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Know Your Supplicant – Support For Channel 165

By George Stafanick, Blog Contributor

I received a call from a fellow engineer who was troubleshooting a voice issue last week. He asked if I could look over data he and his team collected and offer advice to help resolve an intermittent voice issue involving Cisco 7925 VoIP handsets. 


I put my troubleshooting hat on and started to ask questions and review the following:


The Aruba infrastructure was working great with no reported issues till they deployed the Cisco handsets. Their voice network is deployed only on 5GHz. They currently have 100+ Ascom i62 phones running on 5GHz with no reported issues. They deployed 100 Cisco handsets and the issue presented itself on day one.




Resolve intermittent voice connectivity with Cisco 7925 handsets



Voice is choppy in certain areas. Intermittent at times. Issue can be reproduced. Issue is segmented by location. 



No known bugs identical to these issues on Aruba or Cisco side. Cisco 7925 code was deployed 2 revs behind the latest release. Handsets were updated to SR1. Issue was still observed after code upgrade. 






Wifi network is designed with voice best practices --- confirmed 

25 Tx power max on access points--- confirmed 

25% cell overlap --- confirmed 

-65 Cells --- confirmed


Channel assignment (co-channel and adjacent channel) is good --- confirmed 

More than -20 dBm difference between same channels


Conclusion --- Wireless design is solid. Ascom voice is working on 5GHz, no reported issues. 



Spectrum Sweep 

Spectrum analysis conducted was clean no interference on 5GHz --- confirmed 

Noise floor between -90 and -98 --- confirmed 


Conclusion -- Spectrum is clean. 



Code was upgraded to the latest on both the Aruba and Cisco side. 




Aruba Controller --- collected 

Cisco 7925 Handset --- collected 


Aruba Supporting Channel 165:






After closely reviewing all the data collected it was apparent the issue was with the Cisco handset.  After closely looking at the physical layer, data layer and debugs it appeared the Cisco handset didn't support channel 165. The areas complaining about voice issues all had access points assigned to channel 165. A packet capture confirmed the handset wasn't probing on channel 165. 


A Tac case was opened with Cisco and confirmed. The Cisco 7925 doesn't support channel 165. Additionally, it was confirmed Ascom does support channel 165. This further explains why the Ascom handset did not have issues. 







Cisco 7925 -- Not supporting  channel165


PAGE 64 - "Ensure that channel 165 is not enabled in the DCA list as the Cisco Unified Wireless IP Phone 7925G, 7925G-EX, and 7926G do not support this channel."



Ascom - Supporting channel 165


PAGE 3 - 



b/g: 2.4–2.4835 GHz (Ch 1–11) and

a: 5.15–5.35 GHz (Ch 36–64), 5.47–5.85 GHz (Ch 100–165)"





Knowing your client and how it operates is important to understand how they will communicate and roam on the medium. In this real world example, following basic troubleshooting steps we were able to determine the issue was with the Cisco handset relatively quickly. A very important lesson can be gained from this experience. "Know your client." 


Further more, it should also be said that not all clients support all channels. Recently I worked on a Biomed project, a new purchase of pumps were ordered. Upon delivery basic connectivity test were conducted and data sheets were pulled for the wireless NICs. The pump vendor installed cheap wireless cards that didn't support UNII2 or UNII2E channels. 


Have you had a similar expereince ? I would like to know ...