529Networking Configuration GuideAppendix BSilence suppressionThe following describes using silence suppression on half-duplex and full-duplex links:Silence suppression, also known as voice activity detection, reduces bandwidth requirements by asmuch as 50 percent. The following explains how silence suppression functions on a BusinessCommunications Manager network.G.711 and G.729, support Silence suppression.A key to VoIP Gateways in business applications is reducing WAN bandwidth use. Beyondspeech compression, the best bandwidth-reducing technology is silence suppression, also knownas Voice Activity Detection (VAD). Silence suppression technology identifies the periods ofsilence in a conversation, and stops sending IP speech packets during those periods. Telco studiesshow that in a typical telephone conversation, only about 36% to 40% of a full-duplexconversation is active. When one person talks, the other listens. This is half-duplex. There areimportant periods of silence during speaker pauses between words and phrases. By applyingsilence suppression, average bandwidth use is reduced by the same amount. This reduction inaverage bandwidth requirements develops over a 20-to-30-second period as the conversationswitches 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 whetherthe link is half-duplex or full-duplex.Silence suppression on half-duplex linksThe following figure shows the bandwidth requirement for one call on a half-duplex link withoutsilence suppression. Since the sender and receiver share the same channel, the peak bandwidth isdouble the full transmission rate. Because voice packets are transmitted even when a speaker issilent, the average bandwidth used is equal to the full transmission rate.
