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.
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.
American National Standards Institute. The U.S. standardization body. ANSI is a member of the International Organization for Standardization (ISO).
Application Program Interface. A set of calling conventions defining how a service is invoked through a software package.
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.
A form of interactive conferencing that employs voice communications accomplished by using one or all of the followingL telephones, microphones, audio mixers, and echo canceling equipment. When three or more locations are involved, an audio bridge is used.
An audio conference supplemented with visual display, usually employing a computer, software, and modem. Peripheral devices such as a mouse, graphic tablet and fax transmission facilitate the sharing of graphic materials.
The ability of a camera in a videoconferencing environment, to automatically follow, pan and zoom in on the presenter. This technology generally uses audio signal, Chrominance or luminance to accomplish Autotrack and is a function of the camera or a peripheral device.
A terrestrial communications channel linking an earth station to a local switching network or population center.
The ability of a camera to compensate for inadequate lighting in a videoconference environment. Backlight is usually a manual setting in a camera's set up configuration.
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 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.
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.
An abbreviation for Bell Communications Research. Bellcore is the research and coordinating entity that develops standards and ensures centralized services are offered by the BOCs and RBHCs. 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.
A single digital unit of information
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.
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.
The speed of a digital transmission, measured in bits per second.
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.
Allows use of multiple ISDN telephone lines to achieve higher bandwidth videoconference connections.
Bits per second, a unit of measurement of the speed of data transmission and thus of bandwidth.
In videoconferencing vernacular, a bridge connects three or more conference sites so that they can simultaneously communicate. Bridges are often called MCU's - multipoint control 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.
Concatenation of "bridge" and "router". Used to refer to devices which perform both bridging and routing functions.
Information which flows in short intense data groupings (often packets) with relative long silent periods between each transmission burst.
Corporate communications tool involving video transmissions of information via videoconference. Common uses of business television are for meetings, product introductions and training.
A group of eight bits; usually the smallest addressable unit of information in a data memory storage unit.
Also known as far-end camera control, it is the capability to control the functions of the remote location's video camera.
Allows pre-defined camera angles to be programmed into a videoconferencing system.
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.
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.
In the video bridging vernacular, chair control is the ability of a conference participant to control various functions of the MCU (bridge) remotely, either by dialup modem or over the internet.
Coder-Decoder. Videoconferencing hardware that codes the outgoing video and audio signals and decodes the incoming signals. Prior to transmission, the codec converts analog signals to digital signals and compresses the digital signals. Incoming audio and video must be decompressed and converted from digital back to analog.
The act of working together remotely on a computerized presentation, document or application during a videoconference.
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.
Typically self-paced instruction employing a personal computer as the primary platform.
Also known as quadaiture. A bridging function which allows conference participants to see multiple sites simultaneously. Usually set to three and four quads in a "Hollywood Squares" style, often one of the quadrants can be set to be a panoramic view for larger audiences.
The person in charge of scheduling all the aspects of a conference including facility procurement, bridging set up (if any) and network testing.
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.
Reducing the size of a data file by reducing unnecessary information, such as blanks and repeating or redundant characters or patterns.
A conference whereby computer images and applications are shared and collaborated on. Data conferences are usually performed in conjunction with an audio or video conference.
Direct broadcast videoconference. Refers to service that uses videoconferences to broadcast multiple channels of television programming directly to home mounted small-dish antennas.
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.
A videoconference receiver circuit which extracts or "demodulates" the "wanted "signals from the received carrier.
Videoconferencing on a personal computer. Most appropriate for small groups or individuals (compare with room-based videoconferencing). Many desktop videoconferencing systems support document sharing.
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.
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.
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.
A device used to present hard copy materials over a videoconference. Usually consists of a platform with a video camera suspended above it. The camera can be zoomed in and out and often times there are lights attached to illuminate the article being examined.
A feature supported by many desktop videoconferencing systems that allows participants at both ends of a videoconference to view and edit the same computer document.
DSL Internet service offers much higher Internet connection speeds than a regular telephone line. SBC offers several different speed options, up to 200 times faster than a standard dial-up connection.
Digital Video Broadcast, the standard for direct broadcast television in Europe and the U.S. Based on MPEG2 Compression.
An electronic circuit which attenuates or eliminates the echo effect on videoconference telephony links. Echo cancellers are largely replacing obsolete echo suppressors.
A time-delayed electronic reflection of a speaker's voice. This is largely eliminated by modern digital echo cancellers.
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.
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.
A large write-on wipe-off dry erase marker board used to integrate transmitted computer data with hand written/drawn documents. An electronic whiteboard allows a presenter or trainer to write on and manipulate computerized documents such as spreadsheets, graphs and charts then save the marked up documents in a computer format for printing sharing and dispersing.
A device used to electronically alter a signal so that it can only be viewed on a receiver equipped with a special decoder.
A way to run ethernet at 100Mbps on one or two pairs of standard, unshielded telephone copper wire.
The ability of a camera to form a crystal clear image. An out of focus image appears blurry.
