Command Reference Guide

42 ● compression 3Com Router 5000 Family and Router 6000 Family
Command Reference
■ Bytes coded in a time unit = packet assembly interval X media stream bandwidth.
■ Packet length (IP) = IP header + RTP header + UDP header + voice information
length = 20+12+8+data
■ Packet length (IP+PPP) = PPP header + IP header + RTP header + UDP header +
voice information length = 6+20+12+8+data
■ Network bandwidth = Bandwidth of the media stream X packet length / bytes
coded in a time unit
Since IPHC compression is affected significantly by network stability, it cannot achieve
high efficiency unless line is of high quality, network is very stable, and packet loss
does not occur or seldom occurs. When the network is unstable, IPHC efficiency
drops drastically. With best IPHC performance, IP (RTP) header can be compressed to
2 bytes. If PPP header is compressed at the same time, a great deal of media stream
bandwidth can be saved. The following table shows the best IPHC compression
efficiency of codec algorithms with packet assembly interval of 30ms.
Tabl e 10 Compression efficiency of IPHC+PPP header
Only when there is an intersection (a compression mode recognized by both parties)
in the voice compression modes owned by the two communication parties can the
two parties establish normal communication with each other. If there is no
consistency in the compression modes set for the equipment at the two ends
connected to each other, or there is no common compression method, the calling will
fail.
Coding
modes
Bytes
coded in a
time unit
Before compression After IPHC+PPP compression
Packet
length
IP+PPP
Network
bandwidt
h IP+PPP
Packet
length
IP+PPP
Network bandwidth IP+PPP
G.729 30 76 20.3 kbps 34 9.1 kbps
G.723r63 24 70 18.4 kbps 28 7.4 kbps
G.723r53 20 66 17.5 kbps 24 6.4 kbps
G.726r16 60 106 28.3 kbps 64 17.1 kbps
G.726r24 90 136 17.5 kbps 94 25.1 kbps