Cisco 7920 - Unified Wireless IP Phone VoIP, Administration Manual

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Chapter 2 An Overview of the Wireless Network
Components of the VoIP Wireless Network
2-10
Cisco Wireless IP Phone 7920 Administration Guide for Cisco CallManager Release 4.0 and 4.1
OL-7104-01
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QoS Basis Service Set (QBSS)—The access point uses this beacon
information element (IE) to send the channel utilization of the access point to
the Cisco Wireless IP Phone. The phone uses the QBSS value to determine
whether the access point can effectively handle more traffic.
The Cisco Wireless IP Phone associates with the access point with the highest
RSSI and lowest channel utilization values (QBSS) that have matching SSID and
encryption types.
Related Topics
•
Roaming in a Wireless Network, page 2-10
• Securing Voice Communications, page 2-4
•
Wireless Network and Access Point Configuration, page 2-19
Roaming in a Wireless Network
Cisco Wireless IP Phone users have the ability to move from one location in the
premises to another while conversing on the phone. Unlike cellular phones that
have broad coverage, the coverage area for the Cisco Wireless IP Phone is
smaller; therefore, phone users must roam from one access point to another more
frequently. To understand some of the limitations of roaming with wireless IP
phones, the following examples provide information about roaming in the
WLAN.
•
Pre-call Roaming — A Cisco Wireless IP Phone 7920 user powers on the
phone in the office, and the phone associates with the nearby access point.
The user leaves the building and moves to another building where he places
a call. The phone associates with a different access point in order to place the
call from the new location. If the associated access point is within the same
Layer 2 VLAN, the IP address remains the same for the phone. But, if the
roaming phone crosses a Layer 3 boundary with DHCP enabled, the phone
recognizes that it is no longer in the same subnet. The phone requests a new
IP address before it can connect to the network and place the call.
•
Mid-call Roaming — A Cisco Wireless IP Phone 7920 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
the phone authenticates and associates with the new access point. The current
access point hands the call over to the new access point while maintaining
continuous audio connection without user intervention. As long as the access