3Com MSR 50, MSR 30 manual Algorithm, Compression efficiency of IPHC+PPP header

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	Table 672	G.729 algorithm
			Packet		Packet
	Packet	Bytes	length	Network	length	Network
	assembly	coded in a	(bytes)	bandwidth	(bytes)	bandwidth	Coding
	interval	time unit	IP	IP	IP+PPP	IP+PPP	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.
n	■ 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.

Table 673 Compression efficiency of IPHC+PPP header

		Before compression		After IPHC+PPP compression
		Packet
	Bytes	length	Network	Packet length	Network
	coded in a	(bytes)	bandwidth	(bytes)	bandwidth
Codec	time unit	IP+PPP	IP+PPP	IP+PPP	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

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.