Chapter 7 Configuring QoS on an Access Point

Configuration Guidelines

Cisco Wireless IP Phone 7920 Support

The Wireless IP Phone 7920 phone uses Cisco CDP message with Appliance VLAN-ID TLV to inform the access point of its presence. The access point intercepts the CDP messages sent from the client, and if it contains the Appliance VLAN-ID TLV, it should flag the client as VoIP phone client.

If VLAN is enabled, all phone clients should be associated to a single Voice VLAN. If VLAN is not enabled, we advise that all the VoIP packets be classified by using the same user_priority value (6).

The access point always uses DIFS with minimum contention window (CW) value derived from the CWmax and CWmin range parameters to prioritize voice traffic.

Radio Interface Transmit Queues

The radio maintains four priority queues, one for each traffic category, and 802.11e Enhanced DCF (EDCF) to provide differentiate Distributed Coordination Function (DCF) access to the wireless medium. An EDCF-aware access point is assigned distinct pairs of CWmin and CWmax parameters for each traffic category. The CWmin and CWmax parameters can be modified through the CLI.

Adjusting Radio Access Categories

The access point uses the radio access categories to calculate backoff times for each packet. As a rule, high-priority packets have short backoff times.

The default values in the minimum and maximum contention window fields, and in the slot time fields are based on settings recommended in IEEE Draft Standard 802.11e. For detailed information on these values, consult the standard.

We recommend that you use the default settings. Changing these values can lead to unexpected blockages of traffic on your wireless LAN, and the blockages might be difficult to diagnose. If you change these values and find that you need to reset them to the default values.

The values listed in Table 7-2are to the power of 2. The access point computes contention window values by using the equation CW = 2 ** X minus 1, where X is the value in Table 7-3.

Table 7-3

CWmin and CWmax Default Parameters

 

 

 

 

 

 

Priority

 

Fixed slottime Number

CWmin

CWmax

 

 

 

 

 

0

 

6

15

1023

 

 

 

 

 

1

 

2

15

1023

 

 

 

 

 

2

 

1

7

15

 

 

 

 

 

3

 

1

3

7

 

 

 

 

 

Cisco Wireless Router and HWIC Configuration Guide

 

OL-6415-03

7-5

 

 

 

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3D Connexion OL-6415-03 manual Radio Interface Transmit Queues, Cisco Wireless IP Phone 7920 Support
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