This document explains how to get Cisco 8821 and 792x wireless phones (7921G, 7925G, 7926G) to work well in a Cisco Unified Wireless Network.
Important note: Cisco no longer supports the 792x phones. See the following End of Life announcements:
In particular, the 792x phones have not been tested with AP-COS (802.11ac Wave 2 / 802.11ax) APs, nor with 9800 series controllers, and Cisco TAC will not assist with any such deployments. Information on the 792x phones is provided below for historical reference.
Voice over WLAN - a challenging technology
Voice over WLAN (VoWLAN) is one of the most challenging technologies that Cisco provides. For VoWLAN to work satisfactorily - especially in the high-stress environments in which it is deployed, such as healthcare - the network, and the phone, must be able consistently to transport a real-time, bidirectional, securely encrypted audio stream, with almost no dropouts, while the endpoint moves across four dimensions (space and frequency).
Seven basic guidelines to making VoWLAN work well
Though delivering a reliable VoWLAN service is difficult, it is possible, provided that the network provider adheres to the following basic design guidelines.
1. Have solid coverage in 5GHz - and lock 802.11 mode on phones to 5GHz
Your network's ability to perform is fundamentally dependent on a solid physical layer. VoWLAN uses both the 2.4GHz and 5GHz bands. Of these, the 2.4GHz band's lower frequency signals carry further - however, the constrained bandwidth (only three non-overlapping channels) and ever-increasing interference, render 2.4GHz, in most cases, unsuitable for reliable voice. Network providers who want to deliver a reliable VoWLAN service will ensure that their design adheres to the following standard:
Every spot in the coverage area is serviced by at least two viable 5GHz access points, at -67dBm or stronger.
You can easily validate the necessary coverage by setting your phone into site survey mode, and walking throughout your coverage area.
Additionally, AP placement, antenna selection, building construction, etc. must be such that multipath distortion is kept to a minumum. To ensure gap-free roaming, a moving phone must be able to hear each roamed-to AP at least 5 seconds before it needs to roam to it - so place all APs in the middle of halls, at corridor junctions, etc., rather than in blind spots.
2. Run current phone firmware
On the 792x: run 1.4.7 - nothing earlier
1.4.7 firmware or above is strongly recommended, due to the CSCut25250 (Phones stops sending SCCP messages) fix.
On the 8821: run 11.0(6)SR2 -- nothing earlier
The latest image has fixes to several phone related issues like: poor roaming, one way audio, phone freeze/hang/crash and phone deregistration issues. If you encounter any new issues, troubleshooting from the latest firmware will be the best path forward. If any problems with the latest firmware, contact TAC.
Please refer TAC Recommended AireOS for AireOS WLC side code recommendations.
Please refer TAC Recommended IOS-XE for 9800 WLC side code recommendations.
3. If using FlexConnect local switching, enable ARP caching
If using FlexConnect local switching, make sure to enable ARP caching (i.e. the AP ARPing on behalf of the wireless client), for the sake of reliability and phone battery lifetime.
4. Optimize Security for Fast Secure Roaming
WPA2/AES Enterprise with CCKM and/or FT-802.1X is recommended.
WPA2/AES Enterprise provides for the greatest security, and - with a Fast Secure Roaming method - also provides for the best roam times.
For 8821: use WPA2/AES Enterprise with 802.11r (FT over the air)
For 792x: use WPA2/AES Enterprise with CCKM.
Can have both CCKM and FT-802.1X enabled on the WLAN - 792x uses CCKM and 8821 will use FT-802.1X
Note: The phones(882 and 792x) do not support 802.11k and 802.11v and should be disabled.