g711, CUCM, SCCP: ulaw vs. alaw

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Aug 6th, 2009
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Is there any way to define which g711

encoding is preferred on a Skinny/SCCP Cisco Phone or a ATA186?

I was not able to find anything regarding the

phones. In the call manager I can only set

up g711 in a region, but not if it shall

be alaw or ulaw.

The default behavior of the cisco phones I tested was to initiate a call with g711ulaw.

Here in Europe it makes sense do directly use

alaw. I know that the Software MTP can be used

to change in between and this is also in use

for our PSTN GW, but I would like no change in

the RTP packets.

How can I force the sccp phone to directly use g711alaw?

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Nicholas Matthews Thu, 08/06/2009 - 15:24
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As long as your entire IP network is using G711ulaw, you will not be able to tell the difference between ulaw/alaw. The devices that handle the conversion will do it automatically (DSPs).

If you are working with other vendor's equipment, you should be able to set the codec via dial peers on gateways, or through negotiation of the media.


reidg Fri, 09/18/2009 - 03:12
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HI, did you every get an answer to this, I've been scratching my head over this on and off for a couple of weeks, and can't see where you tell cucm to use alaw as default rather than ulaw?

Nicholas Matthews Fri, 09/18/2009 - 07:41
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I don't think there is a way. You have to realize first that it doesn't really matter. Even if your PSTN connectivity is alaw, the gateway is going to automatically switch it over anyways (no additional complexity required).

If your phones are interacting with a gateway you can cause alaw to be used by forcing it on your dial peers, but CUCM will not care about the compand type.

Hope this clarifies.



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