Silence Suppression; Appendix B; Silence Suppression On Half-Duplex Links - Nortel BCM50 Configuration Manual

Appendix B

Silence suppression

The following describes using silence suppression on half-duplex and full-duplex links:
Silence suppression, also known as voice activity detection, reduces bandwidth requirements by as
much as 50 percent. The following explains how silence suppression functions on a Business
Communications Manager network.
G.711 and G.729, support Silence suppression.
A key to VoIP Gateways in business applications is reducing WAN bandwidth use. Beyond
speech compression, the best bandwidth-reducing technology is silence suppression, also known
as Voice Activity Detection (VAD). Silence suppression technology identifies the periods of
silence in a conversation, and stops sending IP speech packets during those periods. Telco 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 suppression, 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 suppression on peak bandwidth requirements differ, depending on whether
the link is half-duplex or full-duplex.

Silence suppression on half-duplex links

The following figure shows the bandwidth requirement for one call on a half-duplex link without
silence suppression. Since the sender and receiver share the same channel, the peak bandwidth is
double the full transmission rate. Because voice packets are transmitted even when a speaker is
silent, the average bandwidth used is equal to the full transmission rate.
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