A
Algorithm
Any
step-by-step problem-solving procedure. Transmission
of compressed
video over a communications network requires sophisticated compression algorithms.
Some video conferencing systems offer both proprietary and standard compression
algorithms.
Analog
Signal
A type of signal that encodes voice, video, or data transmitted over wire or
through the air, and is commonly represented as an oscillating wave. An analog
signal can take any value in a range and changes smoothly between values, as
opposed to digital
signals, which is characterized by discrete bits of information in numerical
steps. An analog signal can transmit analog or digital data.
ADC
Analog-to-Digital Conversion
Process of converting analog signals to a digital representation. DAC represents
the reverse translation.
ATM - A synchronous
Transfer Mode
A high bandwidth, High speed (up to 155 Mbps), controlled-delay fixed-size packet switching
and transmission system integrating multiple data types (voice, video, and
data). Uses fixed-size packets also known as "cells" (ATM is often referred
to as "cell relay").
Asynchronous
Transmission
A mode in which the sending and receiving serial
hosts know where a character begins and ends because each byte is framed
with additional bits, called a start bit and a stop bit. A start bit indicates
the beginning of a new character; it is always 0 (zero). A stop bit marks the
end of the character. The time interval between characters may be of varying
lengths. Synchronous data
uses an external reference clock to unify both ends of the data circuit.
Audio
In video communications, electrical signals that carry sounds. The term is
also used to describe systems concerned with sound with recording and transmission;
speech pickup systems, transmission links that carry sounds, amplifiers
and the like.
Autonomous
System
Internet (TCP/IP) terminology for a collection of gateways (routers)
that fall under one administrative entity and cooperate using a common Interior
Gateway Protocol (IGP).
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B
B Channel
A 56Kbps or 64-kbps channel that carries user data on a line using ISDN D-channel signaling.
B-Mac
A method of transmitting and scrambling television signals. In such transmissions
MAC (Multiplexed Analog Component) signals are time-multiplexed with a
digital burst containing digitized sound, video synchronizing,
authorization, and information.
Bandwidth
Bandwidth is the data capacity of a service, measured in thousands of bits
per second (kbps) or millions of bits per second (Mbps). In video conferencing systems
a larger bandwidth is used to spread or "dither" the signal in order to
prevent interference.
Baseband
The basic direct output signal in an intermediate
frequency based obtained directly from a television
camera, video conference television receiver,
or video tape recorder. Baseband signals can
be viewed only on studio monitors. To display
the baseband signal on a conventional television
set a "modulator" is required to convert the
baseband signal to one of the VHF or UHF television
channels which the television set can be tuned
to receive.
Bit
Error Rate
The percentage of received bits in error compared to the total number of bits
received. A bit error rate of 10-6 means that there is an average of one error
per million bits.
bps
Bits per second
A unit of measurement of the speed of data transmission and thus of bandwidth.
Actually a nested acronym, meaning binary digits per second. (lower case is
significant)
Bps (or
BPS)
(8-bit) bytes per second (upper case is significant)
BRI Basic Rate
Interface
One of two ISDN subscriber "interfaces" in
ISDN. 2 voice (B)
channels at 64 kbps channels and 1 data signal (D) channel at 16 kbps.
The B-channels are for voice, video, and data. The D-channel is
for signaling between telephone company switches and for carrying ISDN user-network
messages.
Bridge
In video conferencing vernacular, a bridge connects three or more conference
sites so that they can simultaneously pass data, voice, or video. video conferencing
bridges are often called MCU's -
multipoint conferencing units. (See router).
Broadband
A way of transmitting large amounts of data, voice, and video that is greater
than telephony networks. In ISDN,
broadband channels support rates above the primary rate (1.544 Mbps or
2.048 Mbps). (See wideband and narrowband)
Business
Television
Corporate communications tool involving video transmissions of information
via video conference. Common uses of business television are for meetings, product
introductions and training.
> A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
C
Camera presets
Allows pre-defined camera angles to be programmed into a video conferencing
system.
CCITT
Consultative Committee for international Telegraphy and Telephony
(Now called the International Telecommunications Union's Telecommunications
Standardization Sector or TSS.) The world's leading telecommunications standards
organization responsible for establishing interoperability standards for communications
systems.
CIF
Common Intermediate Format
An international standard for video display formats developed by TSS. The QCIF
format, which employs half the CIF spatial resolution in both horizontal and
vertical directions, is the mandatory H.261 format. QCIF is used for most desktop
video conferencing applications where head and shoulder pictures are sent
from desk to desk.
