Old CME Install - any special configuration needed to enable G711A

TONY SMITH · ‎09-09-2020

Hi,

We're looking at a pretty ancient CME install, currently configured for G711u only. However most ITSPs over her prefer or require G711a. At the moment it's working, just about, because the ITSP offers the call initially with G711a and G729 only, the CME responds with "488 Not Acceptable Media" then after a couple of tries the ITSP sends the Invite again but with a load of extra codecs including G711u and the call connects.

However if I add G711a to the codec class for the appropriate incoming Dial Peer, I get a different response to the first Invite, "501 Not Implemented".

Is there some other place where G711a needs to be enabled at the system level?

It's an old install as I say, IOS version 12.4(20)T2

Thanks, Tony S

Roger Kallberg · ‎09-09-2020

To my knowledge there are no system wide place where this needs to be set. How does your codec list for the ITSP connection look like?

TONY SMITH · ‎09-10-2020

ITSP incoming dial peer uses a codec class ...

		voice class codec 1
		 codec preference 1 g711ulaw
		 codec preference 2 g711alaw

Incoming Invite offers ..

m=audio 16040 RTP/AVP 8 18 101

Roger Kallberg · ‎09-10-2020

You’re ITSP is offering g711 alaw (8) and g729 (18) and your codec list contains g711 u and a law in that order. As it doesn’t match that’s likely why you get the error. Test to change your codec list so that it matches what you’re ITSP offers and test again.

TONY SMITH · ‎09-11-2020

Thanks. My understanding is that it should connect as long as there is at least one codec in common between the dial peer codec class and the incoming SDP. After all look at the pile of codecs in a CUCM early offer. However that said I can set up a test codec class and apply it only to one specific test dial peer next time I have access to the system.

FYI the reason I kept G711u in the codec list is that the call won't work at all if that's removed. What happens is the ISTP offers G711a and G729 first and repeats that Invite several times, getting an error each time before ending an Invite with more codecs ..

m=audio 8070 RTP/AVP 8 0 18 97 2 99 4 80 96

And the call finally connects as G711u. When we were first asked to look at this system it was configured for G711u only and all calls work in this half-arsed way, each incoming call fails several times before the ITSP finally offers G711u.

What I'm trying to go is get the call to connect first time as G711a. This will also allow them to move to a different ITSP, because as far as I'm aware over here all ITSPs use G711a.

Roger Kallberg · ‎09-11-2020

In theory what you wrote is absolutely spot on. What you see seems to not follow that however. If you want to use G711 alaw make sure that that the phones also has that as an option in their codec list, preferably with the highest preference or maybe even the only codec they have in their list.

TONY SMITH · ‎09-11-2020

I managed to get access sooner than I thought and had a deeper look. It seems CUE is the issue, my test call was routing to voicemail and I can see the following between CME and CUE ...

ITSP->CME Incoming Invite with G711a and G729 as before
CME->ITSP Trying
- CME->CUE Invite G711a only
- CUE->CME Trying, Ringing then 200 OK - But 200 OK has G711u in the SDP
- CME->CUE Bye "cause=65"
CME->ITSP 501 error

So it looks like I need to work out how to either get CUE to support G711a, or get it to use a transcoder. Transcoder is configured and registered, but doesn't get invoked.

Roger Kallberg · ‎09-11-2020

CUE, that was a long time since I had an encounter with that. Likely last time in Brussels 10 years ago before I got my numbers.

Have a look at the dial-peer(s) for the CUE integration. Suspect that it might have one codec set and that that is 711u. If memory serve me CUE only supports this variant of 711, but you could test to see if it would work with alaw, I doubt it will. If so your best option is to see to that a transcoder is invoked.

TONY SMITH · ‎09-11-2020

Thanks. That sequence in my previous post was obtained when codec lists for both inbound and outbound (to CUE) dial peers included both G711a and G711u. If the DP for CUE has G711u only we get the sequence I was originally seeing ..

ITSP->CME Incoming Invite with G711a and G729
CME->ITSP Trying
No attempt to send call to CUE
CME->ITSP 501 error

So it looks like the sticking point will be getting that transcoder correctly invoked for CUE calls.

Nithin Eluvathingal · ‎09-09-2020

Check the codec configured on phone. you can specificaly say codec on phone.