Cisco Unified Wireless Ip Phone 7925G, 7925G-Ex, And 7926G Administration Guide - Cisco 7925G Administration Manual

VoIP Wireless Network
Mid-call Roaming
A wireless IP phone user is actively engaged in a call and moves from one building to another. The
roaming event occurs when the phone moves into the range of a different AP, and then the phone
authenticates and associates with the new AP. The previous AP hands the call over to the new AP while
maintaining continuous audio connection without user intervention. As long as the APs are in the same
Layer 2 subnet, the wireless IP phone keeps the same IP address and the call continues. As a wireless
IP phone roams between APs, it must reauthenticate with each new AP. See
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If the user is roaming across Layer 2 boundaries, then Layer 3 mobility must be enabled to have seamless
roaming, and to preserve an existing call.
If the wireless IP phone user moves from an AP that covers IP Subnet A to an AP that covers IP Subnet
B without Layer 3 mobility enabled, the phone no longer has an IP address or gateway that is valid
within the new subnet and the call can disconnect.
Layer 3 Roaming
Layer 3 roaming is available with Cisco autonomous and unified deployments. With unified deployments,
Layer 3 roaming can be performed using intercontroller roaming or, if AP groups are used, to enable
Layer 3 roaming.
Layer 3 roaming with Cisco Unified Access Points is accomplished by controllers that use dynamic
interface tunneling and requires Layer 3 mobility to be enabled. Clients that roam across controllers
and VLANS can keep their IP address when using the same SSID.
Fast and Secure Roaming
Cisco Centralized Key Management (CCKM) enables authenticated client devices to roam securely
from one AP to another without any perceptible delay during reassociation. With the support of the
CCKM protocol, the wireless IP phone negotiates the handoff from one AP to another easily. During
the roaming process, the phone scans for the nearby APs, determines which AP can provide the best
service, and then reassociates with the new AP. When implementing stronger authentication methods,
such as WPA2 and EAP, the number of information exchanges increases and causes more delay during
roaming. To avoid delays, use CCKM.
CCKM, a centralized key management protocol, provides a cache of session credentials on the Cisco
Unified Wireless LAN Controller or a Wireless Domain Server (WDS). As the phone roams from one
AP to the next, CCKM compresses the number of message exchanges during roaming by providing a
master key stored on the WDS for the AP to use. The reassociation exchange is reduced to two messages,
thereby reducing the off-network or audio gap time.
For details about CCKM, see the "Cisco Fast Secure Roaming Application Note" at:
http://www.cisco.com/en/US/products/hw/wireless/ps4570/prod_technical_
reference09186a00801c5223.html
Note
for information about authentication.
In dual-band WLANs, it is possible to roam between 2.4 GHz bands (802.11b/g)
and 5 GHz bands (802.11a). The phone moves out of range of one AP using
one band and into the range of another that has the same SSID but is using a
different band.

Cisco Unified Wireless IP Phone 7925G, 7925G-EX, and 7926G Administration Guide

WLANs and Roaming
Authentication Methods,
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