To help address the problems of VoIP stability and roaming, an initial Call Admission Control (CAC) scheme
is required. With CAC, QoS is maintained in a network overload scenario by ensuring that the number of
active voice calls does not exceed the configured limits on the AP. The Cisco Unified IP Phone can integrate
layer 2 TSpec admission control with layer 3 Cisco Unified Communications Manager admission control
(RSVP). During times of network congestion, calling or called parties receive a fast busy indication. The
system maintains a small bandwidth reserve so wireless phone clients can roam into a neighboring AP, even
when the AP is at full capacity. After reaching the voice bandwidth limit, the next call is load-balanced to a
neighboring AP without affecting the quality of the existing calls on the channel.
Implementing QoS in the connected Ethernet switch is highly desirable to maintain good voice quality. The
COS and DSCP values that the Cisco Unified IP Phone sets do not need to be modified.
Packet loss occurs during roaming; however, the security mode and the presence of fast roaming determines
how many packets are lost during transmission.
Note
For more information about Voice QoS in a wireless network, see Cisco Unified IP Phone 9971 Wireless
LAN Deployment Guide at: http://www.cisco.com/en/US/products/ps10453/products_implementation_design_
guides_list.html.
The Cisco Unified IP Phone 9971 does not support Video CAC; however, Voice CAC is supported for
WLANs.
Note
Supported Antennas
Some Cisco access points require or allow external antennas. See the following URL for the list of supported
antennas and how these external antennas should be mounted:
http://www.cisco.com/en/US/prod/collateral/wireless/ps7183/ps469/product_data_sheet09186a008008883b.html
The Cisco Aironet Series 1130 and 1140 access points must be mounted on the ceiling because they
possess omnidirectional antennas.
Note
802.11 Standards for WLAN Communications
Wireless LANs must follow the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards that
define the protocols that govern all Ethernet-based wireless traffic. The Cisco Unified IP Phone supports the
following standards:
•
802.11a: Uses the 5 GHz band that provides more channels and improved data rates by using OFDM
technology. Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) support this
standard.
•
802.11b: Specifies the radio frequency (RF) of 2.4 Ghz for both transmitting and receiving data at lower
data rates (1, 2, 5.5, 11 Mbps).
Cisco Unified IP Phone 8961, 9951, and 9971 Administration Guide for Cisco Unified Communications Manager
10.0
50
802.11 Standards for WLAN Communications
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