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Address Mask

A bit mask used to select bits from an Internet address for subnet addressing. The mask is 32 bits long and selects the network portion of the Internet address and one more bits of the local portion. Sometimes called subnet mask.

ANSI

American National Standards Institute. The U.S. standardization body. ANSI is a member of the International Organization for Standardization (ISO).

API

Application Program Interface. A set of calling conventions defining how a service is invoked through a software package.

AppleTalk

A networking protocol developed by Apple Computer for communication between Apple Computer products and other computers. This protocol is independent of what network it is layered on. Current implementation exist for LocalTalk (235 Kbps) and EtherTalk (10 Mbps).

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.

Asynchronous

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.

Analog

A form of transmitting information characterized by continuously variable quantities, as opposed to digital transmission, which is characterized by discrete bits of information in numerical steps. An analog signal is responsive to changes in light, sound, heat and pressure.

Analog-to-Digital Conversion (ADC)

Process of converting analog signals to a digital representation. DAC represents the reverse translation.

ATM

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

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.

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 Syste

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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bps

Bits per second, a unit of measurement of the speed of data transmission and thus of bandwidth.

B channel

The ISDN circuit-switched bearer channels, capable of transmitting 64 Kbps of digitized information.

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.

Backhaul

A terrestrial communications channel linking an earth station to a local switching network or population center.

Bandwidth

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.

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.

Baud

The rate of data transmission based on the number of signal elements or symbols transmitted per second. Today most digital signals are characterized in bits per second.

Basic rate ISDN

See ISDN entry.

Bell Operating Company

Any of the 22 regulated telephone companies that were "spun off" from AT&T during divestiture. The BOCs are grouped into RBHCs-Regional Bell Holding Companies such as Nynex, BellSouth and others.

Bellcore

An abbreviation for Bell Communications Research. Bellcore is the research and coordinating entity that develops standards and ensures the BOCs and RBHCs offer centralized services. It was formed to take the place of Bell Labs which, after divestiture, severed all formal ties with the BOCs. It is owned by the seven RBHCs. This is a tricky position for Bellcore since the RBHCs view each other as competitors and periodically raid each other's service.

Bit

A single digital unit of information.

Bit Error Rate

The fraction of a sequence of message bits that are in error. A bit error rate of 10-6 means that there is an average of one error per million bits.

Bit Map

The total of all bit planes used to represent a graphic. Its size is measured in horizontal, vertical and depth of bits. In a one-bit (monochrome) system there is only one bit plane. As additional planes are added color can be described. Two bit planes yield four possible values per pixel, eight yield 256, and so on.

Bit Rate

The speed of a digital transmission, measured in bits per second.

Blanking

An ordinary television signal consists of 30 separate still pictures or frames sent every second. They occur so rapidly, the human eye blurs them together to form an illusion of moving pictures. This is the basis for television and motion picture systems. The blanking interval is that portion of the television signal which occurs after one picture frame is sent and before the next one is transmitted. During this period of time special data signals can be sent which will not be picked up on an ordinary television receiver.

BRI

(See the ISDN entry).

Bridge

In videoconferencing vernacular, a bridge connects three or more conference sites so that they can simultaneously communicate. Bridges are often called MCU’s - multi-point 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.

Broadband

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.

Broadcast

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

Brouter

Concatenation of "bridge" and "router". Used to refer to devices which perform both bridging and routing functions.

Burtsy data

Information which flows in short intense data groupings (often packets) with relative long silent periods between each transmission burst.

Business Television

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

Byte

A group of eight bits; usually the smallest addressable unit of information in a data memory storage unit.

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Carrier

A term used to refer to various telephone companies that provide local, long distance or value added services; alternately, a system or systems whereby many channels of electrical information can be carried over a single transmission path.

CCD

Charge coupled device used in cameras as an optical scanning mechanism. It consists of a shift register that stores samples of analog signals. An analog charge is sequentially passed along the device by the action of stepping voltages and stored in potential wells formed under electrodes. The charge is moved from one well to another by the stepping voltages.

