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IP Telephony: Packet-based Multimedia Communications Systems by Jean-Pierre Petit, David Gurle, Olivier Hersent

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Chapter 7. Network dimensioning

Simple compressed voice flow model

Voice coders

Most recent vocoders implement a voice activity detection algorithm, which uses significantly less bandwidth in silence periods than in active speech periods. For most coders the activity bitrate is constant, and we call it M. In silence periods, very basic coders will have a null bitrate, but coders such as G.723.1 or G.729 actually send some information to describe the background noise level and other parameters. We will call the bitrate during silence periods m.

m and M values for popular coders are shown in Table 7.1.

Table 7.1. m and M values for popular coders

Codec

Frames/IP packet

M (kbit/s)

m (kbit/s)

G.723.1 (5.3 kbit/s)

4

8

3.73

G.723.1 (6.4 kbit/s)

4

9.07

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