Decoder: A piece
of hardware or software that is used to convert video or audio (typically) from
the digital form used in transmission or storage into a form that can be viewed.
E
E.164: The designation for one of the international format standards for phone numbers. An E.164 alias is simply an alias in phone number format for your video endpoint's IP address. Dialing with E.164 aliases instead of IP addresses helps maintain network security and allows endpoints to maintain dialing continuity if they do not have a static IP address or if their static IP has been changed.
F
Full Duplex: Sending
data in both directions at the same time. Usually higher quality, but requires
more bandwidth. In video conferencing, full duplex will be much more natural
and useable. Cheap speakerphones are half duplex, whereas more expensive ones
are full duplex.
Fractional T-1: FT-1
for 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.
G
G.7xx: A family of
ITU standards for audio compression.
Gatekeeper: In the
H.323 world, the gatekeeper provides several important functions. First, it
controls access to the network, allowing or denying calls and controlling the
bandwidth of a call. Second, it helps with address resolution, making possible
email type names for end users, and converting those into the appropriate network
addresses. They also handle all tracking and billing, call signaling, and the
management of gateways.
Gateway: gateways
provide a link between the H.323 world and other video conferencing system.
A common example would be a gateway to a H.320 (ISDN) video conferencing system.
H
H.261: ITU standard
for video coding for videoconferencing. H.261 is a discrete cosine transform
(DCT) based algorithm for video in the 64kb's to 2mb/s range. All H.323 compliant
videoconferencing system are required to support this codec.
H.263: ITU standard
for video coding within videoconferencing. H.263 offers better compression than
H.261, particularly in the low bitrate range used by modem.
H.320: ITU standard
for videoconferencing over ISDN and fractional T1 lines.
H.323: ITU standard
for videoconferencing over networks that do not guarantee bandwidth, such as
the Internet. H.323 is the standard that this cookbook is recommending that
most users in the education community should be using.
H.324: ITU standard
for video conferencing over standard phone lines.
Half Duplex: A telecommunications
system where data can only flow in one direction at a time. Cheaper speakerphones
are a good example of this, where only one person can talk at a time.
I
IP Address: A 4-byte
number uniquely defining each unit on the Internet. Forms in conjunction with
the Transfer control Protocol (TCP) the TCP/IP
Static IP: A static
or dedicated IP Address is a type of account from an ISP where your computer(s)
are assigned the same IP Address at all times. While this used to be a requirement
for web-site serving, it is usually used today for security purposes.
IP link: The Internet
Protocol. IP is the basic language of the Internet. It was developed by the
government for use in internet-working multiple computer networks together.
J
Jitter: A flickering
on a display screen. Besides a monitor or connector malfunction, jitter can
be caused by a slow refresh rate.
K
L
Luminance: Brightness
M
MCU: In videoconferencing
vernacular, a bridge connects three or more conference sites so that they can
simultaneously communicate. Bridges are often called MCU's - multipoint conferencing
units. In IEEE 802 parlance, a bridge is a device that interconnects LAN's or
LAN segments at the data-link layer of the OSI model to extend the LAN environment
physically. They work with frames (as opposed to packets) of data, forwarding
them between networks. They learn station addresses and they resolve problems
with loops in the topology by participating in the spanning tree algorithm.
Finally, the term bridge can be used in audio conferencing to refer to a device
that connects multiple (more than two) voice calls so that all participants
can hear and be heard.
MPEG:MPEG (Moving
Picture Experts Group) is a series of ISO Standards for digital video and audio,
designed for different uses and day rates.
Multipoint: Communication:
configuration in which several terminals or stations are connected. Compare
to point-to point, where communication is between two stations only.
Multipoint Control Unit
(MCU): A device that bridges together multiple inputs so that more than
three parties can participate in a videoconference. The MCU uses fast switching
techniques to patch the presenter's or speaker's input to the output ports representing
the other participants.
Multipoint Conferencing
Server (MCS)(also MCU): A hardware or software H.323 device that allows
multiple video conferencing (or audio or data) users to connect together. Without
an MCS typically only point to point conferences can take place. Commonly supports
voice activated switching, where whoever is talking is broadcast to all users,
but new systems suppport "Hollywood squares", where multiple windows show each
participant. ITU-T standard H.231 describes the standard way of doing this.
Many current systems only support H.320 (ISDN) but many vendors are working
to upgrade their products to support H.323 (LAN, Internet) as well. In the H.320
space, this functionality is referred to as a multipoint control unit (MCU).
Sometimes these terms are used interchangeably, although they refer to somewhat
different implementations.
N
Network:A group of
stations (computers, telephones, or other devices) connected by communications
facilities for exchanging information. Connection can be permanent, via cable,
or temporary, through telephone or other communications links. The transmission
medium can be physical (copper, wire, fiber optic cable, etc.) or wireless,
for example via satellite.
O
P
Packet: A unit of
information sent across a (packet-switched) network. A packet generally contains
the destination address as well as the data to be sent.
Pan: to pivot a camera
in a horizontal direction, Tilt: is to pivot in the vertical direction.
Pixel: The smallest
element of the computer or television display on the raster scale.
Q
R
Real-Time: The processing
of information that returns a result so rapidly that the interaction appears
to be instantaneous. Telephone calls and videoconferencing are examples of real-time
applications. These kinds of real-time information not only need to be processed
almost instantaneously, but also 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.
S
Switched Port: This
is a type of hub that forwards packets to the appropriate port based on the
packet's address. Since switching hubs forward each packet only to the required
port, they provide much better performance. Most switching hubs also support
load balancing, so that ports are dynamically reassigned to different LAN segments
based on traffic patterns.
T
T1:The transmission
bit rate of 1.544 million bits per second. This is also equivalent to the ISDN
Primary Rate Interface for the U.S. The European T1 or E1 Transmission rate
is 2.048 million bits per second.
T.120: This is the
ITU-T standard (International Telecommunications Union) for document conferencing.
Document conferencing allows two or more people to concurrently view and edit
a document across a network. T>120: is the commonly used name to refer
to a family of distinct standards. Many video conferencing companies were developing
their own implementations of this until Microsoft released its free NetMeeting
software. Now, many companies are using NetMeeting, while perhaps enhancing
it in some way.
U
V
W
X
Y
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