CCITT

Consultative Committee for international Telegraphy and Telephony, (now called the International Telecommunications Union’s Telecommunications Standardization Sector or TSS) An international body responsible for establishing interoperability standards for communications systems. The world’s leading telecommunications standards organization.

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. QCIF displays 176 pixels grouped in 144 non-interlaced luminance lines.

Codec

Coder/decoder system for digital transmission.

Compression

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D-channel

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.

Data compression

Reducing the size of a data file by reducing unnecessary information, such as blanks and repeating or redundant characters or patterns.

DBS

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

Decoder

A television set-top device which enables the home subscriber to convert an electronically scrambled television picture into a viewable signal. This should not be confused with a digital coder/decoder known as a CODEC which is used in conjunction with digital transmissions.

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 on one-quarter of a second.

Demodulator

A videoconference receiver circuit which extracts or “demodulates” the “wanted “signals from the received carrier.

Digital

Conversion of information into bits of data for transmission through wire, fiber optic cable, videoconference, or over air techniques. Method allows simultaneous transmission of voice, data or 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.

Domain

In the Internet, a part of a naming hierarchy consisting of a sequence of names (labels) separated by periods (dots). e.g., rieac.com

DVB

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

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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. Earth Station antennas vary in size from the.2 foot to 12-foot (65 centimeters to 3.7 meters) diameter size used for TV reception to as large as 100 feet (30 meters) in diameter sometimes used for international communications. The typical antenna used for INTELSAT communication is today 13 to 18 meters or 40 to 60 feet.

Echo Canceller

An electronic circuit which attenuates or eliminates the echo effect on videoconference telephony links. Echo cancellers are largely replacing obsolete echo suppressors.

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.

EIRP

Effective Isotropic Radiated Power - This term describes the strength of the signal leaving the videoconference antenna or the transmitting earth station antenna, and is used in determining the C/N and S/N. 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.

Encoder

A device used to electronically alter a signal so that it can only be viewed on a receiver equipped with a special decoder.

Ethernet

A LAN running on coaxial or twisted pair wiring, at 1 or 10 Mbps.

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Fast Ethernet

A way to run Ethernet at 100Mbps on one or two pairs of standard, unshielded telephone copper wire.

Forward Error Correction (FEC)

Adds unique codes to the digital signal at the source so errors can be detected and corrected at the receiver.

Fps

Frames per second.

Fractional T-1

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.

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.

Frequency

The number of times that an alternating current goes through its complete cycle in one second of time. One cycle per second is also referred to as one hertz; 1000 cycles per second, one kilohertz; 1,000,000 cycles per second, one megahertz: and 1,000,000,000 cycles per second, one gigahertz.

FTP

File Transfer Protocol. The Internet protocol (and program) used to transfer files between hosts.

Full-CIF (FCIF)

See CIF

Full-duplex (FDX)

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 live-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.

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Gateway

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.

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H.320

A recommendation of the ITU-T based on Discrete Cosine Transform, CCM and motion compensation techniques. It can be a video system’s sole compression method or supplementary algorithm, used instead of a proprietary algorithm when two dissimilar codecs have need to interoperate. H.320 includes a number of individual recommendations for coding, framing, signaling and establishing connections. It also includes three audio algorithms, G.721, G.722 and G.728.

Handshake

The electrical exchange of predetermined signals by devices wishing to set up a connection. Once completed the transmission begins. Used in video communications by codecs wishing to interoperate whereby they seek out a common algorithm.

Hertz (Hz)

The name given to the basic measure of radio frequency characteristics. An electromagnetic wave completes a full oscillation from its positive to its negative pole and back again in what is known as a cycle. A single Hertz is thus equal to one cycle per second.

Hub

A network's or system's signal distribution point where multiple circuits convene and are connected. Some type of switching or information transfer can then take place. Switching hubs can also be used in Ethernet LAN environments is an arrangement whereby a LAN segment might support only one workstation. This relieves congestion through a process called micro segmenting.

