[asterisk-users] Possible malformed G729B - SID (VAD/DTX) frames from carrier endpoint ?

[Steve Underwood]

  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