Help in Codecs

Unanswered Question
Nov 8th, 2007


Our client is using CME 4.1, they are using a program to record all incoming and outgoing calls the program has U-LAW codec, yet the E1 Pri connection to the PSTN is configured with A-LAW which produces noise when recording the calls.

When i changed the codec to U-LAW on the E1 port using the "Compand-type" command all calls suffered from extreme noise. I cannot use the codec on the dial peers because they are configured as POTS.

Can you please advise me with a way to convert all codecs used on call manager to use U-LAW. Please note that i checked the DSP and it uses g729

Thanks in advance

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4 (2 ratings)
joergwesely Thu, 11/08/2007 - 15:34

I think it's not possible to run the complete system using uLaw.

When your E1 is using ALaw (usually in Europe), you will ever get noise.

You will have to transcode the voice-stream for the recording if it must be uLaw.

How is the recording-system integrated? Is it using a span-port an sniffes the traffic or does it works different?

laithhattab Fri, 11/09/2007 - 03:49


Thanks for your reply, as you predicted the recording system is using a span-port to sniff the voice vlan traffic. Can you please tell me how can i transcode the voice stram as you suggested.

Thanks a lot


Paolo Bevilacqua Fri, 11/09/2007 - 04:20


voice on IP across cisco phones, is always G711u, not matter if a-law or u-law is used on the telco interface.

The router does the companding transcoding transparently, so no issues.

Hope this helps, please rate post if it does!

laithhattab Fri, 11/09/2007 - 04:43

Hi bevilacqua

I agree with you regarding cisco phones codecs and the transcoding but when the teleco interface is involved in the call the codec is transformed into A-LAW which is not supported by the recording software.

Paolo Bevilacqua Fri, 11/09/2007 - 04:57


As I was saying before, no matter the companding used by telco, the phones will use only and exclusively U-law for G.711, unless you have changed to G.729 under ephone config.

Again, the router does the transcoding from A to U and viceversa, without any need for configuration.


This Discussion