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IBS

INTELSAT Business Services.

Indeo

Intel's compression/decompressional algorithm for video playback and CD-ROM applications. Indeo is similar to Motion JPEG. This algorithm has been harnessed by Microsoft in their Video for Windows. IBM in their OS/2, and Apple in their QuickTime. It supports playback at rates of 15 frames per second. These can be viewed full-screen while maintaining the same playback rate.

INTELSAT

The International Telecommunications Satellite Organization operates a network of Satellites for international transmissions.

Interoperability

The ability of electronic components produced by different manufacturers to communicate across product lines. The trend toward embracing standards has greatly furthered the interoperability process.

Intra-LATA

A connection that does not cross over a LATA boundary and one that regulated LECs are allowed to carry on an end-to-end basis.

ISDN - Integrated Services Digital Network.

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 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 2.048 Mps, respectively. Each channel (time slot) is 64Kbps. One channel is reserved as the D channel; the other 23, as bearer channels (23+D).

Isochronous

Pronounced "I-sockronous" Data transmission where timing is derived from the signal carrying the data. No timing or clock lead is provided at the customer interface. In isochronous data transmission, data has no embedded timing - send it slower and it is still valid, only late. Voice and video are intimately tied to timing. With TMD (Time Division Multiplexing) services, there is a direct relationship between the signal rate used to digitize the voice and samples and the bearer channel rate, allowing accurate reconstruction of the voice (or other signals) at the far end. In packet technologies, no such relationship exists. Services like ATM must use care in transferring such signals so timing can be recovered since it cannot be derived from the ATM bearer channel.

ITU

International Telecommunications Union; one of the specialized agencies of the United Nations and founded in 1865 before telephones were invented as a telegraphy standards body.

IEC

Interexchange Carrier. Long-distance carrier providing service between local area exchanges (LECs). AT&T, MCI, and GTE-Sprint are IECs. The services an IEC provides may be interstate or intrastate, as long as it's between local areas.

IXC

Interexchange carrier, long distance service providers in the U.S. that provide inter-LATA service.

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JPEG

ISO Joint Picture Expert Group standard for the compression of still pictures.

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Kbps

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

Kelvin (K)

The temperature measurement scale used in the scientific community. Zero K represents absolute zero, and corresponds to minus 459 degrees Fahrenheit or minus 273 Celsius. Thermal noise characteristics of LNA or LNB used in satellite communications are measured in Kelvins. The lower number the better.

Kilohertz (kHz)

Refers to a unit of frequency equal to 1,000 Hertz.

Ku Band

The satellite frequency range from 10.9 to 17 GHz.

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LAN

Local Area Network, a computer network linking workstations, file servers, printers, and other devices within a local area, such as an office. LANs allow the sharing of resources and the exchange of both video and data.

LATA

Local Access and Transport Areas. The areas within which the Bell Operating and independent telephone companies can provide transport services. Inter-LATA connections must be provided by Interexchange carriers.

LEC

Local exchange company, the local telephone office that provides service between the long-distance carrier and the customer premises. Depending on the location, the LEC may be part of an RBOC (Regional Bell Operating Company) or an independent telephone company, such as GTE or United Telephone. RBOC refers to the seven Bell companies that were part of AT&T before divestiture. Each RBOC is broken down into a number of LECs. For example, the Ameritech RBOC includes Illinois Bell, Ohio Bell, Wisconsin Bell, etc.

Leased Line

A dedicated circuit typically supplied by the telephone company.

Low Noise Amplifier (LNA)

This is the preamplifier between the antenna and the earth station receiver. For maximum effectiveness, it must be located as near the antenna as possible, and is usually attached directly to the antenna receive port. The LNA is especially designed to contribute the least amount of thermal noise to the received signal.

Low Noise Block Downconverter (LNB)

A combination Low Noise Amplifier and downconverter built into one device attached to the feed.

