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Cisco Wireless LAN Controller Configuration Guide
OL-17037-01
Chapter 6 Configuring WLANsWireless Device Access
Configuring WLANs
Guidelines for Configuring QBSS
Follow these guidelines when configuring QBSS on a WLAN:
•
7920 phones are non-WMM phones with limited CAC functionality. The phones look at the channel
utilization of the access point to which they are associated and compare that to a threshold that is
beaconed by the access point. If the channel utilization is less than the threshold, the 7920 places a
call. In contrast, 7921 phones are full-fledged WMM phones that use traffic specifications (TSPECs)
to gain access to the voice queue before placing a phone call. The 7921 phones work well with
load-based CAC, which uses the percentage of the channel set aside for voice and tries to limit the
calls accordingly.
Because 7921 phones support WMM and 7920 phones do not, capacity and voice quality problems
can arise if you do not properly configure both phones when they are used in a mixed environment.
To enable both 7921 and 7920 phones to co-exist on the same network, make sure that load-based
CAC and 7920 AP CAC are both enabled on the controller and the WMM Policy is set to Allowed.
This becomes particularly important if you have many more 7920 users than 7921 users.
Note
Refer to
for more information and configuration instructions for load-based CAC.
Additional Guidelines for Using 7921 and 7920 Wireless IP Phones
Follow these guidelines to use Cisco 7921 and 7920 Wireless IP Phones with controllers:
•
Aggressive load balancing must be disabled for each controller. Otherwise, the initial roam attempt
by the phone may fail, causing a disruption in the audio path.
•
The Dynamic Transmit Power Control (DTPC) information element (IE) must be enabled using the
config 802.11b dtpc enable
command. The DTPC IE is a beacon and probe information element
that allows the access point to broadcast information on its transmit power. The 7921 or 7920 phone
uses this information to automatically adjust its transmit power to the same level as the access point
to which it is associated. In this manner, both devices are transmitting at the same level.
•
Both the 7921 and 7920 phones and the controllers support Cisco Centralized Key Management
(CCKM) fast roaming.
•
When configuring WEP, there is a difference in nomenclature for the controller and the 7921 or 7920
phone. Configure the controller for 104 bits when using 128-bit WEP for the 7921 or 7920.
•
For standalone 7921 phones, load-based CAC must be enabled, and the WMM Policy must be set to
Required on the WLAN.
•
The controller supports traffic classification (TCLAS) coming from 7921 phones using firmware
version 1.1.1. This feature ensures proper classification of voice streams to the 7921 phones.
•
When using a 7921 phone with the 802.11a radio of a 1242 series access point, set the 24-Mbps data
rate to Supported and choose a lower Mandatory data rate (such as 12 Mbps). Otherwise, the phone
might experience poor voice quality.
