On 09/05/2010 04:08 AM, Vikram Ragukumar wrote: > Hello, > > We are in the process of debugging a voice quality issue for a client of > ours that is a VoIP services provider. The client uses a softphone that > runs on a pjsip stack. > > When placing a call using the softphone, it negotiates the use of G729 > codec with the remote endpoint (ptime = 20ms). The endpoint transmits RTP > packets with encoded G729 payload. VAD/DTX is enabled. We see that the > last frame transmitted by the carrier side endpoint, before the beginning > of a period of discontinuous transmission has 20 bytes of payload. We have > verified that VAD/DTX is used by the carrier side endpoint by noting that > there exist successive RTP packets that differ by 1 in their sequence > number but have a timestamp difference> 160 and MARK bits are set in the > RTP header. > > Our understanding is that for G729B, the SID frame that is transmitted > before a period of discontinuous transmission has a size of 2 bytes. > However we see that ALL RTP packets sent by the carrier side end point has > a length of 20 bytes. > > Has anybody else seen this behavior from a carrier side endpoint ? Is > there an RFC or document that specifies Your understanding is correct. You need to infer from the length of the last frame being 2 bytes that it is a SID frame, and SID frames should only ever occur as the last frame in an RTP packet. If the SDP negotiation has agreed to used the annex B (CNG/DTX/VAD) option for G.729 you would normally expect to see a SID frame at the end of transmission. If the SDP negotiation has agree to do CNG/DTX/VAD by another means (which it can do) you won't see those SID frames. Even when annex B is used, I think some systems may miss out the SID frames. The use of proper annex B processing requires additional patent licence payments, and I suspect some people try to fudge things to save a little cost.
Steve -- _____________________________________________________________________ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