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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.

MAN

Metropolitan Area Network.

Margin

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

MBONE

Multicast Backbone. A collection of Internet routers that support IP multi-casting. The MBONE is used as a “broadcast (actually multi-cast) channel” on which various public and private audio and video programs are sent.

Mbps

Megabits per second.

Megahertz (MHz)

Refers to a frequency equal to one million Hertz, or cycles per second.

Microwave

Line-of sight, point-to-point transmission of signals at high frequency. Many CATV systems receive some television signals from a distant antenna location with the antenna and the system connected by microwave relay. Microwaves are also used for data, voice, and indeed all types of information transmission. The growth of fiber optic networks have tended to curtail the growth and use of microwave relays.

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. Modulators are also used by CATV companies to place a baseband video television signal onto a desired VHF or UHF channel. Home video tape recorders also have built-in modulators which enable the recorded video information to be played back using a television receiver tuned to VHF channel 3 or 4.

MPEG

Moving Picture Experts Group. MPEG has established standards for compression and storage of motion video.

Multiplexing

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

Multi-point

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

Multi-point Control Unit

(MCU) A device that bridges together multiple inputs so that more than three parties can participate in a videoconference. The MCU uses fast switching techniques to patch the presenters or speaker’s input to the output ports representing the other participants.

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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 (copper, wire, fiber optic cable, etc.) or wireless, for example via satellite.

Node

A concentration point in a network where numerous trunks come together at the same switch.

Noise

Any unwanted and unmodulated energy that is always present to some extent within any signal.

NT-1

Network Termination type 1. The NT-1 is a device which converts the two-wire line (or “U” interface) coming from your telephone company into a 4-wire line (or “S/T” interface). The NT- 1 is physically connected between the ISDN board of your videoconferencing system and your ISDN phone line. The NT- 1 supports network maintenance functions such as loop testing. Check to see if your ISDN equipment requires an external NT-1 to operate.

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.

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OLE

Object linking and embedding. This is a function where all objects you copy and paste to the notebook using the Paste Special command are automatically linked with their origination applications. You or your colleague can double-click on an object and the application associated with that object launches automatically in that person's PC (provided it is installed). Using the application, that person can edit the object. The snapshot in the notebook is updated in both the local and the remote PC’s. In addition, you can update the original file from which you copied the object, if you choose.

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Packet Switching

Data transmission method that divides messages into standard-sized packets for greater efficiency of routing and transport through a network.

PAL Phase Alternative Line System

The European TV standard based upon 50 cycles per second electrical system and 625 lines per frame and 25 Frames per Second. (NTSC, the North American standard is based on 30 frames per second) (French use SECAM)

Pan

To pivot a camera in a horizontal direction, tilt is to pivot in the vertical direction.

PBX

Private Branch Exchange. A telephone switch, usually located on a customer's premises, connected to the telephone network but operated by the customer. A PBX provides pooled access to a given number of inside (extension) lines in a smaller number of outside lines (trunks). Often, outgoing calls are dialed directly, incoming calls are handled by an operator or switched automatically by the PBX software.

Pixel

The smallest element of the computer or television display on the raster scale.

POTS

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

PRI

See ISDN.

PTT

Post Telephone and Telegraph Administration Refers to operating agencies directly or indirectly controlled by governments in charge of telecommunications services in most countries of the world.

Pulse

Code Modulation A time division modulation technique in which analog signals are sampled and quantized at periodic intervals into digital signals. The values observed are typically represented by a coded arrangement of 8 bits of which one may be for parity.

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QCIF

See CIF

QPSK - Quadrature Phase Shift Keying

System of modulating a videoconference signal.

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Rain Outage

Loss of signal at Ku or Ka Band satellite frequencies due to absorption and increased sky-noise temperature caused by heavy rainfall.

RBOC

Pronounced "R-BOCK". Regional Bell Operating Company, one of the seven Bell companies that resulted from the AT&T divestiture. Each RBOC is broken down into a number of LECs. The RBOCs are Nynex, BellAtlantic, Bell South, Southwestern Bell, Ameritech, US West, and Pacific Telesis.

