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QoS for WLANs

Some applications require the network to provide higher levels of services for proper performance. Quality of service (QoS) is used to assist this effort of adding varying levels of priority to traffic. The use of voice, video, and other very time-sensitive applications are driving this requirement into the WLAN products.

QoS on wired networks provides enhanced network service in several ways, including the following:

Dedicated bandwidth for critical users and applications

Limiting jitter and latency (required by real-time traffic)

Minimizing network congestion

Shaping network traffic to smooth the traffic flow

Setting network traffic priorities

In the WLAN arena, not all of these advantages can be achieved because of the overall protocol and distinct differences between wired and wireless networks.

Wireless QoS Deployment Schemes

In the past, WLANs were used to transport low-bandwidth, data application traffic such as bar code information. Today, with the expansion of WLANs into enterprise and mainstream networking applications, WLANs are used to handle high-bandwidth, data-intensive applications as well as time-sensitive voice and multimedia applications. This requirement has led to the necessity for wireless QoS.

Several vendors support proprietary wireless QoS schemes for voice applications. To speed up the rate of QoS adoption and to support multivendor applications, the IEEE 802.11e standard will provide a unified approach to wireless QoS. The completion of the 802.11e standard should take place sometime in 2005, with available support on WLAN products following soon afterward. The QoS that is defined in 802.11e will provide methods for both upstream and downstream traffic, and will define ways to support options for power savings on clients while maintaining a level of QoS.

Downstream and Upstream QoS

Figure 9-10 illustrates the definition of upstream and downstream QoS. Radio downstream QoS refers to wireless traffic leaving the AP and traveling to the WLAN clients. Radio upstream QoS refers to traffic originating in the client and moving to the AP. In most APs today, radio downstream QoS is the only QoS supported. Support is currently proprietary or in development by WLAN vendors for radio upstream QoS. Both upstream as well as downstream QoS over the wireless are covered in the 802.11e specification.

Ethernet downstream refers to wired traffic traveling to the AP. QoS can be used and applied at this point in the network to prioritize and rate-limit traffic to the AP. Configuring Ethernet downstream QoS is not discussed in this book.

Ethernet upstream refers to the wired traffic leaving the AP traveling to the network. The AP provides the classification of the traffic from the AP to the upstream network.

QoS and Network Performance

The performance improvement from the use of QoS might not be easily detected on a lightly loaded network. If latency, jitter, and packet loss are noticeable when the network is lightly loaded, it indicates a network fault or that an application's requirements are not a good match for the network.

QoS will start to provide improved application performance as traffic on the network increases. QoS helps to keep latency, jitter, and packet loss for selected traffic types within acceptable limits by providing downstream prioritization from the AP.

Until such time that 802.11e is completed and becomes available on clients, radio upstream (client) traffic is treated as best effort. A client has to compete with other clients for (radio upstream) transmission and compete for the radio waves with (radio downstream) transmission from the AP. Under certain traffic conditions, a client can experience radio upstream congestion and the performance of QoS-sensitive applications might be unacceptable despite the QoS features on the AP.

The same rules for deploying QoS in a wired network apply to deploying QoS in a WLAN. It is imperative that you understand the type of traffic and the requirements of that traffic as it moves across your network. The protocols, the application's sensitivity to delay, and traffic bandwidth all play an important part in using QoS and placing priority on the types of traffic.

It is also important to investigate the network to verify any limitations imposed by wired network infrastructure components (switched, routers, or hubs) that could impact the use of QoS. For example, having advanced QoS on the AP might be of little use if plugged into a 10-Mbps hub.

Also keep in mind that QoS does not create additional bandwidth, it just provides additional control for how the bandwidth is allocated.