconferencing on a personal computer. Most appropriate for small groups or individuals. Many desktop videoconferencing systems support document sharing. (See Room-based videoconferencing).

Digital Signal
A way of sending voice, video, or data that reconstructs the signals using binary codes (1s and 0s) for transmission through wire, fiber optic cable, videoconference, or over air techniques. Digital audio/video signals represented by discrete variations (in voltage, frequency, amplitude, location, etc.) can be transmitted faster and more accurately than analog signals.

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

Document sharing
A feature supported by many videoconferencing systems that allows participants of a videoconference to view and edit the same computer document.

Domain Name
In the Internet, a part of a naming hierarchy consisting of a sequence of names separated by periods (dots) that corresponds to the network number in the IP address. In the symbolic name john@videoconferencingbridging.com, the domain name is videoconferencingbridging.com.

DTE
Data Terminal Equipment
As defined in the RS-232 specification, equipment to which DCE (Data Communications Equipment) is connected, such as a videoconference terminal, LAN bridge or router.

Dual 56
Two switched 56 calls made between videoconferencing equipment to allow data transfer at 112 kbps. The videoconferencing equipment performs a two-channel inverse-multiplexing procedure to assure channel alignment.

DVB
Digital Video Broadcast
The standard for direct broadcast television in Europe and the U.S. Based on MPEG2 Compression.

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E

Earth Station
The term used to describe the combination or antenna, low-noise amplifier (LNA), down-converter, and receiver electronics used to receive a signal transmitted by a videoconference.

Echo-cancellation
Process which attenuates or eliminates the acoustic echo effect on videoconference calls. Echo cancellors are largely replacing obsolete echo suppressers.

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

Echo suppression
To reduce annoying echoes in the audio portion of a videoconference, it silences all sound when 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.

EIRP
Effective Isotropic Radiated Power
This term describes the strength of the signal leaving the videoconference antenna or the transmitting earth station antenna. The transmit power value in units of dBW is expressed by the product of the transponder output power and the gain of the videoconference transmit antenna.

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F

FCIF/QCIF
Standards-based formats for communicating between videoconferencing systems from different vendors. QCIF is one quarter of the resolution of FCIF.

FDX
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.

Fractional T1
Service offering data rates between 64 kbps (DS0 rate) and 1.536 Mbps (DS1 rate), in specified intervals of 64 kbps. 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.

Frame rate
Frequency in which video frames are displayed on a monitor, typically described in frames-per-second (fps). Higher frame rates improve the appearance of video motion.

Frame store
A system capable of storing complete frames of video information in digital form. This system is used for television standards conversion, computer applications incorporating graphics, video walls and various video production and editing systems.

Full duplex audio
2-way audio simultaneously transmitted and received without any interference or "clipping." A common feature of room-based videoconferencing systems.

Full-motion video
In the videoconferencing world, the term "full-motion video" is often used and 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 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 fps at 112 Kbps.

Full motion video is equivalent to broadcast television video with a frame rate of 30 fps for NTSC signals or 25 fps for PAL signals. Images are sent in real time and motion is continuous. Also known as continuous-motion video.

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G

Gateway
Gateways are points of entrance to and exit from a communications network. Viewed as a physical entity, a gateway is that node that translates between two otherwise incompatible networks or network segments.

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H

H.320 / H.323
Sets of widely-used CCITT video compression standards describing methods to allow videoconferencing systems from different manufacturers to interoperate. They include a number of individual recommendations for coding, framing, signaling and establishing connections. Three audio algorithms, G.721, G.722 and G.728, are also included in the standards.

Half duplex audio
2-way audio transmitted and received in turn (rather than simultaneously) so only one site can speak at a time. Contrast with full duplex audio.

Handshake
Prior to a videoconferencing transmission, the codecs exchange predetermined electrical signals that allow them to interoperate by they seeking out a common algorithm.

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I

Internet Gateway
A gateway for accessing the Internet, which is loosely defined as the complex of wide area networks (WANs) joining government, university, corporate and private computers (nodes) in a vast web of network interconnection.

IP
Internet Protocol
IP Address
An address that uniquely identifies each host on a network or Internet.
An IP address has a length of 32 bits, and is divided into four 8-bit parts, each separated by a period, as in 149.122.3.30. This kind of notation is called dotted decimal notation. Each part can consist of a number between 1 and 255.

In addition to an IP address, you can use a symbolic (domain) name provided by Domain Name Services (DNS) to designate an Internet address.

ISDN
Integrated Services Digital Network
An international standard for end-to-end digital transmission of voice, data, and signaling. In a videoconference it is a system that provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.

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K

Kbps
Kilobits per second.
Refers to transmission speed of 1,000 bits per second.

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L

LAN
Local Area Network
A network that interconnects devices over a geographically small area, typically in one building or a part of a building. The most popular LAN type is Ethernet. LANs allow the sharing of resources and the exchange of both video and data.

LAN/WAN Connectivity
This is the practical set of tools, from OS layer protocols to support services, that make a remote access device an effective link between LANs and WANs. An effective remote access server must include a host of communications and translation protocols to fulfill this function.

