Glossary of Terms
A B C D E F G H I K L M N P Q R S T U V W X
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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
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.
Process of converting analog signals to a digital representation. DAC
represents the reverse translation.
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").
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.
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.
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).
A 56Kbps or 64-kbps channel that carries user data on a line using ISDN
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,
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.
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.
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.
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
Bps (or BPS). (8-bit) bytes per second (upper case is significant)
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
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).
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)
Corporate communications tool involving video transmissions of information
via videoconference. Common uses of business television are for meetings,
product introductions and training.
Allows pre-defined camera angles to be programmed into a videoconferencing
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.
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.
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.
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.
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 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.
Direct broadcast videoconference
Refers to a service that uses videoconferences to broadcast multiple channels
of television programming directly to home mounted small-dish antennas.
A channel that carries WAN synchronization information on a line using
ISDN D-channel signaling.
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.
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. Many desktop videoconferencing systems support document
sharing. (See Room-based videoconferencing).
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.
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.
A feature supported by many videoconferencing systems that allows participants
of a videoconference to view and edit the same computer document.
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 email@example.com,
the domain name is videoconferencingbridging.com.
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
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.
Digital Video Broadcast
The standard for direct broadcast television in Europe and the U.S. Based
on MPEG2 Compression.
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.
Process which attenuates or eliminates the acoustic echo effect on videoconference
calls. Echo cancellors are largely replacing obsolete echo suppressers.
A time-delayed electronic reflection of a speaker's voice. This is largely
eliminated by modern digital echo cancellation.
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.
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.
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.
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.
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.
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.
2-way audio simultaneously transmitted and received without any interference
or "clipping." A common feature of room-based videoconferencing
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.
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.
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.
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.
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.
Prior to a videoconferencing transmission, the codecs exchange predetermined
electrical signals that allow them to interoperate by they seeking out
a common algorithm.
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.
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 18.104.22.168. This kind of notation
is called dotted decimal notation. Each part can consist of a number between
1 and 255.
to an IP address, you can use a symbolic (domain) name provided by Domain
Name Services (DNS) to designate an Internet address.
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.
Kilobits per second.
Refers to transmission speed of 1,000 bits per second.
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.
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.
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.
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.
Media Access Control
A system of rules used to move data from one physical medium to another.
MAC (A, B, C, D2)
Color video transmission system. Subtypes refer to the various methods
used to transmit audio and data signals.
The amount of signal in dB by which a satellite videoconference system
exceeds the minimum levels required for operation.
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
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.
Megabits per second
The process of manipulating the frequency or amplitude of a carrier in
relation to an incoming video, voice or data signal.
A device which modulates a carrier. Modulators are found as components
in broadcasting transmitters and in videoconference transponders.
Techniques that allow a number of simultaneous transmissions over a single
Communication configuration in which several terminals or stations are
connected. Compare to point-to-point, where communication is between two
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.
Videoconference with more than two sites. The sites must connect via a
video bridge. (Compare with point-to-point videoconference.)
A low-capacity communications circuit/path. It usually implies a speed
of 56Kbps or less. (Contrast with wideband and broadband)
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).
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.
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))
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.
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))
Videoconference between two sites. (See Multipoint videoconference.)
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
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
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.
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.
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)
Videoconferencing service offered to the public on a fee-for-usage basis.
Common reference to the CCITT standards (H.261 et. al.) which describe
methods to allow for videoconferencing system interoperability.
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.
Quadrature Phase Shift Keying
System of modulating a videoconference signal.
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.
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.
(Group) based videoconferencing
Videoconferencing using a sophisticated system. Appropriate for large
groups (See Desktop videoconferencing).
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
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.
An EIA standard for a 37-pin data communications connector, usually used
with RS-422 or RS-423 electrical specifications.
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
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).
The V.35, RS-499, or X.21 port on the MAX.
Host Port Module
A module on the MAX that connects to a serial host through its serial
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.
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.
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.
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.
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)
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.
In North America, a digital channel which communicates at 45 Mbps, or
28 T1 lines.
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.
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.
A work-at-home computer user who connects to the corporate LAN backbone
using remote access technologies.
The earth station used to transmit signals for a satellite videoconference.
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.
Computerized switching system which allows multipoint 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.
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.
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.
A medium-capacity communications circuit/path. It usually implies a speed
from 64Kbps to 1.544Mbps. (Contrast with broadband and narrowband)
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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