Cisco CISCO1401 Software Guide - Page 162

Understanding QoS for Wireless LANs Chapter 13 Configuring QoS Understanding QoS for Wireless LANs Typically, networks operate on a best-effort delivery basis, which means that all traffic has equal priority and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an equal chance of being dropped. When you configure QoS on the bridge, you can select specific network traffic, prioritize it, and use congestion-management and congestion-avoidance techniques to provide preferential treatment. Implementing QoS in your wireless LAN makes network performance more predictable and bandwidth utilization more effective. When you configure QoS, you create QoS policies and apply the policies to the VLAN configured on your bridge. If you do not use VLANs on your network, you can apply your QoS policies to the bridge's Ethernet and radio ports. QoS for Wireless LANs Versus QoS on Wired LANs The QoS implementation for wireless LANs differs from QoS implementations on other Cisco devices. With QoS enabled, bridges perform the following: • They do not classify packets; they prioritize packets based on DSCP value, client type (such as a wireless phone), or the priority value in the 802.1q or 802.1p tag. • They do not match packets using ACL; they use only MQC class-map for matching clauses. • They do not construct internal DSCP values; they only support mapping by assigning IP DSCP, Precedence, or Protocol values to Layer 2 COS values. • They carry out EDCF like queuing on the radio egress port only. • They do only FIFO queueing on the Ethernet egress port. • They support only 802.1Q/P tagged packets. Bridges do not support ISL. • They support only MQC policy-map set cos action. To contrast the wireless LAN QoS implementation with the QoS implementation on other Cisco network devices, see the Cisco IOS Quality of Service Solutions Configuration Guide at this URL: http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/fqos_c.html Impact of QoS on a Wireless LAN Wireless LAN QoS features are a subset of the proposed 802.11e draft. QoS on wireless LANs provides prioritization of traffic from the bridge over the WLAN based on traffic classification. Just as in other media, you might not notice the effects of QoS on a lightly loaded wireless LAN. The benefits of QoS become more obvious as the load on the wireless LAN increases, keeping the latency, jitter, and loss for selected traffic types within an acceptable range. QoS on the wireless LAN focuses on downstream prioritization from the bridge. These are the effects of QoS on bridge traffic: • The radio downstream flow is traffic transmitted out the bridge radio to another bridge. This traffic is the main focus for QoS on a wireless LAN. • The Ethernet downstream flow is traffic sent from a switch or a router to the Ethernet port on the bridge. If QoS is enabled on the switch or router, the switch or router might prioritize and rate-limit traffic to the bridge. 13-2 Cisco Aironet 1400 Series Wireless Bridges Software Configuration Guide OL-4059-01