g711ulaw and g711alaw

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Jan 26th, 2006
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I have a stange problem. When trying to internetwork between a ccme and a callmanager. I noticed that when phones connected to my callmanager cluster call each other , they do it at g711alaw, then as soon as the called party picks up they negociate at g.711ulaw. I saw this by pressing ? twice on an ip phone.

My question is that why do phones try to negociate at g711alaw when the phone is ringing?


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Steven Smith Thu, 01/26/2006 - 07:40
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The codec negotiation does not happen until the other party picks up the phone. Codec negotiation occurs after the other party has picked b/c of call forwards that are possible that use different codecs and b/c of shared lines that could be using different codecs. I have not tried this, but it sounds like a default codec that is on the phone at that time.

aalejo Thu, 01/26/2006 - 07:48
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Are you calling between IP Phones (Ip phone in CME and IP Phone in CCM)?

How the CME is configured?

Do you see G711alaw during rinback?


NAVIN PARWAL Thu, 01/26/2006 - 07:58
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i will make the senario a little simple.

When i call between 2 phones connected to the callmanager, while the bell is rining i hit ? on the ip phones and see that the codec is g711alaw and then as soon as they pick up the phone it goes it g711ulaw.

My question is why is the initial codec g711alaw in the callmanager? can we change this?

aalejo Thu, 01/26/2006 - 08:27
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Because you are receiving ringback inband from the CCME using G711alaw (Callmanager does not provide Ringback). After call is connected, codec is negociated between CCM and CME. CCM defaults to G711ulaw (CallManager does not have a way to negociate between G711ulaw a G711alaw, defaults to G711ulaw). CME have some commands (enable by default) that allows to find out if the remote party is a CCM.

Your question is: why the gateway is providing ringback using G711alaw?

For answering this question I will need CCME configuration.

NAVIN PARWAL Thu, 01/26/2006 - 08:45
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both the phones are connected to the callmanager, during ringback the default codec is alaw, after the called party picks up the codec is ulaw, my question is why is the ringback codec alaw in callmanager, anyplace i can see these settings?



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