Nortel VoIP Gateway, Configuration Manual

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Norstar VoIP Gateway Configuration Guide
Appendix C
Silence compression
This section describes using silence compression on half duplex and full duplex links:
• “Silence Compression on Half Duplex Links” on page 168
• “Silence compression on Full Duplex Links” on page 170
• “Comfort noise” on page 172
Silence compression reduces bandwidth requirements by as much as 50 per cent. This section
explains how silence compression functions on a Norstar VoIP Gateway network. For information
about enabling silence compression on the VoIP Gateway, refer to
“Configuring the Channel
Settings” on page 76 .
G.723.1 and G.729, Annex B support Silence compression.
A key to VoIP gateways in business applications is reducing WAN bandwidth use. Beyond speech
compression, the best bandwidth-reducing technology is silence compression, also known as Voice
Activity Detection (VAD). Silence compression technology identifies the periods of silence in a
conversation, and stops sending IP speech packets during those periods. Telephone company
studies show that in a typical telephone conversation, only about 36% to 40% of a full-duplex
conversation is active. When one person talks, the other listens. This is half-duplex. There are
important periods of silence during speaker pauses between words and phrases. By applying
silence compression, average bandwidth use is reduced by the same amount. This reduction in
average bandwidth requirements develops over a 20-to-30-second period as the conversation
switches from one direction to another.
When a voice is being transmitted, it uses the full rate or continuous transmission rate.
The effects of silence compression on peak bandwidth requirements differ, depending on whether
the link is half-duplex or full duplex.