Adds unique codes to the digital signal at the source so errors can be detected and corrected at the receiver.
Frames per second.
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.
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. Broadcast TV (full motion video) is 30 frames-per-second.
A telecommunications network based on frame relay technology. Data is multiplexed. Contrast with Packet-Switching Network.
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.
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.
Standards-based formats for communicating between videoconferencing systems from different vendors. QCIF is one quarter of the resolution of FCIF
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.
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.
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Designers of VT systems know that you may be able to fool the eye, but you can't trick the ear. Perhaps the most important aspect of a good videoconference is audio quality. If voices come through gargly or haltingly, or if lip-synch is obviously delayed, users will be turned off; you'll have a hard time convincing them that videoconferencing is a good idea. Studies have shown that 100-ms delays are detectable, but tolerable; 250-ms delays are annoying; and 450-ms or greater delays are unacceptable. Spurred not only by visual telephony, but also by a big market for good quality IP telephony, the industry has settled on the G.7xx audio algorithms shown in the following table. An ideal algorithm consumes the lowest amount of transmission bandwidth while delivering a wide audio frequency spectrum. In VT systems, bandwidth not used by audio can be made available to improve picture quality. |
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| R = required for this architecture O = optional |
G.722 was introduced as a "high fidelity" audio option. (See G.711)
G.728 offered the option of compressing audio to only 16 Kbps so that remaining bandwidth could be allocated to video. (See G.728)
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.
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.
A term used to describe some emerging applications that allow an electronic work space for collaborative work, sharing of ideas, and group process. Groupware is able to store, sort, and organize the inputs of participants to support group evaluation and consensus building.
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.
H.323 is the international standard and the market leader for IP Telephony. H.323 networks in production today carrying hundreds of millions (perhaps billions) of minutes per month. H.323 has proven to be an extremely scalable solution that meets the needs of both service providers and enterprises, with H.323 products ranging from stacks and chips to wireless phones and video conferencing hardware.
H.323 is a standard published by the ITU, which is responsible for standardizing the communication systems in use around the world. Companies such as Cisco, Clarent, Genuity, iBasis, ITXC, Lucent, Microsoft, Pagoo, PictureTel, Polycom, RADVision, Siemens, Sonus Networks, VocalTec, and many others have embraced H.323.
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.
The originating location of a videoconference. Usually the site where the speaker or presenter is located.
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.
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.
The inverse multi-plexor uses up to 4 low-cost ISDN-Basic Rate lines to
provide a composite bandwidth of up to 512 kbps. So, the IMUX is well suited for
applications where a high bandwidth connection is needed to support bursty or
infrequent traffic, like videoconferencing, medical image transfer or disaster
recovery.
The IMUX transmits data over either the RS-530 or V.35 interface and performs at
synchronous data transfer rates from 56 kbps to 512 kbps. At rates over 64 kbps,
the BONDING Mode 1 inverse multiplexing protocol synchronizes data over up to
eight 64 kbps B channels. ISDN network termination is designed into the IMUX,
eliminating the need and expense of up to 4 NT-1s.
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.
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.
Short for Internet Protocol
An identifier for a computer or device on a TCP/IP network. Networks using the TCP/IP protocol route messages based on the IP address of the destination. The format of an IP address is a 32-bit numeric address written as four numbers separated by periods. Each number can be zero to 255. For example, 1.160.10.240 could be an IP address.
Within an isolated network, you can assign IP addresses at random as long as each one is unique. However, connecting a private network to the Internet requires using registered IP addresses (called Internet addresses) to avoid duplicates.
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." There are two types of ISDN. Basic Rate Interface (BRI) and Primate Rate Interface (PRI). ISDN BRI 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. 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).
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.
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.
Interexchange carrier, long distance service providers in the U.S. that
provide inter-LATA service.
ISO Joint Picture Expert Group standard for the compression of still pictures.
Kilobits per second. Refers to transmission speed of 1,000 bits per second.
Refers to a unit of frequency equal to 1,000 Hertz.
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.
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.
Long Distance Carrier
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.
A dedicated circuit typically supplied by the telephone company.
A test done by the phone company to make sure the customer is within the maximum distance of 18,000 feet from the central office that services that customer. Note however that ISDN service could be available at a longer distance than that with a mid-span repeater.
Multiplexed analog component color video transmission system. Subtypes refer to the various methods used to transmit audio and data signals.
Metropolitan Area Network.
The amount of signal in dB by which a satellite videoconference system exceeds the minimum levels required for operation.
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.
Megabits per second.
Multipoint Control Unit (Bridge)
Refers to a frequency equal to one million Hertz, or cycles per second.
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.
The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or data signal.
Moving Picture Experts Group. MPEG has established standards for compression and storage of motion video.
Techniques that allow a number of simultaneous transmissions over a single circuit.
Communication configuration in which several terminals or stations are connected. Compare to point-to-point, where communication is between two stations only.
(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.
The ability of a videoconferencing system to halt transmission of outgoing audio. There is usually a button on the remote control labeled audio mute. Audio mute allows the facilitation of side-bar conversations without interrupting the other sites during the videoconference.
