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Re: [Sip] G723.1H and G723.1L SDP negotiation

"Kevin P. Fleming" <kpfleming@digium.com> Mon, 01 March 2010 13:27 UTC

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Date: Mon, 01 Mar 2010 07:11:37 -0600
From: "Kevin P. Fleming" <kpfleming@digium.com>
Organization: Digium, Inc.
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To: Elison Niven <elison.niven@matrixtelesol.com>
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Cc: sip@ietf.org
Subject: Re: [Sip] G723.1H and G723.1L SDP negotiation
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Elison Niven wrote:

> I know this has been discussed and asked a lot of times in the past but
> still I find no affirmative solution.
> 
> For a UA that supports both G7231-H and G7231-L, SHOULD it specify both the
> codecs in the offer that it sends?
> 
> If the UA is willing to specify both the codecs in the offer that it sends
> (because it specifies two different codecs in its user interface), what
> should be the method of specifying the codecs in the SDP body?

The G.723.1 recommendation mandates that a receiver be willing and able
to accept frames in both bitrates at any time, without any signaling
required to switch between them. Given that requirement, there is no
need to indicate both bitrates in the SDP and use separate payload
numbers; an offer can include that endpoint's *preferred* bitrate (using
the bitrate= parameter), but the endpoint must be prepared to accept
frames in both bitrates regardless.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
skype: kpfleming | jabber: kfleming@digium.com
Check us out at www.digium.com & www.asterisk.org

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