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Configuring G.711 A-law in cisco callmanager 5

Posted on 2007-11-22
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We are using Cisco callmanager 5 in the UK with an MGCP gateway connecting to the PSTN via an E1 PRI.

The problem is, whenever we make a call and check the codec being used, it says it is using G711u i.e. U-Law. As far as I understand it, the PSTN in the UK uses A-law. How can we configure callmanager to use A-law between the gateway and the IP Phones? As it stands it works, it just bugs me why we have to use u-law.
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Question by:cstosgale
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by:Reid Palmeira
ID: 20353794
If you have an MGCP gateway, then you're PSTN termination should be coming there. beyond that point (on your LAN) the codec that you use for VoIP is pretty much up to you, doesn't matter much at all since it's only VoIP from the MGCP gateway to the endpoint. As long as the phones and CM setup both support it.
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by:cstosgale
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Good point, so why is Europe so keen on using A-law as opposed to U-law? Is this because the encoding used in E1 ISDN is closer to G.711 A-law than to U-Law?

And why is there no way of choosing which variant you want to use in callmanager? I'm guessing your codec choice in callmanager will affect what codec is used between the endpoints as it is an mgcp gateway.
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Reid Palmeira earned 500 total points
ID: 20354853
the codec negotiation is part of the session setup so you can look at the SDP packets and see exactly how the CM and the MGCP gateway are negoitating but my guess is that in the list of supported codecs that both devices supports g.711u is earlier than g.711a so it just negotiates to that codec. So when they're comparing lists the two sides just pick u-law first.

as for why the difference, there's a decent wikipedia entry http://en.wikipedia.org/wiki/G.711 and it's not so much the E1/T1 differences, just a different algorithm. I've seen a-law in Japan as well which seems to contradict what wikipedia says.

You should be able to set that list of supported codecs in CM but I'm not sure about the MGCP gateway, depends on make/model.
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