Subject: Re: Question: packing several frames in a single RTP packet

From: [email-masked]
Date: Mon, 11 Sep 2000 11:33:56 -0500

Later drafts of RFC 1890 resolve ambiguities even for variable frame length
audio codecs without size indicators (which are not sample-based).  For
example, take G.729A/B.  It has two types of frames, a 10 byte active, and
a 2 byte Annex B comfort noise.  The latest draft profile specifies that
the payload must consist of zero or more active frames followed by zero or
one comfort noise frame.

Vince Virgilio

Orion Hodson <[email-masked]>@cs.ucl.ac.uk on 09/10/2000 04:50:22 AM

Sent by:  [email-masked]

To:   Jonathan Rosenberg <[email-masked]>
cc:   "'Guler, Amos'" <[email-masked]>, "[email-masked]'"
      <[email-masked]>
Subject:  Re: Question: packing several frames in a single RTP packet

<[email-masked]>Jonathan

R
osenberg writes:
> This may be of theoretical interest only. In practice, I think that all
> frame based codecs are either (1) the same frame length, (2) indicate
within
> the frame itself what size or type it is, when the lengths are not the
same.
> Its only for frames that have variable length AND don't indicate their
> length that you  need worry about this. Does anyone know of any frame
codecs
> which are a problem?

VDVI is variable length and has no size indicator.  Since it's a very
low cost codec this isn't a major deal - it's 160 16-entry table
searches per frame.  It's slightly unusual because it is a sample
based codec cast that's effectively cast into a frame based format for
unambiguous decoding.  I can't think of any other's off hand.

- Orion.

---

From: Orion Hodson <[email-masked]>
Date: Mon, 11 Sep 2000 18:58:50 +0100

<[email-masked]>[email-masked] wri
> the payload must consist of zero or more active frames followed by zero or

The current draft (draft-ietf-avt-profile-new-09.txt) has the wording
about relatively prime payload sizes.  I believe the wording about
mixing CN and coded frames is specific to G.729, and the combination
of speech and CN frames.

G.729 and it's annexes cover several different bit-rates and
correspondingly different frame sizes.  Different payload identifiers
are used for the different bit-rate representations and therefore
different speech frame sizes cannot be packetized within packets the
same packet.  This solution is an implicit realization of Amos's point
and is one way of solving the problem should it occur with other
codecs.

- orion.