Response Group calls to Skype4B client work, Polycom VVX Phones Fail with CAC enabled

By | September 3, 2016

Figured I’d post this one quickly and hopefully it will help someone else, I don’t have time to replicate the issue in my lab so don’t have any screenshots more detailed info I’m afraid

I recently had an interesting one where a customer had deployed Skype4B I with another integrator in a hurry and had 100’s of sites with less than 2Mbit (yes, Mbit) of traffic with 10+ users behind it.

Normally this wouldn’t be such an issue but the CAC policies had been defined by the existing integrator to only allowed a maximum of 51kbps per call. This is fine for SILK calls with a bit of forward error correction as the SILK codec uses stupidly low amounts of bandwidth, but as this customer was deploying Polycom VVX phones as each site. we are now involving the Response Group Service and Polycom VVX Phones in the same call leg. As the VVX phones don’t support SILK the RGS service is trying to use G711 which as anyone that’s been around in the telephony world knows, has a 64kbit payload. (not including overhead)

Interestingly once the CAC limit per session was raised to 64kbit calls flowed fine. Which indicates that one of the devices isn’t calculating the TCP and SRTP overheads very well, because if we include all the overheads the call *should* be around 102kbps.

The symptoms with this issue was that the call wouldn’t even appear at the VVX phone at all whilst the soft client was ringing.

To see if calls to your VVX phone are failing due to CAC issues, head on over to the phones log via the web interface and look for the following lines.

0901110224|ice |4|00|ICE Bandwidth check result: insufficient bandwidth for the call reported
0901110224|so |4|00|SoMediaSessEvLclAnswerSdpAvailable in Offering state – call disconnect due to failed ICE bandwidth check

Hope this saves someone else some hassle because it was a bit of a weird one for me.

If you’re looking for more info on codec bandwidth usage there is a rather lengthy article on Technet here 

Scroll to the bottom of the article for the interesting bits

Update: Microsoft has a great bandwidth calculator for download here:


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