CODEC
COder-DECoder
In the video conferencing world, a video codec converts analog
video signals from a video camera to digital
signals for transmission over digital circuits, and then converts the digital
signals back to analog signals for display.
Compressed
video
When the vast amount of information in a normal TV transmission is squeezed
into a fraction of its former bandwidth by a codec, the resulting compressed
video can be transmitted more economically over a smaller carrier. Some information
is sacrificed in the process, which may result in diminished picture and sound
quality.
An uncompressed NTSC signal
requires about 90 Mbps of throughput, greatly exceeding the speed of all but
the fastest and shortest of today's networks. Squeezing the video information
can be accomplished by reducing the quality (sending fewer frames in a second
or displaying the information in a smaller window) or by eliminating redundancy.
Compression
Compression is a technique that reduces the quantity of bandwidth or bits required
to encode a block of information so that it occupies less space on a transmission
channel or storage device and a fundamental concept of video communications.
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D
DBS
Direct broadcast video conference
Refers to a service that uses video conferences to broadcast multiple channels
of television programming directly to home mounted small-dish antennas.
D
Channel
A channel that carries WAN synchronization information
on a line using ISDN D-channel
signaling.
Delay
The time it takes for a signal to go from the sending station through the video conference
to the receiving station. This transmission delay for a single hop video conference
connection is very close to one-quarter of a second.
Demodulator
A video conference receiver circuit which extracts or "demodulates" the "wanted" signals
from the received carrier.
Desktop
video conferencing
video conferencing on a personal computer. Most appropriate for small groups
or individuals. Many desktop video conferencing systems support document
sharing. (See Room-based
video conferencing).
Digital
Signal
A way of sending voice, video, or data that reconstructs the signals using
binary codes (1s and 0s) for transmission through wire, fiber optic cable,
video conference, or over air techniques. Digital audio/video signals represented
by discrete variations (in voltage, frequency, amplitude, location, etc.) can
be transmitted faster and more accurately than analog
signals.
Distance
learning
The incorporation of video and audio technologies
so that students can "attend" classes
and training sessions that are being presented at a remote location. Distance
learning systems are usually interactive and are becoming a highly-valuable
tool in the delivery of training and education to widely-dispersed students
or in instances where the instructor cannot travel to the student's site.
Document
sharing
A feature supported by many video conferencing systems that allows participants
of a video conference to view and edit the same computer document.
Domain
Name
In the Internet, a part of a naming hierarchy consisting of a sequence of names
separated by periods (dots) that corresponds to the network number in the IP
address. In the symbolic name john@videoconferencingbridging.com, the domain
name is videoconferencingbridging.com.
DTE
Data Terminal Equipment
As defined in the RS-232 specification,
equipment to which DCE (Data Communications Equipment) is connected, such as
a video conference terminal, LAN bridge or router.
Dual
56
Two switched 56 calls made between video conferencing equipment to allow data
transfer at 112 kbps. The video conferencing equipment performs a two-channel
inverse-multiplexing procedure to assure channel alignment.
DVB
Digital Video Broadcast
The standard for direct broadcast television in Europe and the U.S. Based on
MPEG2 Compression.
> A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
E
Earth Station
The term used to describe the combination or antenna, low-noise amplifier (LNA),
down-converter, and receiver electronics used to receive a signal transmitted
by a video conference.
Echo-cancellation
Process which attenuates or eliminates the acoustic echo effect on video conference
calls. Echo cancellors are largely replacing obsolete echo suppressers.
Echo
Effect
A time-delayed electronic reflection of a speaker's voice. This is largely
eliminated by modern digital echo cancellation.
Echo
suppression
To reduce annoying echoes in the audio portion of a video conference, it silences
all sound when on by temporarily deadening the communication link in one direction.
Unfortunately, not only the echo is stopped but also the remote end's new speech,
which results in clipping.
EIRP
Effective Isotropic Radiated Power
This term describes the strength of the signal leaving the video conference
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 video conference
transmit antenna.
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F
FCIF/QCIF
Standards-based formats for communicating between video conferencing systems
from different vendors. QCIF is one quarter of the resolution of FCIF.
FDX
Full-duplex
Two-way, simultaneous transmission of data; a communication protocol in which
the communications channel can send and receive data at the same time. Compare
to half-duplex, where information can only be sent in one direction at a time.
Fractional
T1
Service offering data rates between 64 kbps (DS0 rate) and 1.536 Mbps (DS1
rate), in specified intervals of 64 kbps. It is typically provided by a carrier
in lieu of a full T-1 connection and is a point-to-point arrangement. A specialized
multiplexer is used by the customer to channelize the carrier's signals.
