Endpoints sending audio as an RTP stream are not required to send packets during silent periods. The capability to stop sending RTP packets during silent periods is known as “Silence Suppression” or VAD (Voice Activity Detection). Whether to use Silence Suppression is usually a configuration option on endpoints.
When processing a stream of RTP packets, here is what RFC 3389 has to say about detecting Silence Suppression:
RTP allows discontinuous transmission (silence suppression) on any audio payload format. The receiver can detect silence suppression on the first packet received after the silence by observing that the RTP timestamp is not contiguous with the end of the interval covered by the previous packet even though the RTP sequence number has incremented only by one. The RTP marker bit is also normally set on such a packet.
September 17, 2011
Phone System 