3D Connexion OL-6415-03 User Manual

CHAPTER

7

Configuring QoS on an Access Point

This chapter describes how to configure quality of service (QoS) on an access point. QoS provides
preferential treatment to certain traffic at the expense of other traffic. Without QoS, the access point
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.

Recommended Reading

The following information is recommended for gaining a better understanding of QoS as it applies to
voice deployments in a wireless environment:

Wireless Quality-of-Service Deployment Guide

http://www.cisco.com/en/US/products/hw/wireless/ps430/prod_technical_reference09186a008014449

8.html

Cisco Wireless IP Phone 7920 Design and Deployment Guide

http://www.cisco.com/en/US/products/hw/phones/ps379/products_implementation_design_guide_boo
k09186a00802a029a.html

Understanding QoS for Wireless LANs

Typically, networks operate on a best-effort delivery basis that 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 QoS is configuredonanaccesspoint,youcanselectspecific network traffic, prioritize it, and use
congestion-management and congestion-avoidance techniques to provide preferential treatment.
Implementing QoS in a wireless LAN makes network performance more predictable and bandwidth
utilization more effective.

Configuring QoS creates and applies policies to the VLANs configured on the access point. If VLANs
are not used on the network, QoS policies can be applied to the 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, access points perform the following:

• Does notclassify packets; it prioritizes 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.

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Understanding QoS for Wireless LANs

• Does not construct internal DSCP values; it only supports mapping by assigning IP DSCP,

precedence, or protocol values to Layer 2 CoS values. Table 7-1 lists the class of service (CoS)
values as they map to DSCP values.

Table 7-1 CoS Values Mapped to DSCP Values

CoS Value DSCP Value

110
218
326
434
546
648
756

• Carries out Enhanced DCF (EDCF)-like queuing on the radio egress port only.

• Support only 802.1Q/P tagged packets. Access points do not support ISL.

• Support only Cisco Modular QoS CLI (MQC) policy-map set cos action.

• Prioritize the trafficfrom voice clients (such as Symbol phones) over traffic from other clients when

the QoS Element for Wireless Phones feature is enabled.

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

Chapter 7 Configuring QoS on an Access Point

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 access point 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 access point:

• The radio downstream flow is traffic transmitted out the access point radio to a wireless client

device. This traffic is the main focus for QoS on a wireless LAN.

• The radio upstream flow is traffic transmitted out the wireless client device to the access point. QoS

for wireless LANs does not affect this traffic.

• The Ethernet downstream flow is traffic sent from a switch or a router to the Ethernet port on the

access point. If QoS is enabled on the switch or router, the switch or router might prioritize and
rate-limit traffic to the access point.

• The Ethernet upstream flow is traffic sent from the access point Ethernet port to a switch or router

on the wired LAN. The access point does not prioritize traffic that it sends to the wired LAN based
on traffic classification.

Figure 7-1 shows the upstream and downstream traffic flow.

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Chapter 7 Configuring QoS on an Access Point

Figure 7-1 Upstream and Downstream Traffic Flow

Configuration Guidelines

Radio

downstream

Client

device

Radio

upstream

Precedence of QoS Settings

When you enable QoS, the access point queues packets based on the CoS value for each packet. If a
packet matches one of the filter types based on its current precedence, the packet is classified based on
the matching filter and no other filters are applied.

Table 7-2 shows the precedence of QoS filters. Precedence number zero is the highest.

Table 7-2 Precedence of QoS Filters

Precedence Filter Type

0 Dynamicly created VoIP client filter. Traffic from voice clients takes

priority over other traffic regardless of other policy settings. This setting
takes precedence over all other policies, second only to previously
assigned packet classifications.

1 User configured class-map match clause (except match any). QoS Policies

configured for and that apply to VLANs or to the access point interfaces
are third in precedence after previously classified packets and the QoS
Element for Wireless Phones setting

2 User configured class-map match any clause (match VLAN). If a default

classification for all packets on a VLAN is set, that policy is fourth in the
precedence list.

Access

point

Ethernet

downstream

Ethernet

upstream

Wired
LAN

81732

Configuration Guidelines

Before configuring QoS on an access point, you should be aware of this information:

• Be familiar with the traffic on your wireless LAN. If you know the applications used by wireless

client devices, the sensitivity of applications to delay, and the amount of traffic associated with the
applications, configuring QoS improves performance.

• QoS does not create additional bandwidth on a wireless LAN; it helps control the allocation of

bandwidth. If there is enough of bandwidth on your wireless LAN, it might not be necessary to
configure QoS.

An access point is essentially a Layer 2 transparent bridge between wired and wireless networks.
Typically,bandwidth on wireless side constrains the the wired side. For example, 802.11b offers 6 Mbps
half duplex and 100baseT offers 100 Mbps full duplex.

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