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■ Packet assembly interval is the duration to encapsulate information into a voice
packet.
■ 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.
Two communication parties can communicate normally only if they share some
identical coding/decoding algorithms. If the codec algorithm between two
connected devices is not consistent, or the two devices share no common
coding/decoding algorithms, the calling will fail.
Table 672 G.729 algorithm
Packet
assembly
interval
Bytes
coded in a
time unit
Packet
length
(bytes)
IP
Network
bandwidth
IP
Packet
length
(bytes)
IP+PPP
Network
bandwidth
IP+PPP
Coding
latency
10 ms 10 50 40 kbps 56 44.8 kbps 10 ms
20 ms 20 60 24 kbps 66 26.4 kbps 20 ms
30 ms 30 70 18.7 kbps 76 20.3 kbps 30 ms
G.729 algorithm: media stream bandwidth 8 kbps, minimum packet assembly interval 10 ms.
Table 673 Compression efficiency of IPHC+PPP header
Codec
Bytes
coded in a
time unit
Before compression After IPHC+PPP compression
Packet
length
(bytes)
IP+PPP
Network
bandwidth
IP+PPP
Packet length
(bytes)
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
G.726r32 120 166 44.3 kbps 124 33.1 kbps
G.726r40 150 196 52.3 kbps 154 41.1 kbps