Latency
The minimum time required to move data from one point to another. Once latency is present, it cannot be optimized. The cause has to be removed (as in using an internal device rather than an external one to remove the latency caused by the serial port). To maximize throughput, use the highest bandwidth available.

Leased Lines
A circuit rented for exclusive use twenty-four hours a day, seven days a week from a telephone company. The connection exists between two predetermined points and cannot be switched to other locations.

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M

MAC
Media Access Control
A system of rules used to move data from one physical medium to another.
MAC (A, B, C, D2)

Multiplexed analog component
Color video transmission system. Subtypes refer to the various methods used to transmit audio and data signals.

Margin
The amount of signal in dB by which a satellite videoconference system exceeds the minimum levels required for operation.

MAX
Media Access Exchange
It supports up to 32 host ports or direct Ethernet connections and up to 8 Mbps to the network. It supports multiple applications, including remote LAN access, leased line backup and individual videoconferencing units, as well as connecting videoconference MCUs to the digital dial-up network.

MBONE
Multicast / Multimedia Backbone
A collection of Internet routers that support IP multi-casting. The MBONE is used as a "broadcast" channel on which various public and private audio and video programs are sent.

Mbps
Megabits per second

Modulation
The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or data signal.

Modulator
A device which modulates a carrier. Modulators are found as components in broadcasting transmitters and in videoconference transponders.

Multiplexing
Techniques that allow a number of simultaneous transmissions over a single circuit.

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

MCU
Multipoint Control Unit
Videoconferencing equipment that allows more than three individual videoconference units to connect together to form a multiparty videoconference session. The MCU uses fast switching techniques to patch the presenters or speaker's input to the output ports representing the other participants.

Multipoint Videoconference
Videoconference with more than two sites. The sites must connect via a video bridge. (Compare with point-to-point videoconference.)

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N

Narrowband
A low-capacity communications circuit/path. It usually implies a speed of 56Kbps or less. (Contrast with wideband and broadband)

Network
A group of stations (computers, telephones, or other devices) connected by communications facilities for exchanging information. Connection can be permanent, via cable, or temporary, through telephone or other communications links. The transmission medium can be physical (i.e. fiber optic cable) or wireless (i.e. satellite).

NT1
Network Terminator Type 1
The NT-1 is physically connected between the ISDN board of your videoconferencing system and your ISDN phone line and converts the two-wire line coming from your telephone company into a 4-wire line. And provides network maintenance functions such line maintenance access, timing, and echo cancellation. NT1s may be built into other pieces of equipment or stand alone.

NTSC
National Television Standards Committee
United States' standard for scanning television signals that has been adopted by numerous other countries. Frames are displayed at 30 frames per second. (Other standards: PAL (Europe) and SECAM (France/former USSR))

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P

Packets
A block of information sometimes called a cell, frame, data unit, service unit, or signaling unit. Although each of these elements do have unique attributes, in essence, all are packets.

PAL
Phase Alternative Line System
The European TV standard for scanning television signals. Frames are displayed at 25 frames per second. Used in most European countries. (Other standards: NTSC (USA) and Secam (France/Former USSR))

Point-to-point videoconference
Videoconference between two sites. (See Multipoint videoconference.)

POP
Point of Presence
This is a point-of-presence of an Internet service provider, used to facilitate remote users' access to the range of applications and IP addresses in the internetwork.

PPP
Point-to-Point Protocol
Provides a standard means of encapsulating data packets sent over a single-channel WAN link. It is the standard WAN encapsulation protocol for the interoperability of bridges and routers. PPP is also supported in workstations, allowing direct dial-up access from a personal computer to a corporate LAN or Internet Service Provider. Using PPP ensures basic compatibility with non-Ascend devices. Both the dialing side and the answering side of the link must support PPP.

PRI
Primary Rate Interface
An ISDN subscriber line, consisting of twenty-three 64 kbps B channels in North America (thirty 64 kbps channels elsewhere) and one 64 kbps D channel, used for signaling purposes.

Promiscuous Mode
A Bridging parameter mode that determines that the Ethernet controller in an Ascend unit accepts all packets and passes them up the protocol stack for a higher-level decision on whether to route, bridge, or reject them. This mode is appropriate if you are using an Ascend unit as a bridge.

Proprietary compression algorithm
A vendor-specific algorithm for compression of a video signal. A videoconferencing system using a proprietary algorithm can only communicate with a remote site using the same algorithm. Many vendors also adhere to standard compression algorithms to facilitate communication across platforms. (i.e .H.320)

Public room
Videoconferencing service offered to the public on a fee-for-usage basis.

Px64
Common reference to the CCITT standards (H.261 et. al.) which describe methods to allow for videoconferencing system interoperability.

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Q

QoS
Quality of Service
Interactive video conferencing requires a high QoS. QOS is important as it determines the quality of your video call. Low quality of service results in latency and a jerky picture with poor and inconsistent audio quality.

QPSK
Quadrature Phase Shift Keying
System of modulating a videoconference signal.

