mgcp gateway fixed codec selection

Unanswered Question
Sep 18th, 2009

is it possible when using a mgcp controlled gateway to set the codec preference for the voip side of the call.

I know regions can be used, and the intraregion default in service parameters as well. But i'm really talking here about setting a preference for g711 alaw as opposed to ulaw.

thanks in advance

I have this problem too.
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Brandon Buffin Fri, 09/18/2009 - 05:57

You can use the "mgcp codec " command to change the default codec on the gateway. Default is g711ulaw.

Hope this helps.


reidg Fri, 09/18/2009 - 06:23

Brendan thanks for that.

I had tried that but didn't the calls still stay at g711 ulaw??? any idea?

Brandon Buffin Fri, 09/18/2009 - 06:42

I'm not sure you can force CUCM to use alaw on the IP side of the call. You can force this on a digital interface by using the PCM type setting. CUCM can do alaw, but I believe ulaw is its first choice, so if the gateway it is interfacing with is capable of ulaw, that's what it chooses. If the gateway were not capable, it would then choose alaw. Same thing with the gateway.


reidg Fri, 09/18/2009 - 06:57

so I suppose the question becomes, how do you disable ulaw or a mgcp gateway to force the use of alaw?

Nicholas Matthews Fri, 09/18/2009 - 07:47

As I mentioned in my other thread - there really is on reason to make this a-law. The gateway will do the switchover automatically, and it doesn't take any more DSP resources than it would for a alaw-alaw situation.


reidg Mon, 09/21/2009 - 00:19

As a general rule i'd agree with you, but I have a requirement to interface with a 3rd party piece of kit, that support alaw, so the current default situation of using ulaw is forcing the use of additional dsp resources..

Nicholas Matthews Mon, 09/21/2009 - 04:39

You may run into problems on transfers and such then. I don't think there is a way to force this with regions / MGCP.

When the call is set up to this device, CUCM should still be able to negotiate alaw, it's just always a second priority.



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