Frame
rate
Frequency in which video frames are displayed on a monitor, typically described
in frames-per-second (fps). Higher frame rates improve the appearance of video
motion.
Frame
store
A system capable of storing complete frames of video information in digital
form. This system is used for television standards conversion, computer applications
incorporating graphics, video walls and various video production and editing
systems.
Full
duplex audio
2-way audio simultaneously transmitted and
received without any interference or "clipping." A
common feature of room-based
video conferencing systems.
Full-motion
video
In the video conferencing world, the term "full-motion video" is often used
and misunderstood. video conferencing 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. video conferencing vendors often
use "full-motion video" to refer to any system that isn't still-frame. Most
video conferencing systems today run 10 to 15 fps at 112 Kbps.
Full
motion video is equivalent to broadcast television
video with a frame rate of 30 fps for NTSC signals
or 25 fps for PAL signals.
Images are sent in real time and motion is
continuous. Also known as continuous-motion
video.
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G
Gateway
Gateways are points of entrance to and exit from a communications network.
Viewed as a physical entity, a gateway is that node that translates between
two otherwise incompatible networks or network segments.
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H
H.320 / H.323
Sets of widely-used CCITT video compression standards
describing methods to allow video conferencing systems from different manufacturers
to interoperate. They include a number of individual recommendations for coding,
framing, signaling and establishing connections. Three audio algorithms,
G.721, G.722 and G.728, are also included in the standards.
Half
duplex audio
2-way audio transmitted and received in turn (rather than simultaneously) so
only one site can speak at a time. Contrast with full duplex audio.
Handshake
Prior to a video conferencing transmission, the codecs exchange
predetermined electrical signals that allow them to interoperate by they seeking
out a common algorithm.
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I
Internet Gateway
A gateway for
accessing the Internet, which is loosely defined as the complex of wide area
networks (WANs)
joining government, university, corporate and private computers (nodes) in
a vast web of network interconnection.
IP
Internet Protocol
IP
Address
An address that uniquely identifies each host on a network or Internet.
An IP address has a length of 32 bits, and is divided into four 8-bit parts,
each separated by a period, as in 149.122.3.30. This kind of notation is called
dotted decimal notation. Each part can consist of a number between 1 and 255.
In addition to an IP address, you can use a symbolic (domain)
name provided by Domain Name Services (DNS) to designate an Internet address.
ISDN
Integrated Services Digital Network
An international standard for end-to-end digital transmission of voice, data,
and signaling. In a video conference it is a system that provides simultaneous
voice, video, and text transmission between individual desktop
video conferencing systems and group
(room) video conferencing systems.
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J
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K
Kbps
Kilobits per second.
Refers to transmission speed of 1,000 bits per second.
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L
LAN
Local Area Network
A network that interconnects devices over a geographically small area, typically
in one building or a part of a building. The most popular LAN type is Ethernet.
LANs allow the sharing of resources and the exchange of both video and data.
LAN/WAN
Connectivity
This is the practical set of tools, from OS layer protocols to support services,
that make a remote access device an effective link between LANs and WANs.
An effective remote access server must include a host of communications and
translation protocols to fulfill this function.
Latency
The minimum time required to move data from one point to another. Once latency
is present, it cannot be optimized. The cause has to be removed (as in
using an internal device rather than an external one to remove the latency
caused by the serial port). To maximize throughput, use the highest bandwidth
available.
Leased
Lines
A circuit rented for exclusive use twenty-four hours a day, seven days a week
from a telephone company. The connection exists between two predetermined points
and cannot be switched to other locations.
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M
MAC
Media Access Control
A system of rules used to move data from one physical medium to another.
MAC
(A, B, C, D2)
Multiplexed analog component
Color video transmission system. Subtypes refer to the various methods used
to transmit audio and data signals.
Margin
The amount of signal in dB by which a satellite video conference system exceeds
the minimum levels required for operation.
MAX
Media Access Exchange
It supports up to 32 host ports or direct Ethernet connections and up to 8
Mbps to the network. It supports multiple applications, including remote LAN access,
leased line backup and individual video conferencing units, as well as connecting
video conference MCUs to
the digital dial-up network.
MBONE
Multicast / Multimedia Backbone
A collection of Internet routers that
support IP multi-casting. The MBONE is used as a "broadcast" channel on which
various public and private audio and video programs are sent.
Mbps
Megabits per second
Modulation
The process of manipulating the frequency or amplitude of a carrier in relation
to an incoming video, voice or data signal.
Modulator
A device which modulates a carrier. Modulators are found as components in broadcasting
transmitters and in video conference transponders.
