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Note: Any references to roam or roaming throughout the remainder of this document is in the context
of mid−call roaming.
As it relates to roaming, the Cisco 7920 IP Phone currently only supports Layer 2 roaming. This means that a
Cisco 7920 IP Phone can roam between APs that have coverage for a single Layer 2 subnet, but calls are
dropped when the phone roams into the AP coverage for another Layer 2 network.
Layer 2 Roaming
Layer 2 Roaming occurs when a WLAN device, for example the Cisco 7920, physically moves enough that its
radio associates to a different AP. The original and the updated AP offer coverage for the same IP subnet, so
that the WLANs client are still valid after the roam.
The Cisco 7920 performs an Layer 2 roam for the following reasons:
When the Cisco 7920 initially boots, this is considered a roaming event because it is associating with
a new AP.
•
The Cisco 7920 does not receive beacons from the associated AP, and therefore believes that the AP
is no longer available. If three consecutive beacons are not received by the Cisco 7920, and its unicast
packet to the AP is not acknowledged, the Cisco 7920 begins the roaming process to another AP.
•
The Cisco 7920 periodically scans for better APs. Because the initial startup is considered to be a
roaming event, all client stations have roamed at least once. After the roam process has completed, the
client station maintains the list of eligible roam targets. When all AP information is received (channel
update, current AP update), the phone evaluates its current AP with the list of eligible roam targets. If
conditions change on the current AP (RSSI low or QBSS high) so that one of the APs in the client's
stored list now appears to be a better choice than the current AP, the phone starts a handoff procedure
to associate with the AP.
•
The Service Set Identifier (SSID) or encryption type on the Cisco 7920 has changed.•
Once one of the events above occurs, the Layer 2 roaming process proceeds as follows:
The Cisco 7920 looks at its list of eligible roam targets (APs with a matching SSID or encryption
type) and chooses the best candidate. The Cisco 7920 then attempt to associate and authenticate to
1.
Cisco − Wireless IPT Design Guide for the Cisco 7920 IP Phone