Subject: [AVT] G.729 Annex B in RTP

From: Steve Underwood <[email-masked]>
Date: Wed, 22 Oct 2008 23:03:17 +0800

Hi,

I have a question about the use of G.729 Annex B (DTX/VAD/CNG) in RTP.

RFC3551 says an RTP packet may contain a variable number of 10ms G.729 
frames, and the last frame can be the little 2 byte SID frame to 
commence a period of no transmission. Now, let's say we set the 
packetisation to 40ms, and the first 10ms frame of audio we process 
produces a SID frame. We send an RTP packet containing just this. Let's 
say the next 10ms frame is declared to be voice, and a 10 byte G.729 
frame is produced. The next frame is declared to be another start of 
silence, so a SID frame is produced. We send the 10 bytes + 2 bytes as 
an RTP frame, but its now only 20ms since the last RTP packet. Our DTX 
feature, rather than reduce the data rate, has actually created a peak. 
Under difficult conditions it looks like G.729 might produce bursts of 
high traffic in this way.

Does anyone apply any algorithms to mitigate this behaviour?

Regards,
Steve

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