Network Layer: OSI Layer 3
Provides upper layers with independence from the
data transmission and switching technologies used
to connect systems; responsible for establishing,
maintaining, and terminating connections. Respon-
sible for routing, switching, and subnetwork access
across the entire OSI environment.
Network Time Protocol (NTP)
An Internet standard protocol (RFC 1305) built on
top of UDP/IP, port 123, that assures accurate local
timekeeping with reference to international time
standards (radio, satellite, atomic clocks) via the
Internet. The protocol is capable of synchronizing
distributed clocks within milliseconds over long
time periods.
NTSC and PAL resolutions
In North America and Japan, the NTSC standard
(National Television System Committee) is the
predominant analog video standard, while in Europe
the PAL standard (Phase Alternation by Line) is
used. Both standards originate from the television
industry. NTSC has a resolution of 480 horizontal
lines, and a frame rate of 30 fps. PAL has a higher
resolution with 576 horizontal lines, but a lower
frame rate of 25 fps. The total amount of information
per second is the same in both standards.
Packet
Generic term used to describe a contiguous device
of data with attached control information. Does not
distinguish between frames and cells.
Parity
A data-checking technique, which uses an extra bit.
If you send one extra-bit for each data bits to make
sure your data got there intact, add and send one
more bit - a parity bit. Bit parity could be odd or
even. If it is even, you assign 0 to the parity bit in
case when the sum of data bits is a even number
and 1 if it is odd. It is odd, if the logics is reversed.
After the data bits has arrived, the system will sum
data bits and check if it matches the eve-or-oddness
predicted by the parity byte.
PAT (Program Association Table)
Data appearing in packets having PID code of zero
that the MPEG decoder uses to determine which
programs exist in a Transport Stream. PAT points to
PMT, which, in turn, points to the video, audio, and
data content of each program.
PSI (Program Specifi c Information)
Information that keeps track of the different pro-
grams in an MPEG transport stream and in the
elementary streams in each program. PSI includes
PAT, PMT, NIT, CAT, ECM, and EMM.
PMT (Program Map Tables)
The tables in PAT that point to video, audio, and data
content of a transport stream.
PCR (Program Clock Reference)
The sample of the encoder clock count that is sent in
the program header to synchronize the decoder clock.
PCRI (Interpolated Program Clock Reference)
A PCR estimated from a previous PCR and used
to measure jitter.
PID (Program Identifi er)
A 13- bit code in the transport packet header.
PID 0 indicates that the packet contains a PAT PID.
PID 1 indicates a packet that contains CAT.
The PID 8191 (all 1s) indicates null (stuffi ng)
packets. All packets belonging to the same elemen-
tary stream have the same PID.
Ping (Packet InterNet Groper)
Ping is a program that “bounces” a request off of
another computer over a network to see if the
remote computer is still responding. If the ping
comes back, the remote computer is still alive.
Point-to-point
A virtual circuit, usually circuit switched, established
between exactly two end points. End point technol-
ogy is typically some bit serial interface, although a
wide range of line capacity could be used.
Protocol
A formal description of messages to be exchanged
and rules to be followed for two or more systems to
exchange information.
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 inconsis-
tent audio quality.
Ring
A network topology in which all nodes are connected
in a closed loop. Transmission is permitted by
passing a token around the ring; a node copies out
messages addressed to it and copies in messages
addressed to others. Message receipt verifi cation
can also be performed with the token.
RMON (Remote Network Monitoring Management
Information Base - RFC 1271)
An agent (embedded or stand-alone) that can
continuously collect performance, fault, and confi gu-
ration statistics, perform diagnostic operations,
operate autonomously, generate management traps,
and return collected data to a management station.
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Glossary