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Avoiding transcoding


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Hi team,

I would like to know if there's some option i can set to avoiding transcoding where ever is possible.

I have snom phones and snom one yellow @ latest version.

By default, I like g711. Because I receive calls through two BRI Patton. But on itsp, I would like to use g729 on phones too.

Is there some way to set the trunk's codec as the preferred one even for phones?



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Of course the PBX tries to negotiate a codec that does not require transcoding. But keep in mind, it is not only A and B-party that need to understand the codec, it must also be understood by the PBX (otherwise such features as call recording or barge/listen in would be impossible). The way SDP works, it is not always easy, especially when things like T.38 may also be in the game.


I would say as a general rule, either give as much freedom to the PBX to pick the best codec or just have everyone just offer one codec. The latter approach is to avoid negotiations at all; and it works great as long as all parties support that codec.

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I understand that, but what I mean is something like this:


When snom phone got a call from Patton isdn, both will use g711

When snom phone make a call by ITSP, both will use g729


Phone -- g711 --- PBX --- g711 --- Patton BRI

Phone --- g729 --- PBX --- g729 -- VoIP provider


Patton BRI trunk only allow g711

VoIP trunk only allow g729

Snom phone has both codec enabled, with g711 as first choice


In every call, snom phone use g711 only


How to tell phone to automatically switch preferred codec?

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Well, the only way to get this working is to set the codec preference for the trunks. It is still not so easy, lets just assume for a second that the trunk provider sends 180 Ringing without SDP when you start the call and the phone receives 183 with a SDP. In that case, the PBX has to make a decision. It is legal in SIP to change the codec later, but unfortunately there are a lot of problems on the interoperability side (we tried that a couple of years ago and it was a big problem for support). So thats how you end up in transcoding situations. Re-INVITE would be another option, but again we had a lot of interop-problems here so we kept the call flow as simple as possible (only in the case of a attended transfer we do a Re-INVITE).


If you have to transcode from G.711 to G.729, especially on a trunk, the situation could be worse. The quality should be still reasonable good for a G729 call; and if you dont have too many trunk calls, then the CPU load should also be okay. Having something where you have to transcode e.g. from GSM to G729 will have much worse audio quality. Honestly, I would keep it the way it is right now, it would not get much better if you let the phone do the transcoding (from linear to G.729).

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I thought it was very "simple" for PBX to tell the phones "sorry, I just have g729" in specific situations.

I was expecting negotiating was something useful for these type scenery.

Could be a future feature that can be set only on some supported phones?

So, it would be better to switch all calls to g729. Is there some limitation on that codec with yellow version?

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Unfortunately, G.729A is part of the patent war that you see right now in the cell phone world. We have made a license agreement, so no problem there :rolleyes: . However, the way G.729 is licensed is by the number of calls. Because we cannot license "unlimited" calls (and we have to pay per call), there are certain limits on how many calls you can have in parallel. And this is also the reason why the free edition cannot have G.729 calls.


We thought for a long time to support also other low-rate codecs that are free. However, wich one? iLBC? speex? At the end of the day, we are depending what the service providers offer and it seems that we are pretty much stuck with G.711 and G.729.

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