g711 alaw to ulaw translation?

Answered Question
Oct 6th, 2009

Hi all, I have mgcp gateways that are connected via to our pbx's. The gateway configuration on the cucm has the codec on the pri interface as alaw. But i've noticed when I do the 'show voice call sum' on the gateway, it shows ulaw in use. I assume that this is showing the ip side of the call? But if that is the case is there transcoding happening on the router between alaw and ulaw automatically?



I have this problem too.
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Correct Answer by Paolo Bevilacqua about 7 years 3 months ago

Yes, it happens automatically. This is not considered transcoding, but simple compunding conversion.

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Correct Answer
Paolo Bevilacqua Tue, 10/06/2009 - 02:25

Yes, it happens automatically. This is not considered transcoding, but simple compunding conversion.

reidg Tue, 10/06/2009 - 02:35

thanks for that, havn't heard this phrase before though.

Could I ask a follow on: if the cucm is also interacting with third party piece of kit over a sip trunk, and the bit of kit only uses alaw. I've noticed that the cucm stays involved in the call, rather than passing off to the endpoints, i.e. 3rd party kit and gateway, unlike when its to a phone, when it does drop out. And the call leg from the cucm to the gateway uses ulaw, whilst the sip trunk uses alaw, do you think this is why i'm seeing mtp resources being used on the cucm, or is it just that the gateway is non rfc2833 compliant.

Paolo Bevilacqua Tue, 10/06/2009 - 02:50

I think if you was to use H.323 or SIP you would be able to free up CUCM when the device is involved.

So far I've seen that g711u is the de facto standard for voip and everything works with it. Just try.

btmulgrew Wed, 10/07/2009 - 14:42

Hi Grant - I think a CUCM MTP may be involved in the ulaw to alaw conversion -from the latest SRND:


An MTP can be used to transcode G.711 a-law audio packets to G.711 mu-law packets and vice versa, or it can be used to bridge two connections that utilize different packetization periods (different sample sizes).


reidg Thu, 10/08/2009 - 00:14

Hi Brian, yes thats how I read it as well. thanks


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