Multiplexing
Techniques that allow a number of simultaneous transmissions over a single
circuit.
Multipoint
Communication configuration in which several terminals or stations are connected.
Compare to point-to-point, where communication is between two stations
only.
MCU
Multipoint Control Unit
video conferencing equipment that allows more than three individual video conference
units to connect together to form a multiparty video conference session. The
MCU uses fast switching techniques to patch the presenters or speaker's input
to the output ports representing the other participants.
Multipoint
video conference
video conference with more than two sites. The sites must connect via a video
bridge. (Compare with point-to-point
video conference.)
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N
Narrowband
A low-capacity communications circuit/path. It usually implies a speed of 56Kbps
or less. (Contrast with wideband and broadband)
Network
A group of stations (computers, telephones, or other devices) connected by
communications facilities for exchanging information. Connection can be
permanent, via cable, or temporary, through telephone or other communications
links. The transmission medium can be physical (i.e. fiber optic cable)
or wireless (i.e. satellite).
NT1
Network Terminator Type 1
The NT-1 is physically connected between the ISDN board
of your video conferencing system and your ISDN phone line and converts the
two-wire line coming from your telephone company into a 4-wire line. And provides
network maintenance functions such line maintenance access, timing, and echo
cancellation. NT1s may be built into other pieces of equipment or stand alone.
NTSC
National Television Standards Committee
United States' standard for scanning television signals that has been adopted
by numerous other countries. Frames are displayed at 30 frames per second.
(Other standards: PAL (Europe)
and SECAM (France/former
USSR))
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O
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P
Packets
A block of information sometimes called a cell, frame, data unit, service unit,
or signaling unit. Although each of these elements do have unique attributes,
in essence, all are packets.
PAL
Phase Alternative Line System
The European TV standard for scanning television signals. Frames are displayed
at 25 frames per second. Used in most European countries. (Other standards: NTSC (USA)
and Secam (France/Former
USSR))
Point-to-point
video conference
video conference between two sites. (See Multipoint
video conference.)
POP
Point of Presence
This is a point-of-presence of an Internet service provider, used to facilitate
remote users' access to the range of applications and IP
addresses in the internetwork.
PPP
Point-to-Point Protocol
Provides a standard means of encapsulating data packets sent
over a single-channel WAN link.
It is the standard WAN encapsulation protocol for the interoperability of bridges and routers.
PPP is also supported in workstations, allowing direct dial-up access from
a personal computer to a corporate LAN or
Internet Service Provider. Using PPP ensures basic compatibility with non-Ascend
devices. Both the dialing side and the answering side of the link must support
PPP.
PRI
Primary Rate Interface
An ISDN subscriber
line, consisting of twenty-three 64 kbps B
channels in North America (thirty 64 kbps channels elsewhere) and one 64
kbps D channel,
used for signaling purposes.
Promiscuous
Mode
A Bridging parameter mode that determines
that the Ethernet controller in an Ascend
unit accepts all packets and
passes them up the protocol stack for a higher-level decision on whether
to route, bridge,
or reject them. This mode is appropriate if you are using an Ascend unit
as a bridge.
Proprietary
compression algorithm
A vendor-specific algorithm for compression of
a video signal. A video conferencing system using a proprietary algorithm can
only communicate with a remote site using the same algorithm. Many vendors
also adhere to standard compression algorithms to facilitate communication
across platforms. (i.e .H.320)
Public
room
video conferencing service offered to the public on a fee-for-usage basis.
Px64
Common reference to the CCITT standards
(H.261 et. al.) which describe methods to allow for video conferencing system
interoperability.
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Q
QoS
Quality of Service
Interactive video conferencing requires a high QoS. QOS is important as it
determines the quality of your video call. Low quality of service results in
latency and a jerky picture with poor and inconsistent audio quality.
QPSK
Quadrature Phase Shift Keying
System of modulating a video conference signal.
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R
Real-Time
The processing of information that returns a result so rapidly that the interaction
appears to be instantaneous. video conferencing is an example of a real-time
application. This kind of real-time information not only needs to be processed
almost instantaneously, but it needs to arrive in the exact order it's sent.
A delay between parts of a word, or the transmission of video frames out of
sequence, makes the communication unintelligible.
Receiver (Rx)
An electronic device which enables a particular video conference signal to be
separated from all others being received by an earth
station, and converts the signal format into a format for video, voice
or data.
Room
(Group) based video conferencing
video conferencing using a sophisticated system. Appropriate for large groups
(See Desktop
video conferencing).
Router
A device or setup that finds the best route between any two networks, even
if there are several networks to traverse. Like bridges,
remote sites can be connected using routers over dedicated or switched
lines to create WANs.
