[asterisk-users] SIP Codec negotiation

Joshua Colp jcolp at digium.com
Fri May 11 08:10:56 CDT 2018

On Fri, May 11, 2018, at 10:07 AM, Steve Edwards wrote:


> So, without examining the RTP, you cannot tell which codec was actually 
> used?

>From an Asterisk perspective "core show channel" will also show you what is currently flowing.

> In the above example, even though the INVITE/SDP says they prefer gsm over 
> ulaw and the OK/SDP says I prefer ulaw over gsm, they can choose to use 
> gsm or ulaw?


> Can it be asymmetrical? They send gsm and I send ulaw?

Technically, yes. In practice it's a bit iffy - specifically because some DSPs in devices won't allow it - they require a single codec be in use for each direction.

Joshua Colp
Digium, Inc. | Senior Software Developer
445 Jan Davis Drive NW - Huntsville, AL 35806 - US
Check us out at: www.digium.com & www.asterisk.org

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