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Default Codec used by CUCM

Gordon Ross
Level 9
Level 9

I've noticed that CUCM likes to use the G711 u-Law codec on the wire. Being in the UK, things here tend to be a-Law.  Because of CUCM's liking for u-Law, things often get transcoded from a-Law, to u-Law, and then back to a-Law.  Is there any way to tell CUCM to prefer to use a-Law on the wire, rather than u-Law ? (This is for both handsets and MGCP gateways)  GTG

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3 Replies 3

Steven Holl
Cisco Employee
Cisco Employee

Unless you have a connection to a VoIP PSTN provider that is offering you only g711alaw and not g711ulaw, you don't need to do g711alaw in your network.

The POTS connections will be sent in as g711alaw, sure.  But as soon as they hit the DSP, it will convert them to whatever you want.  Hence a PSTN gateway doesn't care whatsoever if it is doing PSTN----alaw---IOS-gw----alaw----CUCM or PSTN----alaw---IOS-gw----ulaw----CUCM.  If you do g711ulaw inside your LAN in the UK, you won't have any issues.  It onl'y presents itself as a problem if you have a SIP ITSP or something along those lines, where they NEED to see g711alaw from you.

g711ulaw is preferred over alaw in the LAN for Cisco UC, since there are products that only do ulaw, like CUE.  Doing ulaw everywhere will ensure you don't need to invoke transcoders which would otherwise be unnecessary.

Hello,

With PSTN = yes I agree,

But actualy, the fashion want to use the SIP trunk. So how to prioritize the G711A (A-law) over G711U (U-Law).

It's very weird if CUCM, an expensive solution, can't have solution to change the codec preference in CUCM, without using Transcoder or CUBE.

regards,

Antra

Just to clarify, you don't need transcoding for that. Software MTP does the translation from g711ulaw to g711alaw and vice versa.

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