We are having problems with our 7920 phones as well as Spectralink phones roaming. We have no problems with our data terminals roaming. The phones and terminals are on separate Vlans on the same aps. The aps are running 12.03t. RSSI is above 20. Separate channels 1,6,11. The phone will totally drop off the network when it tries to roam.
There were some issues w/ radio cutting in and out w/ 12.03T. It is recommended to use 12.2(15)ja, which would require a migration to IOS. Also want to ensure that the 7920 has the latest code (01.07) from CCO, where scanning and roaming has been improved. Also need to ensure there is adequate RF overlap of non-overlapping channels for seamless roaming. Refer to http://www.cisco.com/en/US/products/hw/phones/ps379/products_white_paper0900aecd800f6d97.shtml for more info.
I do not know if you checked this yet but at the recent Cisco Networkers Convention in Toronto I have learned that there should be 20% cell overlap for VOIP as opposed to the usual 10% overlap for a regular data network.
The problem started when we upgraded our AP's from 350's to 1231's. The way that I understood it was that when running multiple SSIDs on a 1231 AP it assigns a MAC to each SSID. On the 350's it just sent out all 0's. With the older versions of firmware on the 7920's (pre 2.0) they would try to roam and would be looking for 0's for the next AP. With the 1231's sending out a specific MAC they wouldn't see a next hop so they would stay associated to where they started until they totally lost coverage.
At least that is what I got out of the explanation I was given.
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