Specifications

2-15
Cisco Unified Wireless IP Phone 7921G Administration Guide for Cisco Unified CallManager Release 4.1, 4.2, 5.0
OL-10802-02
Chapter 2 An Overview of the Voice Over IP Wireless Network
Components of the VoIP Wireless Network
Note If a user leaves the WLAN coverage area and then comes back into the
same WLAN area, the phone must reconnect to the network. By pressing
a key on the phone, the user activates the phone and increases the
scanning rate to speed up reconnecting to the 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 access point, and then the phone
authenticates and associates with the new access point. The previous access
point hands the call over to the new access point while maintaining
continuous audio connection without user intervention. As long as the access
points are in the same Layer 2 subnet, the unified IP
phone keeps the same IP
address and the call continues. As a unified IP
phone roams between access
points, it must re-authenticate with each new access point. See the
“Authentication Mechanisms in the Wireless Network” section on page 2-19
for information about authentication.
If the unified IP phone user moves from an access point that covers
IP
Subnet A to an access point that covers IP Subnet B, 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—With the release of the Cisco Catalyst 6500 Series
Wireless LAN Services Module (WLSM), the Cisco
Unified Wireless
IP
Phone 7921G now supports Layer 3 roaming for autonomous mode access
points. For details about the Cisco WLSM, refer to the product
documentation available at:
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/cfgnotes/wls
m_1_1/index.htm
Layer 3 roaming with lightweight mode access points is accomplished by
controllers that use dynamic interface tunneling. 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 access point
to another without any perceptible delay during reassociation. With the
support of CCKM protocol, the wireless
IP phone is able to negotiate the
handoff from one access point to another more easily. During the roaming
process, the phone must scan for the nearby access points, determine which