Wmm wi fi qos

Quality of Service in a Wireless Environment

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Table Of Contents

Configuring QoS in a Wireless Environment

This chapter describes how to configure quality of service (QoS) on your Cisco wireless interface. With this feature, you can provide preferential treatment to certain traffic at the expense of other traffic. Without QoS, the device offers best-effort service to each packet, regardless of the packet contents or size. It sends the packets without any assurance of reliability, delay bounds, or throughput.

This chapter consists of these sections:

•Understanding QoS for Wireless LANs

•Configuring QoS

•QoS Configuration Examples

Understanding QoS for Wireless LANs

By default, 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 device, you prioritize network traffic, creating QoS policies and applying the policies to the VLAN configured on your device. Implementing QoS in your wireless LAN makes network performance more predictable and bandwidth utilization more effective. If you do not use VLANs on your network, you can apply your QoS policies to the Ethernet and radio ports.

Note When you enable QoS, the device uses Wi-Fi Multimedia (WMM) mode by default.

QoS for Wireless LANs Versus QoS on Wired LANs

The QoS implementation on wireless LANs differs from QoS implementations on wired networks. With QoS enabled, bridges:

•Do not classify packets; they prioritize packets based on differentiated services code point (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 modular quality of service (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 Enhanced Distributed Coordination Function (EDCF)-like queuing on the radio egress port only.

•They do only FIFO queuing on the Ethernet egress port.

•They support only 802.1Q/P tagged packets. Bridges do not support InterSwitch Link Protocol (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/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/index.htm

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 device 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 device. These are the effects of QoS on network traffic:

•The radio downstream flow is traffic transmitted out the device radio to another bridge. This traffic is the main focus for QoS on a wireless LAN.

•The radio upstream flow is traffic received on the device radio from another bridge. QoS for wireless LAN does not affect this traffic.

•The Ethernet downstream flow is traffic sent from a switch or a router to the Ethernet port on the device. If QoS is enabled on the switch or router, the switch or router might prioritize and rate-limit traffic to the device.

•The Ethernet upstream flow is traffic sent from the device Ethernet port to a switch or router on the wired LAN. The device does not prioritize traffic that it sends to the wired LAN based on traffic classification.

Precedence of QoS Settings

When you enable QoS, the device queues packets based on the Layer 2 class of service value for each packet. The device applies QoS policies in this order:

1. Packets already classified—When the device receives packets from a QoS-enabled switch or router that has already classified the packets with non-zero 802.1Q/P user_priority values, the device uses that classification and does not apply other QoS policy rules to the packets. An existing classification takes precedence over all other policies on the device.

Note A Cisco device always acts on tagged 802.1P packets that it receives over the radio interface, even if a QoS policy has not been configured.

2. Policies you create on the device—QoS Policies that you create and apply to VLANs or to the device interfaces are second in precedence after previously classified packets.

3. Default classification for all packets on VLAN—If you set a default classification for all packets on a VLAN, that policy is third in the precedence list.

Using Wi-Fi Multimedia Mode

When you enable QoS, the device uses Wi-Fi Multimedia (WMM) mode by default.

The following features of the WMM specification are supported:

•Addition of the WMM information element to associate request frames

•Addition of the WMM parameter element to the beacon, probe response and association response frames

•Addition of the QoS control field to data frames

•Support for setting the field sent in the WMM parameter element (per access class):

–contention window (CW) min

–CW max

–Arbitration Interframe Space (AIFS)

–Admission control required

–Transmit opportunity (TXOP) size

•Separate transmit sequence numbers for each access class and for frames that do not have the QoS control field

•Separate duplicate sequence number checking lists on receive for each access class and for frames that do not have the QoS control field

•No ACK frames for QOS control fields that do not require ACK

•Negotiation of WMM capability with client on reassociation

•Support for burst transmission of multiple frames in a transmit opportunity

•Support for the WMM specified backoff procedure

•Support for the WMM retransmit procedure

•Addition of 802.1d priority for WMM enabled clients

•Support for separate Temporal Key Integrity Protocol (TKIP) replay detection counters on receive for each access class and for frames that do not have the QOS control field

The following features of the WMM specification are supported only on Cisco 3201 WMIC:

•Transmission of a packet with the no ACK required bit set in the QoS control field

•End of service period (EOSP) bit in the QoS control field

•Management action frames

•Traffic Specification (TSPEC) element

•Admission control procedure

•Enforcement of admission control required field

•Triggered power save delivery

Configuring QoS

QoS is enabled by default. This section describes how to configure QoS on your device. Before configuring QoS on your device, you should be aware of this information:

•The most important guideline in QoS deployment is to be familiar with the traffic on your wireless LAN. If you know the applications used by wireless client devices, the applications’ sensitivity to delay, and the amount of traffic associated with the applications, you can configure QoS to improve performance.

•QoS does not create additional bandwidth for your wireless LAN; it helps control the allocation of bandwidth. If you have plenty of bandwidth on your wireless LAN, you might not need to configure QoS.

Layer2 QoS (RBCP and Voice)

Cisco devices can support wireless voice clients that are 802.11e compliant, transmitting and receiving frames with the Layer 2 802.1p priority bits set.

Between the host router and wireless device there is an Router Blade Control Protocol (RBCP) that monitors the health of a device by sending periodic keep alive packets. The device sends layer2 802.1q tagged RBCP packets with the highest priority (priority 7) in the keep alive packets to ensure the keep alive packets make it to the host router, even on a congested network.

The Layer 2 RBCP packet is shown in the following diagram.

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