RS-232-C
A set of EIA standards specifying various electrical and mechanical characteristics
for interfaces between computers, terminals, and modems. The standard applies
to both synchronous and asynchronous
binary data transmission at rates below 64 kbps.
RS-449
An EIA standard for a 37-pin data communications connector, usually used with
RS-422 or RS-423 electrical specifications.
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S
Secam
A color television system developed by the French and used in the former USSR.
Secam operates with 625 lines per picture frame and 50 cycles per second. It
is incompatible with the European PAL system
or the U.S. NTSC system.
Serial
Host
A device, such as a video conferencing codec,
that is connected to a serial host port communicating over a point-to-point
link. To a serial host, the MAX appears
to be a cable or DCE (Data Communications Equipment).
Serial
Host Port
The V.35, RS-499,
or X.21 port
on the MAX.
Serial
Host Port Module
A module on the MAX that
connects to a serial host through its serial host port.
Standard
compression algorithm
An algorithm convention
for compression of
a video signal. Adherence to standards allows communication among a wide variety
of video conferencing systems, though not with the same clarity as two similar
systems using a proprietary algorithm. H.320
/H.323 are the most widely accepted standards in use today.
Switched
56
A dial-up network-based service providing a data channel operating at a rate
of 56 kbps. Also a type of network access line, used to provide access to switched
56 network services. SDSAF
Switched Digital Services Applications Forum
A consortium of equipment vendors, service vendors, and users, with the goal
of advancing the state of switched digital services.
SVC
Switched Virtual Circuit
A path over a packet-switched network that appears to be a dedicated circuit,
but in fact the connection only stays up as long as needed, and then ends.
Synchronization
In serial data transmission, a method of ensuring that the receiving end can
recognize characters in the order in which the transmitting end sent them,
and can know where one character ends and the next begins. Without synchronization,
the receiving end would perceive data simply as a series of binary digits
with no relation to one another. Synchronous communication relies on a
clocking mechanism to synchronize the signals between the sending and receiving
machines. (See Asynchronous
Transmission)
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T
T1
In North America, T1 service delivers 1.544 Mbps, whereas ISDN service
delivers 128 kbps. The data travels over the line at the same speed, but for
T1 lines, the capacity is twelve times that of ISDN. Typically channelized
into 24 DS0s, each capable of carrying a single voice conversation or data
stream. The European T1 or E1 transmission rate is 2.048 million bits per second.
T3
DS-3
In North America, a digital channel which communicates at 45 Mbps, or 28 T1
lines.
T1
PRI line
A T1 line that uses 23 B
channels for user data, and one 64 kbps D
channel for ISDN D-channel
signaling. This type of PRI line is a standard in North America, Japan, and
Korea.
TDMA
Time division multiple access.
Refers to a form of multiple access where a single carrier is the shared by
many users. Signals from earth
stations reaching the video conference consecutively are processed in time
segments without overlapping.
Telecommuter
A work-at-home computer user who connects to the corporate LAN backbone
using remote access technologies.
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U
Uplink
The earth
station used to transmit signals for a satellite video conference.
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V
V.35
Commonly used to describe electrical characteristics and connector characteristics
for a high speed synchronous interface
between DTE and
DCE. Originally V.35 described a 48 kbps group band modem interface with
electrical characteristics defined in an appendix. Although V.35 is considered
obsolete and no longer published by the CCITT,
its legacy lives on in the data communications world in the form of the electrical
characteristics originally described in the appendix.
Video
bridge
Computerized switching system which allows multipoint
video conferencing.
video conferencing
Communication across long distances with video
and audio contact that may also include graphics
and data exchange. Digital video transmission
systems typically consist of camera, codec (coder-decoder),
network access equipment, network, and audio system.
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W
WAN
Wide Area Network
A data network typically extending a LAN outside
a building or beyond a campus. Typically created by using bridges or routers to
connect geographically separated LANs. WANs include commercial or educational
dial-up networks such as CompuServe, InterNet and BITNET.
Whiteboarding
A term used to describe the placement of shared
documents on an on-screen "shared
notebook" or "whiteboard." video conferencing software includes tools that enable
you to work with familiar tools to mark up the electronic whiteboard much like
you do with a traditional wall mounted board.
Wideband
A medium-capacity communications circuit/path. It usually implies a speed from
64Kbps to 1.544Mbps. (Contrast with broadband and narrowband)
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X
X.21
A set of CCITT specifications
for an interface between DTE
and DCE for synchronous operation
on public data networks. Includes connector, electrical, and dialing specifications.
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Y
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