I found this Cisco document is very useful to make 7925 Deployment better in 802.11a band. I hope information is fairly up to date (as I did not check each individual bug status as at now in May 2013) since it was documented late last year (Oct 2012). Here is the content of it without comments (you should read those as well)
1. Have solid coverage in 5GHz – and lock your phones to 802.11a
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 minimum. 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 1.4.3 or above – nothing earlier
The 1.4.2 firmware introduced a greatly improved scanning and roaming algorithm in the 792x phones. 1.4.3 introduces other critical fixes. If you are experiencing audio problems with your phones – never, ever run any firmware prior to 1.4.3.
Be aware that, for newer hardware revisions of the 792x phones, it is not possible to run any firmware prior to 1.4(3)SR1 – see the release notes. Therefore, for new or RMAd phones, you can’t run the bad old firmware, even if you want to.
3. Access points in local mode, not Flexconnect (H-REAP) local switching
Flexconnect (H-REAP) local switching suffers from the following severe bug:CSCtz31572 HREAP local switching – ARP , broadcast key on standalone transition
This bug causes one-way audio, and affects all 7.0 and 7.2 CUWN releases. Until this bug is fixed, be sure always to use local mode APs, never Flexconnect local switching, for your VoWLAN clients. If your WAN link between the APs and the WLC is high latency or low bandwidth, then install a WLC at the site where the phones are.
4. Use WPA2/AES EAP/CCKM or static WEP – never PSK
WPA2/AES Enterprise with CCKM is the recommended security scheme for 792x phones – you should never use any other security method. You may use Local Authentication on the WLC, if you do not want to use an external RADIUS server. (When using CCKM, use the WLC command “config wlan security wpa akm cckm timestamp-tolerance 5000” to increase the likelihood of performing a fast roam.) (Also see the CCKM Client Disconnect Bugs in 7.0/7.2 tip.)
If security is not a concern, then static WEP will work well.
Do not use WPA/WPA2 Preshared Key (PSK.) This is because the 792x phones suffer from the following unresolved bug, which prevents WPA key exchanges from completing:
CSCtt38270 7925 sometimes takes 1+ second to respond to WPA M1 key message
Until this bug is resolved, WPA PSK deployments will experience audio gaps of up to several seconds intermittently at roam time.
5. Optimize channels, power, and data rates
- use at least 8 channels, at most 12
- use channels from UNII-1 (36-48), UNII-2 (52-64), UNII-2 Extended (100-140), and/or UNII-3 (149-161 but not 165)
- if coverage is weak, avoid channels with lower power limits
- if radar detection is frequent, avoid the DFS channels (UNII-2, UNII-2 extended)
- in 5GHz, use a minimum power level of at least 11dBm
- in all 5GHz deployments but the very densest ones, you can simply set a power level of 1 (maximum)
- data rates:
- set 6Mbps as the lowest mandatory rate, be sure that 12 and 24Mbps are enabled
- remember to make any changes on all WLCs in the RF group
6. Enable continuous scan mode (in CUCM)
If continuous scan mode is not enabled, then the phone will not scan vigorously unless in call. This means that it may drop incoming calls and pages.
7. Configure all QoS, and everything else, exactly as documented in the 7925 DG
Go through the entire 7925G Deployment Guide, and configure the phones and the wireless network exactly as per its recommendations (with the exception of any of the guidance above.) In particular, make sure that all QoS configurations are set as per best practice, throughout your wireless and wired network.
With strict adherence to every single one of the above guidelines, there is a high probability that your VoWLAN service will meet your clients’ performance expectations.