The timer receive-rtcp command uses library-based detection and the receipt of either Real-Time Protocol (RTP) or RTCP packets is considered activity on a call. Silence detection occurs only if there are no packets received for both RTP and RTCP.
When the ip rtcp report interval and timer receive-rtcp commands are used, the gateway uses RTCP report detection, rather than RTP packet detection, to determine whether calls on the gateway are still active or should be disconnected. RTCP report detection is therefore more reliable than RTP packet detection because there can be periods during voice calls when one or both parties are not sending RTP packets.
One common example of a voice session in which no RTP is sent is when a caller dials into a conference call and mutes that endpoint. If voice activity detection (VAD, also known as silence suppression) is enabled, no RTP packets are sent while the endpoint is muted. However, the muted endpoint continues to send RTCP reports at the interval specified by the ip rtcp report interval command.
The timer receive-rtcp timer
argument (or m
factor for multiplication factor) is multiplied by the interval that is set using the ip rtcp report intervalcommand. If no RTP or RTCP packets are received during the calculated interval, the call is disconnected. The gateway signals the disconnect to the VoIP network and the time-division multiplex (TDM) network so that upstream and downstream devices can clear their resources. The gateway sends a Q.931 DISCONNECT message to the TDM network and a SIP BYE or H.323 ReleaseComplete message to the VoIP network to clear the call when the timer expires. The Q.931 DISCONNECT message is sent with a cause code value of 3 (no route) for SIP calls and a cause code value of 41 (temporary failure) for H.323 calls. No Q.931 Progress Indicator (PI) value is included in the DISCONNECT message.
To show timer-related output for SIP calls, use the debug ccsip events command. To show timer-related output for H.323 calls, use the debug cch323 h225command.