A low-capacity communications circuit/path. It usually implies a speed of 56Kbps or less. (Contrast with wideband and broadband)
The Industry Standard in data collaboration software. The software allows multiple users to connect their computers together for the purpose sharing documents, collaborating on spreadsheets, budgets and other software driven applications.
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.
The Network Termination Device is required to connect an ISDN terminal equipment to an ISDN line. The NTD connects to the two-wire line (twisted pair copper wiring) that your telephone company has assigned for your ISDN service. Your ISDN service (in North America) will not work if the NTD's plug is not connected to a working electrical outlet. However if your router has a U-interface, the NTD is built-in to the hardware.
Any unwanted and unmodulated energy that is always present to some extent within any signal.
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
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.
Also referred to as business television, it is a form of conferencing that employs satellite distribution of a video program, usually live rather than pre-recorded, from one origination site, the satellite uplink for transmission, the satellite transponder, the satellite downlink equipment, and television monitors for viewing. Audiences at the receive sites may interact with the presenters at the origination site via telephone, fax, or data keypads, but usually do not interact via video.
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)
To pivot a camera in a horizontal direction, tilt is to pivot in the vertical direction.
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.
Picture-in-picture
The smallest element of the computer or television display on the raster scale.
Point-to-point Videoconference
A single location-to-location videoconference where the originating site calls the remote site directly. This type of videoconference is established in much the same way as a telephone call.
(Plain Old Telephone Service): The basic telephone service - standard single line telephones, telephone lines, and access to the public switched network. There are no added features, such as call waiting or call forwarding, with POTS.
Microsoft computer software product used to create slides and interactive presentations. Charts and graphs can be created and imported and manipulated to achieve attractive and interesting business presentations.
(Primary Rate Interface) A larger aggregate than a BRI, a PRI will consist of 24 channel's (T1) or 31 channel's (E1). In either case one channel is reserved for call signaling. For T1s the D-channel is the 24th channel while the E1s use the 16th channel for signaling.
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.
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.
Videoconferencing service offered to the public on a fee-for-usage basis.
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.
See CIF
Quality Of Service
QPSK - Quadrature Phase Shift Keying
System of modulating a videoconference signal.
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.
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.
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.
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.
Videoconferencing using a sophisticated system. Appropriate for large groups (compare to desktop videoconferencing).
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.
A device used to convert a computer's RGB output into a standard composite or S-VHS NTSC video signal. The video output is usually plugged into the videoconference system so presenters can transmit their computer software over the videoconference as a standard video signal.
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.
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.
A single graphical or computer image usually used to demonstrate a point given in a lecture or training session. Most videoconference systems have the ability to transmit a slide during the course of a videoconference.
A still frame image sent by a location over a videoconference. Image may be a video image or a graphic slide. The videoconference system usually has a button which automatically sends the snapshot and restores motion video.
The range of electromagnetic radio frequencies used in transmission of voice, data and television.
Multipoint conferences involving locations limited to different connection speeds. The bridge will connect multiple sites at the bandwidths those sites are able to accept. Typical speed match conferences may include a series of normal bandwidth locations (eg. 384Kbps) and a location that only operates at 128Kbps or perhaps is on a switched 56 network.
(Service Profile Identifier): The ISDN switch needs to have a unique identification number for each ISDN set to which it sends calls and signals. SPID's are particular to North American BRI implementations. SPID's allow multiple ISDN devices, such as voice and data, to share the local loop while supporting the multiple services simultaneously. SPID's identify the services that are ordered from the carrier.
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.
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.
Standard Compression Algorithm
An algorithm convention for compression of a video signal. Adherance 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 is the most widely accepted standard in use today.
See Video Streaming
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 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.
In North America, a digital channel which communicates at 45.304 Mbps.
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.
The ability of a camera to move in a vertical manner either manually or via remote control.
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.
Generic term for telephone company. Can refer to an Interexchange Carrier or to the Local Exchange Carrier.
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.
The earth station used to transmit signals for a satellite videoconference.
Computerized switching system which allows multipoint videoconferencing.
Communication across long distances with video and audio contact that may also include graphics and data exchange.
Video Streaming is the ability to play video and audio smoothly over the internet or an intranet in either a real-time or archived manner. Often, data collaboration is used in conjunction with video streaming to create a remote interactive meeting or training environment.
A clear acetate sheet used on an overhead projector to present information graphics or drawings.
In Multipoint bridging vernacular, voice activated switching allows any given endpoint to switch become the broadcaster of the videoconference by sensing voice energy from the site that is talking.
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.
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.
A medium-capacity communications circuit/path. It usually implies a speed from 64Kbps to 1.544Mbps. (Contrast with broadband and narrowband)
Transmission via radio waves or satellite. Wireless transmission is likely to be used in many computer networks of the future.
A set of packet switching standards published by the CCITT.
The ability of a camera's lens configuration to move closer or further away from its subject while maintaining its ability to focus.