By its very nature, the IP (Internet Protocol) standard sends information in packets over the
Internet, which are then transmitted and reassembled for the end user. When sending control
commands or data, lossless transmission is needed over the IP packet network. Transmission
control protocol (TCP) runs atop the IP network guaranteeing that the data packets are lossless,
arriving error free and in their proper sequence. Hence the connotation TCP/IP. A drawback to
this TCP protocol is that transmissions may be delayed to ensure accuracy. Thus TCP/IP or LAN
networks may induce latency effects. This standard, while good for data transmission is bad for
sending continuous video or audio data streams over the Internet.
To allow more contiguous flow of audio and video data over IP networks a lossy, unreliable
(versus reliable TCP) protocol has been developed called User Datagram Protocol. Under the
H.323 standard the IP stack uses the unreliable User Datagram Protocol for audio and video
that promises nothing more than best effort delivery . This means that video and audio will
be transmitted with possible gaps in the data stream (creating possible jumps or delayed video
with audio that may briefly drop out or modulate). The quality of DVC depends on the size of
the network pipline, the speed of the network, the current load on the network, and the speed and
efficiency of the desktop terminal. Within the networking community the H.323 standard then
does not provide a guaranteed Quality of Service (QoS). While this may sound less than perfect,
the advantage is that networks can now support DVC without changing the current network
infrastructure.
Codecs
In essence the H.323 standard is comprised of several protocol standards. The audio codec that
compresses and decompresses the transmission of audio is G.711. Several other audio codecs are
supported in H.323 dependent on network traffic; some like G.723 which transfer audio at bit
rates lower than 56 kbs (preferred for <56K connectivity). For a more indepth description of
audio codecs as a function of connectivity or quality transmission refer to Intel’s description at:
http://www.intel.com/proshare/conferencing/h323/endpts.htm
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Komentarze do niniejszej Instrukcji