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R

Real-Time
The processing of information that returns a result so rapidly that the interaction appears to be instantaneous. Videoconferencing is an example of a real-time application. This kind of real-time information not only needs to be processed almost instantaneously, but it needs to arrive in the exact order it's sent. A delay between parts of a word, or the transmission of video frames out of sequence, makes the communication unintelligible.

Receiver (Rx)
An electronic device which enables a particular videoconference signal to be separated from all others being received by an earth station, and converts the signal format into a format for video, voice or data.

Room (Group) based videoconferencing
Videoconferencing using a sophisticated system. Appropriate for large groups (See Desktop videoconferencing).

Router
A device or setup that finds the best route between any two networks, even if there are several networks to traverse. Like bridges, remote sites can be connected using routers over dedicated or switched lines to create WANs.

RS-232-C
A set of EIA standards specifying various electrical and mechanical characteristics for interfaces between computers, terminals, and modems. The standard applies to both synchronous and asynchronous binary data transmission at rates below 64 kbps.

RS-449
An EIA standard for a 37-pin data communications connector, usually used with RS-422 or RS-423 electrical specifications.

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S

Secam
A color television system developed by the French and used in the former USSR. Secam operates with 625 lines per picture frame and 50 cycles per second. It is incompatible with the European PAL system or the U.S. NTSC system.

Serial Host
A device, such as a videoconferencing codec, that is connected to a serial host port communicating over a point-to-point link. To a serial host, the MAX appears to be a cable or DCE (Data Communications Equipment).

Serial Host Port
The V.35, RS-499, or X.21 port on the MAX.

Serial Host Port Module
A module on the MAX that connects to a serial host through its serial host port.

Standard compression algorithm
An algorithm convention for compression of a video signal. Adherence to standards allows communication among a wide variety of videoconferencing systems, though not with the same clarity as two similar systems using a proprietary algorithm. H.320 /H.323 are the most widely accepted standards in use today.

Switched 56
A dial-up network-based service providing a data channel operating at a rate of 56 kbps. Also a type of network access line, used to provide access to switched 56 network services.

SDSAF
Switched Digital Services Applications Forum
A consortium of equipment vendors, service vendors, and users, with the goal of advancing the state of switched digital services.

SVC
Switched Virtual Circuit
A path over a packet-switched network that appears to be a dedicated circuit, but in fact the connection only stays up as long as needed, and then ends.

Synchronization
In serial data transmission, a method of ensuring that the receiving end can recognize characters in the order in which the transmitting end sent them, and can know where one character ends and the next begins. Without synchronization, the receiving end would perceive data simply as a series of binary digits with no relation to one another. Synchronous communication relies on a clocking mechanism to synchronize the signals between the sending and receiving machines. (See Asynchronous Transmission)

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T

T1
In North America, T1 service delivers 1.544 Mbps, whereas ISDN service delivers 128 kbps. The data travels over the line at the same speed, but for T1 lines, the capacity is twelve times that of ISDN. Typically channelized into 24 DS0s, each capable of carrying a single voice conversation or data stream. The European T1 or E1 transmission rate is 2.048 million bits per second.

T3
DS-3
In North America, a digital channel which communicates at 45 Mbps, or 28 T1 lines.

T1 PRI line
A T1 line that uses 23 B channels for user data, and one 64 kbps D channel for ISDN D-channel signaling. This type of PRI line is a standard in North America, Japan, and Korea.

TDMA
Time division multiple access.
Refers to a form of multiple access where a single carrier is the shared by many users. Signals from earth stations reaching the videoconference consecutively are processed in time segments without overlapping.

Telecommuter
A work-at-home computer user who connects to the corporate LAN backbone using remote access technologies.

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U

Uplink
The earth station used to transmit signals for a satellite videoconference.

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V

V.35
Commonly used to describe electrical characteristics and connector characteristics for a high speed synchronous interface between DTE and DCE. Originally V.35 described a 48 kbps group band modem interface with electrical characteristics defined in an appendix. Although V.35 is considered obsolete and no longer published by the CCITT, its legacy lives on in the data communications world in the form of the electrical characteristics originally described in the appendix.

Video bridge
Computerized switching system which allows multipoint videoconferencing.

Videoconferencing
Communication across long distances with video and audio contact that may also include graphics and data exchange. Digital video transmission systems typically consist of camera, codec (coder-decoder), network access equipment, network, and audio system.

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W

WAN
Wide Area Network
A data network typically extending a LAN outside a building or beyond a campus. Typically created by using bridges or routers to connect geographically separated LANs. WANs include commercial or educational dial-up networks such as CompuServe, InterNet and BITNET.

Whiteboarding
A term used to describe the placement of shared documents on an on-screen "shared notebook" or "whiteboard." Videoconferencing software includes tools that enable you to work with familiar tools to mark up the electronic whiteboard much like you do with a traditional wall mounted board.

Wideband
A medium-capacity communications circuit/path. It usually implies a speed from 64Kbps to 1.544Mbps. (Contrast with broadband and narrowband)

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X

X.21
A set of CCITT specifications for an interface between DTE and DCE for synchronous operation on public data networks. Includes connector, electrical, and dialing specifications.

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