Subject: [Sip] SIP AMR Codec (RFC3267) out-of-band bitrate change

From: Sheetal Khemani <[email-masked]>
Date: Mon, 1 Nov 2004 14:02:56 -0500 (EST)

Hi,

I want to know if AMR mode changes are supported via SIP/SDP (possibly
using a new off in a reINVITE).

RFC3267 doesn't mention any parameter in the SDP to change the bitrate
during the call using SIP. It allows the mode-set parameter that tells the
encoder the subset of modes/bitrates it can use.  It doesn't allow the
specification of the bitrate the other end needs to use/switch-to.

I came across some 3GPP docs (3GPP TS 26.236: Packet switched
conversational multimedia applications; Transport protocols) that suggest
using the bandwidth paramter, but that seems to be specific to QoS and I
am not sure if this applies to the SIP standard per se.

Please let me know if anyone has any insight on this topic.

Thanks
Sheetal

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---

From: Magnus Westerlund <[email-masked]>
Date: Tue, 02 Nov 2004 10:35:54 +0100

Hi,

There is no explicit out-of-band signalling for mode changes defined for 
  the RTP payload format for AMR. Mode changes within the active 
mode-set is expected to be made in two different ways.

1. The sender already posses the information that it needs to switch and 
simply does this. The main information source is expected to congestion 
control. Thus information from RTCP or DCCP can be used to allow the 
sender to determine the need for reducing or increasing the rate.

2. The inband Code Mode Request (CMR) field. This field is intended to 
be used in gateway cases. For example a session between a IP client and 
a circuit switched mobile client. The rules and recommendation on how 
the CMR field should be used are present in section 3.9 of RFC 3267.

I would also like to clarify that a sender is able to at any point 
switch codec mode (within the active mode-set) during a session, no 
external signalling is needed.

So to be able to change the bit-rate there is only the need to know that 
  you need to change the rate. Do you have a need for indication that is 
based on external indication, and not based on available bit-rate?

Cheers

Magnus

-- 

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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