[asterisk-users] G.729 and Voicemail

[Jeff LaCoursiere]

On Fri, 9 Oct 2009, Gordon Henderson wrote:

> On Fri, 9 Oct 2009, Kevin P. Fleming wrote:
>
>> Gordon Henderson wrote:
>>
>>> All deskphnoes I've ever bought support g729 natively. They also all
>>> support G711. The wholesale termination services I use all support g729
>>> too. The only fly in the oinkment is local PSTN connections which are
>>> obviously g711a. Now if voicemail would just blindly store in the incoming
>>> format, then that would be nice - VM files would be stored in g729 or
>>> g711, or whatever else comes in. It's just data afterall - why even think
>>> about transcoding? (Although I'm sure the answer will be something to do
>>> with trying to play g729, etc. attachments from an email or switching
>>> codecs mid-call from playing the prompts to playing the message)
>>
>> You missed my point; let's say a call comes in over your PSTN
>> connection, goes to voicemail, and they leave a voicemail, in G.711a
>> format. Then, one of your internal users places a call to the voicemail
>> system from their G.729 phone to retrieve their messages; now for that
>> message to be played back, it must be sent to the phone in G.729 format,
>> because that's what format the phone and Asterisk chose when the call
>> was placed.
>
> No - I understood what you were saying, but maybe I should have said that
> all deskphones undesrstand many codecs, not just g729. (E.g. Snom,
> Grandstream, etc. all support many codecs and I'm not going to criple them
> by turning them off)

> So when on a PSTN call, they speak g711, when on a
> VoIP trunk call they speak g729 -

I would be very surprised if that were true.  Your phones speak many 
codecs, but they negotiate with asterisk on registration which one they 
will be using.  They don't switch codecs based on the remote channel 
(which they don't even know about).  Today, if your phones are negotiating 
729 on registration, you are definitely transcoding calls to/from the 
PSTN.

>  when an incoming VoIP call goes to voicemail it records in whatever 
> format the incoming trunk is in, so when the deskphone plays it back, it 
> gets the file in whatever format it's stored in and can play it.

Again the phone won't switch negotiated codecs to match the file - 
transcoding will simply take place if they don't match.

>
> However, I don't think that is going to work for the reasons I said above,
> (changing codecs during one call) although it would be nice if it did.
>
> It's basically a PITA when mixing PSTN and VoIP. Fortunately a lot of my
> customers are VoIP only, so now knowing that I can store voicemail in g729
> format, I'll go off and try that and save myself some bandwidth.
>

Thats ounds like the best plan!

j

> Gordin
>
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