Real-Time

The processing of information that returns a result so rapidly that the interaction appears to be instantaneous. Telephone calls and videoconferencing are examples of real-time applications. These kinds of real-time information not only need 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. The telephone network is designed for real-time communication.

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.

RGB

Red, green, blue. The additive used in color video systems. Color television signals are oriented as three separate pictures: red, green and blue. Typically, they are merged together as a composite signal but for maximum quality and in computer applications the signals are segregated.

RS-232-C

A set of standards specifying various electrical and mechanical signals for interfaces between computers, terminals, and modems. In personal computer world, the original PC and PC/XT used the 25-pin RS-232 connector. With the introduction of the AT-class personal computers, the serial connector was reduced to the 9-pin version.

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Scrambler

A device used to electronically alter a signal so that it can only be viewed or heard on a receiver equipped with a special decoder.

Secam

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

Signal to Noise Ratio (S/N)

The ratio of the signal power and noise power. A video S/N of 54 to 56 dB is considered to be an excellent S/N, that is, of broadcast quality. A video S/N of 48 to 52 dB is considered to be a good S/N at the headend for Cable TV.

Single-Channel-Per-Carrier (SCPC)

A method used to transmit a large number of digital signals over a single satellite transponder. Digital Business Television typically uses an SCPC transmission format.

Spectrum

The range of electromagnetic radio frequencies used in transmission of voice, data and television.

Splitter

A passive device (one with no active electronic components) which distributes a television signal carried on a cable in two or more paths and sends it to a number of receivers simultaneously.

Spread Spectrum

The transmission of a signal using a much wider bandwidth and power than would normally be required. Spread spectrum also involves the use of narrower signals that are frequency hopped through various parts of the transponder. Both techniques produce low levels of interference Between the users. They also provide security in that the signals appear as though they were random noise to unauthorized earth stations. Both military and civil videoconference applications have developed for spread spectrum transmissions.

Synchronization(Sync)

The process of orienting the transmitter and receiver circuits in the proper manner in order that they can be synchronized. Home television sets are synchronized by an incoming sync signal with the television cameras in the studios 60 times per second. The horizontal and vertical hold controls on the television set are used to set the receiver circuits to the approximate sync frequencies of incoming television picture and the sync pulses in the signal then fine-tune the circuits to the exact frequency and phase.

Switched 56

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.

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10Base-T

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

T1

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.

T.120

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.

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.

TELCO

Generic term for telephone company. Can refer to an Interexchange Carrier or to the Local Exchange Carrier.

Telecommuting

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.

Telemedicine

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.

Transmitter

An electronic device consisting of oscillator, modulator and other circuits which produce a radio or television electromagnetic wave signal for radiation into the atmosphere by an antenna.

Transponder

A combination receiver, frequency converter, and transmitter package, physically part of a communications satellite. Transponders have a typical output of five to ten watts, operate over a frequency band with a 36 to 72 megahertz bandwidth in the L, C, Ku bands. Communications satellites typically have between 12 and 24 onboard transponders although the INTELSAT VI at the extreme end has 50.

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Uplink

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

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VSAT

Very small aperture terminal. Refers to small earth stations, usually in the 1.2 to 2.4 meter range. Small aperture terminals under 0.5 meters are sometimes referred to Ultra Small Aperture Terminals (USAT's)

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WAN

Wide Area Network. A communications network that services a geographic area larger than that served by a local area network or metropolitan area network. 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.” 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.

Wireless

Transmission via radio waves or satellite. Wireless transmission is likely to be used in many computer networks of the future.

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X.25

A set of packet switching standards published by the CCITT.

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Y/C

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.

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ZTerm

Shareware terminal emulator/modem program for Macintosh

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Last Update: 06/07/01 Copyright ©1999 by International Trade Data Network - ALL RIGHTS RESERVED