79
bits are 1110) and the multicast IPv6 address is
selected in a range of ff00::/8 (the upper 8 bits are
all 1).
MTU (maximum transmission unit)
The maximum IP packet size value. The optimum
value is decided by Network. If the value is set too
low, the transmission overhead increases, which
leads to a decrease in transmission speed. If it is
too high, the packet is split up, which may cause
a decrease in transmission speed and make
normal transmission impossible.
NTP (Network Time Protocol)
Communications protocol to synchronize the time
of the clocks of devices on the network with the
time distributed by the NTP server. Uses UDP
port number 123.
P frame (predicted frame)
A frame compressed by the inter-frame prediction
of video image compression technology.
Conducts prediction using the previous frame. To
play an image of this frame, the data of the
previous frame must be saved.
Port Number
An identification number to specify to which
application the received data should be given in
the receiving terminal on Network. The range 0 to
65535 is usable. Numbers from 0 to 1023 are
called well-known ports. sPort numbers to the
application registered here can be abbreviated
and if abbreviated, ports registered there are
supposed to be used (ex: HTTP is TCP/no. 80,
POP3 is TCP/no. 110). Numbers 1024 to 49151
are called registered ports, and registered by
general users. Numbers of 49152 to 65535 are
ports users can freely use. Both well-known and
registered ports are controlled by the IANA
(Internet Assigned Numbers Authority). However,
in theory, provided the transmitting end can
determine to which port number of the receiving
terminal to send, any port number can be used,
and for example, operations such as making
HTTP of TCP/no. 81 and POP3 of TCP/118, are
also possible. However, unless you have a
special reason, to avoid confusion, it's desirable
to operate in line with the abovementioned policy.
RTP (real-time transport protocol)
Real-time transport protocol. Used to distribute
sound and video images in real time. UDP is used.
Focusing on real-time rather than data accuracy.
Thus, packets which do not arrive by the
expected time are destroyed (the image instantly
breaks up) and it keeps playing only with the data
which has arrived.
RTSP (real time streaming protocol)
Real-time streaming protocol. Normally No. 554 is
used. Controls real-time data distribution. By
using authentication, viewing is also controlled.
Streaming
One of the data distribution forms on Network.
Plays after downloading. With this technology, live
distribution via the Network medium has become
possible.
Subnet mask
A bit sequence to separate the Network address
and host address from the IP address. For
example, if the subnet mask is 255.255.255.0 in
IPv4, the former 24 bits of the IP address are the
Network address, the latter 8 bit the host address.
TTL (time to live)
Lifetime of the packet to circulate on Network.
Every transit of a node on the Network such as a
router and so on, reduces the total by one, and
when reaching 0, the packet is destroyed. This
prevents packets remaining in the Network due to
setting errors and so on of the Network.
Unicast
A system to transmit data only to a specified sole
terminal. When transmitting the same data to
multiple terminals by unicast, the data quantity to
transmit is doubled by the number of terminals
and a load is placed on the Network.
URL (Uniform Resource Locator)
A string to uniquely specify a resource place on
Internet.
VBR (variable bit rate)
Variable bit rate. A system to maintain a certain
image quality by fixing the compression ratio of
images in video image compression. Depending
on the input image, the bit rate will vary.
