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CUBE Transcoding Design Question

James Hawkins


I work in the United Kingdom where SIP providers typically use G.711alaw as the codec of choice for calls. Sometimes where the calling party selects G.729 the SIP provider passes this straight through so this needs to be dealt with too.

I am implementing a Cisco Unified Contact Center Express system for a customer and certain features (mainly recording) mandate that this system use G.711ulaw.

This means that inbound calls via the SIP provider must be converted to ulaw by the CUBE. I have set up LTI based transcoding and this works ok but the DSP resource requirement for transcoding all inbound calls is very expensive in terms of PVDM resource required. The current CUBE is an ISR 4431 with PVDM4-256. If I configure the LTI transcoding so that it can deal with G729 and G711alaw as inbound codecs to be transcoded to G711ulaw the maximum number of calls supported is 192 which is below the 250 channels supported by the trunk.

I am looking for suggestions to resolve this. Possible solutions I have though of include:

  1. Only transcode calls to contact centre pilot numbers (I could do this by placing those numbers in an E.164 pattern map and setting up a dial-peer that with that as the destination with the codec set as ulaw). The idea behind this is that only calls to contact centre numbers would be transcoded with other calls passed through the CUBE with the codec used for the inbound call leg. I dislike this for the reason that every time a UCCX pilot number is created it would have to be added to the CUBE config. I am not sure if there are any other issues to consider with this approach?
  2. Use an MTP rather than transcoder to convert between alaw and ulaw. I would be looking at using a software MTP configured on the CUBE but am unsure whether this can convert between alaw and ulaw or whether a DSP based MTP is required for that.

I would be interested to learn how others have tackled this issue. 

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