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How and why to implement Wi-Fi QoS

Quality of service, as network pros know, prioritizes apps as they cross the network. But Wi-Fi QoS needs to be considered, too, in relation to the overall network.

Quality of service on any type of network seeks to achieve the same goals, namely to ensure that latency-sensitive applications have priority over applications that can tolerate delay and congestion. In other words, it's all about the end-user experience.

For example, my voice over Wi-Fi call might be miserable if my network traffic hits momentary delays, but my web browsing likely won't be perceptively affected. This premise holds true for wired and wireless networks alike.

Complete books and book chapters have detailed QoS in networking. But, in this article, let's explore the basics of QoS for wireless networks.

The key ingredients of Wi-Fi QoS

For Wi-Fi QoS to work properly, three things need to support it. The client device, the wireless network itself and the application in question need to be designed to recognize and use QoS underpinnings in their configurations. Generally, Wi-Fi QoS comes from the 802.11e standard, adapted specifically to Wi-Fi through Wi-Fi Multimedia, or WMM, which enhances QoS by prioritizing data packets according to four categories: voice, video, best effort and background.

Wireless LAN (WLAN) vendors can prioritize network traffic in multiple ways. Though the basic constructs are fairly common, the terminology may differ in different product sets.

In the screenshot below, Cisco references QoS profiles as Platinum, Gold, Silver and Bronze. Other vendors simply use voice, video, best effort and background to assign a priority. This configuration assigns a standards-defined priority to a WLAN network by a service set identifier. You may also choose to apply custom bandwidth controls per wireless client or implement application-specific QoS settings if your equipment is capable of identifying specific applications.

Cisco wireless QoS
Cisco WLAN QoS references four profiles: Platinum, Gold, Silver and Bronze. Other vendors use voice, video, best effort and background to assign a priority.

Advantages and disadvantages of Wi-Fi QoS

When certain applications need a highly reliable WLAN environment -- like Vocera voice badges in a hospital, for example -- careful network design and QoS implemented by skilled professionals are often the difference between success and failure. When QoS works as intended, priority apps get through first because those are the rules of the network. QoS essentially works as an insurance policy: When the important apps need bandwidth reservations ahead of other traffic, they will get it.

But QoS can be difficult to get right. When trying to meet the goals of QoS, you could cause other problems that can be hard to troubleshoot. QoS is not a silver bullet, because the design of the WLAN is just as important.

In many cases, it's not enough just to configure QoS on the WLAN; you often have to extend it through the entire connecting LAN for the advantages to be realized. Both the LAN and WLAN sides of the network may have different approaches to QoS, and it can get interesting trying to stich all the pieces together, especially in a multivendor environment.

Wi-Fi QoS for home networks

Many consumer-grade routers are marketed as being QoS-capable. At home, the prioritization tends to be focused on gaming that needs the lowest latency you can provide, and perhaps video.

It's important to remember you can only affect the performance of these applications on your own network, within your own environment, and not beyond. Traffic between devices on your network gets QoS treatment, anything heading to the internet does not. When the network traffic leaves your router and hits the internet, it all tends to get munged together in a big best-effort mix.

One of the best things you can do to reduce latency for any application is to run it on a solid wired network connection and eliminate the variability of Wi-Fi.

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