[cisco-voip] Help on Codec negotiation

[Paul Long]

Jiten,

Several points:

1. H.323 does not require symmetric codecs and has only recently
provided a way to specify them. I don't know how widely supported this
new feature is.

2. The "codec negotiation," as you put it, is not really a negotiation,
as in coming to an agreed set of common capabilities. The capability
exchange is merely a way for each endpoint to unilaterally tell the
other endpoint what it can receive (and transmit, although that's rather
useless information).

3. An H.323 endpoint is not _required_ to select any particular codec
among the remote endpoint's list of capabilities, as they are merely
presented in order of preference.

4. The only way to absolutely assure symmetric codecs is to specify a
single codec in your TCS. (I've also had to do this when talking to
Cisco's SIP phones, so it's not just an H.323 thing.)

Paul

-----Original Message-----
From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Jiten RAJ
Sent: Thursday, March 03, 2005 8:53 AM
To: cisco-voip at puck.nether.net
Subject: [cisco-voip] Help on Codec negotiation

Hi,
I've some problem to understand the negotiation of
codec in H323. In our lab, we have two endpoints who
exchange their capability set this way :

TCS(receive G711, receive G729) --------->
<---------TCS(receive&transmit G729, receive&transmit
G711)

TCSacq------>
<---------TCSacq

OLC(G711)-------->
<---------OLC(G729)

I'm not understanding why they do not open the
channels in both direction using the same codec ?

Thank's for your help.
Jiten