Cisco Cisco Access Registrar 4.2 Specifikace stáhnout pdf (Strana 293)

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Cisco Wireless LAN Controller Configuration Guide

OL-13826-01

Chapter 6 Configuring WLANsWireless Device Access

Configuring WLANs

Configuring QoS Enhanced BSS

The QoS Enhanced Basis Service Set (QBSS) information element (IE) enables the access points to

communicate their channel usage to wireless devices. Because access points with high channel usage

might not be able to handle real-time traffic effectively, the 7921 or 7920 phone uses the QBSS value to

determine if they should associate to another access point. You can enable QBSS in these two modes:

• Wi-Fi Multimedia (WMM) mode, which supports devices that meet the 802.11E QBSS standard

(such as Cisco 7921 IP Phones)

• 7920 support mode, which supports Cisco 7920 IP Phones on your 802.11b/g network

The 7920 support mode has two options:

–

Support for 7920 phones that require call admission control (CAC) to be configured on and

advertised by the client device (these are typically older 7920 phones)

–

Support for 7920 phones that require CAC to be configured on and advertised by the access

point (these are typically newer 7920 phones)

When access point-controlled CAC is enabled, the access point sends out a Cisco proprietary

CAC Information Element (IE) and does not send out the standard QBSS IE.

You can use the controller GUI or CLI to configure QBSS. QBSS is disabled by default.

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 Chapter 4 for more information and configuration instructions for load-based CAC.

• If your WLAN contains both 1000 series access points and Cisco 7920 wireless phones, do not

enable WMM mode or access point-controlled CAC. The information sent by 1000 series access

points in the WMM and AP-CAC-LIMIT QBSS information elements is inaccurate and could result

in degradation of voice quality 7920 wireless phones. However, you may use client-controlled CAC

in networks containing both 1000 series access points and 7920 wireless phones.

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Cisco Cisco Access Registrar 4.2 Specifikace Strana 293