Subject: Re: RTP payload types in draft-ietf-avt-profile-new-08.txt

From: Stephen Casner <[email-masked]>
Date: Thu, 13 Jul 2000 22:29:56 -0700 (Pacific Daylight Time)

Referring back to a discussion in April:

On Thu, 20 Apr 2000, Orion Hodson wrote:

> G.729 is different because the frame type is not included within each
> frame and several distinct frame types exist between the original
> recommendation and it's annexes (audio at 6.4, 8, and 11.8 kbit/s, and
> CNG).  Distinct payload identifiers must be advertised and negotiated
> for each of the available encodings as receivers cannot decode the
> data otherwise.

The RTP profile currently only specifies G.729/A at 8kb/s, in which
the data frames are 10 bytes and the CNG frames are 2 bytes.  Frame
packing rules are specified to allow the CNG frames to be
distinguished by the overall payload length.

I presume that the frame sizes are different for the G.729 annexes
with specify operation at 6.4 and 11.8kb/s, and I agree that different
payload identifiers must be used for these.  It would be possible to
select operation at one of these other rates using an SDP a=fmtp
attribute with a bitrate=6.4 parameter, but I agree that it would be
more appropriate to use a different encoding name such as G729C (or
whatever the annex letter is).

However, I don't know even know what the annex letters are because I
don't track this activity in the ITU.  Nobody has proposed the
addition of these encodings in the RTP profile nor provided the frame
size description and packing rules.  Does Freg Burg (who provided the
existing G.729 information) or anyone else want to do this?  It may be
best to do in a separate payload format specification rather than
trying to add it to the existing profile draft.

Another part of that same discussion was about G.723 for which the
bitrate can change on the fly.  As was discussed, it may be desired to
specify a preference for one bitrate and/or use of VAD in a capability
negotiation.  Therefore, I have added to the rtp-mime draft optional
parameters for G.723 to specify bitrate and annexa=yes/no.  Similarly,
I have added an optional parameter annexb=yes/no for G.729 to specify
that VAD is used or desired.

Or has some other convention been proposed in the SIP community?

							-- Steve