Specifications

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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
Wireless voice users are mobile and often roam across a campus or between floors
in a building while they are connected to a call. In contrast, data users remain in
one place or occasionally move to another location. The ability to roam while
maintaining voice session continuity is one of the advantages of wireless voice;
therefore, RF coverage needs to include areas not usually covered for data, such
as stairwells, elevators, quiet corners outside conference rooms, and passage
ways.
To assure good voice quality and optimal RF signal coverage, you must perform
a site survey that determines settings suitable to wireless voice. The survey results
provide information for the design and layout of the WLAN for voice, such as
access point placement, power levels, and channel assignments.
After deploying and using wireless voice, you should continue to perform post
installation site surveys. When you add a group of new users or install more
equipment or stack large amounts of inventory, you are changing the wireless
environment. You must verify that the access point coverage is still adequate for
optimal voice communications. See the
“Site Survey Verification” section on
page 2-31 for more information.
Associating to an Access Point
At startup, the Cisco Unified Wireless IP Phone 7921G uses its radio to scan for
access points with Service Set Identifiers (SSIDs) and encryption types that it
recognizes. The phone builds and maintains a list of eligible access point targets
and uses the following variables to determine the best access point with which to
associate.
• Received Signal Strength Indicator (RSSI)—The phone uses this value to
determine the signal strength of available access points within the RF
coverage area. The phone attempts to associate with the access point with the
highest RSSI value.
• QoS Basic Service Set (QBSS)—The access point uses this beacon
information element (IE) to send the channel utilization of the access point to
the unified IP
phone. The phone uses the QBSS value to determine whether
the access point can effectively handle more traffic.
Note QBSS is not supported when using Wi-Fi 802.11a.