Codec Selection - How does it work?
 

I'm trying to figure out how codec selection works from end to end. Here's the situation. My Draytek 2100V is setup for g726 (32 KB). I've also setup my voxalot account and indicated that my provider supports g726 in the codec field.

Now, when I make a call to my work number in Sydney what exactly happens? How do the codecs get chosen at voxalot and at the VSP?

The 2100V has a status page that supposedly reflects what's happening to the call and the codec being used. When I made the call this morning it indicated that g729 was being used even though I specified the 2100V to use g726. Obviously some negotiation was taking place to select the right codec.

So what's exactly happening?

As a supplementary question, what are valid parameters for codecs in the providers list?

Hi,

The default value is a complete list of supported Codecs. VoXaLot processes them left to right (leftmost being number 1 preference). It is very important to use the correct separator and Codec values.

You can also remove Codecs and change the order as required.

Ok,

So if the list was alaw;g729;g723 and my ATA was set to use alaw, then what would happen if voxalot routed the call to a VSP that only supported g723?

I assume as follows???

SIP protocal used to communicate call to Voxalot. Voxalot attempts to negotiate call with the VSP (after call plan) at alaw (fails), g729 (fails), g723 succeeds. Correct?

When Voxalot receives the call from my ATA, does it know that my ATA is set to g723?

Perhaps someone can give me pointers to a more technical paper on the SIP protocol?

By the way, what is the EXACT list of acceptable codecs that Voxalot accepts?