These are some of the common roaming issues with the Cisco 7920 phones and the solutions:
- The phone does not roam when placed directly under the Access Point (AP). The phone is most likely not reaching the roaming differential thresholds for the Received Signal Strength Indicator (RSSI) and Channel Utilization (CU). Adjust the power settings on the APs. The phone is not receiving beacons or probe responses from AP.
- The phone roams too slowly. Make sure the Cisco 7920 Wireless IP phone has another acceptable AP as a roaming option in the phone's site survey. The next AP ideally should have an RSSI value of 35 or greater
for roaming. Check the Cisco Catalyst 4000 series switch. Supervisor Engine 2 (SUP2) modules can cause significant delays. Upgrade to SUP2+ or SUP3 instead.
- The phone loses connection to Cisco CallManager when roaming. Check authentication for a possible WEP mismatch. The phone is capable of seamless Layer 2 (L2) roaming only, unless WLSM is configured. Ensure that the new AP is not serving a different IP subnet. If using Light Extensible Authentication Protocol (LEAP), check that TCP ports are not blocked by filters on the AP. Port 1645 is used for the Access Control Server (ACS), and port 1812 is used for other RADIUS servers. Verify that the associated AP has IP connectivity to Cisco CallManager. Check RF signal strength.
- The phone loses voice quality while roaming. Check for low RSSI on the destination AP. Channel overlap might be insufficient. The phone must have time to hand off the call smoothly before it loses its signal with the original AP. The signal from the original AP might be lost.
Note: When multiple Basic Service Set Identifiers (BSSIDs) are configured on the AP, Cisco 7920 wireless IP phones must run firmware version 1.09 or later. Upgrading the firmware of the phone resolves the roaming issue.
Refer the following URL Cisco Wireless IP Phone 7920 Design and Deployment Guide for more information
Roams between access points continuously
Cisco 7920 Wireless Phones