ISDN E1 G711alaw to G711ulaw

[Carl Ratcliffe]

Hi Support Community

• CUCM version 8.6
• We have a MGCP gateway that has a UK E1 circuit.
• The CUCM gateway configuration has PCM Type = alaw
• The region is set to G711
• "show voice call status" shows that G711ulaw is used
• RTMT is not showing any software MTP being invoked and no hardware MTP / transcode is being invoked

My understanding is that E1 circuits use G711alaw but the VOIP leg of the call is using G711ulaw. I understand that this will be because in CUCM 8.6 ulaw takes precedence over alaw in the region codec preference list however I can see no haedware or software that is completing the G711alaw to G77alaw conversion.

Am I missing something or is this done automatically using the DSP channels that are assigned to the pri group timeslots ?

Thanks, Carl Ratcliffe

Preston-Lancashire-England

[Suresh Subramanian]

Hi Carl,

Yes, It is the PVDM DSPs assigned to the PRI convert the VoIP to TDM signalling.

[Carl Ratcliffe]

Hi Suresh

Thanks for your response.

I understand its the DSP's that do all the TDM to VoIP conversion but this was specifically related to G711alaw converted to G711ulaw.

So are we saying if a TDM calls comes in as G711alaw the 1 channel of DSP will not only convert from TDM to VoIP but it will also change the codec from G711alaw to G711ulaw at the same time without any extra resources ?

Thanks, Carl Ratcliffe

Preston-Lancashire-England

[Kevin Monteiro]

attach the "show tech".

-Kevin

[Kevin Monteiro · Accepted solution]

Carl,

When you configure "pri-group time-slots 1-31" your using 31 channels of your PVDM.

Lets take a example, You got your self a "PVDM3-64" 64 indicates the channels.

Now if you configure a full-blown "E1-PRI" then minus  64 channels from 31 ie. would be 33 channels left to use on the router  which can be used for configuring another "E1-PRI" or a hardwareTranscoder/MTP/Conferencing and Yes once you assign the command "pri-group time-slots" the dsp assigns the channels automatically.

Eventually what would matter is the connectivity from the gateway (MGCP) to the CUCM, which is eventually speaks on G711ulaw as per the region setting,  Whats negotiated at the TDM (E1-PRI) does not matter and don't confuse your self with it.

Here is a similar post.where a person had the same concern:

https://supportforums.cisco.com/discussion/12200871/e1t1-codec-companding-g711ulaw-or-g711alaw

Rate the post accordingly.

-Kevin

[Carl Ratcliffe]

Hi Kevin

Thanks for your response, I did read that post previously. Maybe I am over thinking it !

I was always under the impression that if G711alaw needs converting to G711ulaw then a minimum of a software MTP is required - maybe this is just in a VoIP leg which is were im getting confused.

I was then thinking about the call coming into the E1 which I know uses G711alaw, I also know that the pri group timeslots assign DSP resources to convert TDM to VoIP. I was just taking it one step further trying to figure out what is converting alaw to ulaw but from the answers this is negotiated at the TDM stage then depending on the next leg is down to regions and transcoders if required.

The reason for my question in the first place is because when using the DSP calculator and specifying E1 links it always asks you for codec complexity, so just to be 100% clear does the TDM only negotiate between alaw and ulaw and anything else would need a transcoder ? What I mean by this is say we have a CME with local IP phones using G722 only ( only an example ), if the call then comes into the E1 im assuming TDM cant negotiate G722 as it can with alaw and ulaw so a transcoder will be required on top of the pri group timeslot DSP's ?

So in summary E1/T1 can automatically compand between G711alaw and G711ulaw.

Thanks, Carl Ratcliffe

Preston-Lancashire-England

[anilkumar.cisco]

Hello Team,
Any answer or feedback on this query..

Just the same thing came in my mind as well?
Best Regards
Anil Singh