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New codec on the block – recording failures and CUCM 11.x

The inspiration for this post comes from spending way too much time trying to solve an issue that would have taken only minutes had I been aware of the key information I’m generously bestowing upon my fine readers today.

The problem started with reports that just 8841 phones weren’t successfully recording. I happened to have a spare 8841 phone, so I set up a line, configured recording parameters, and began testing. Sure enough – the recording failed on the spare phone.  I also happened to have a 7942 phone, set up an extension and the recording worked just fine.  Note that I had previously examined the endpoint configurations like recording server profile, Built in Bridge enabled, recording calling search space, and recording server setup for the extension.

Thinking this might be related to a SIP versus SCCP issue, I employed the RTMT to see if the audio for the SIP calls was being forked and sent to the recording server. I was able to drill into the SIP call placed to the recording server, check out the Call Flow Diagram, and confirm the recording server was invited to the party.  If you’ve not done this before, just need to log into the RTMT and navigate to Real Time Monitoring Tool -> Voice/Video -> Session Trace Log View -> Real Time Data. While snooping around all the SIP calls to the recording server, I noticed some successes belonging to 8841 phones.  Intrigued that my problem might not be model dependent, I hopped over to the recording server to see which extensions on 8841 phones *did* record successfully.  Which is where things began to make even less sense – only intermittent failures on some 8841 phones in question, others never recorded at all.

Using my any-excuse-for-a-packet-capture philosophy, I setup two 8841 phone endpoints to allow spanning to PC port and fired up Wireshark.  My test recording failed, but my packet analysis hit pay dirt.  When filtering for RTP, I saw PT=OPUS in the Info column.  Immediately, I had my answer.

I was vaguely aware the Opus codec was a thing, but I previously had no idea that 8841 phones supported the OPUS codec and that CUCM 11.X enabled the OPUS codec automagically (thanks?) – all information I gleaned from this link.

Knowing that recording servers historically hate anything that isn’t g.711 or maybe g.729, I immediately proceeded to follow the instructions from the aforementioned link to find and disable Opus for recorded phones. I previously did this for g.722 many years ago, which is why this solution stung a little, my not being aware there was a new codec on the block and to have preemptively avoided this issue entirely.

While looking at the Opus parameter, I couldn’t help but notice iSAC was new to me as well.  Sifting through my packet captures, I found RTP streams using that codec as well and so disabled it for recorded phones, too.

Below are a couple of screen shots of what you can expect to see in the SIP/SDP packets if you are experiencing this same issue. Hopefully this saves you a bit of leg work should you have some recording failures after an 11.x upgrade.

Feel free to send thanks in the form of flowers, coffee, and cheesecake.

Published 10/18/2018

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