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Deal with interference on a WLAN

Keep your aiwaves clear.

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I can't tell you how many times in the past 15 years someone has asked me about radio frequency (RF) interference. Even today, some still believe that wireless LANs simply can't work because of interference (meaning they think throughput and reliability are at unacceptable levels).

Of course, we now have proof from hundreds of millions of more-than-satisfied Wi-Fi users who seldom complain about interference. But while throughput and reliability aren't big issues today, that doesn't mean that interference isn't an issue. Indeed, it is - but since interference tends to be intermittent in most cases, most users simply don't notice.

Over time, though, network managers will need to have a strategy for dealing with interference because the number of WLANs, and especially other devices using the unlicensed bands, is growing rapidly. But there's good news here; it is relatively easy to deal with the interference challenge, and it's going to get easier.

Interference is not noise

First, let's define interference. It's often used synonymously with "noise," but noise is a phenomenon inherent to the known universe and particularly electronic components like amplifiers. Noise is thus the province primarily of engineers building radios.

Interference, on the other hand, is the presence of two signals in close proximity at the same time and at or near the same frequency, where one signal overwhelms the other by virtue of being louder (having more relative power, or amplitude) or where the two conflict with each another. Interference is usually infrequent and bursty in nature; in most cases, it won't even be noticed because automatic retransmission of damaged packets compensates. But more severe interference can cause a correspondingly severe, or even total, degradation of throughput; unacceptable voice quality; or unwatchable video.

There are several ways to deal with interference, generally falling into one of two domains. The first of these is protocol-related, and, indeed, 802.11-based WLANs are designed to operate in the presence of interference. The use of spread-spectrum radio techniques, essentially mandated in the unlicensed bands, also helps reduce interference to some degree. But protocols and spread-spectrum radio can carry us only so far.

This brings up the second approach, which is energy-related. This technique is very useful (if not essential) in dealing with non-Wi-Fi sources of interference, because Wi-Fi radios can't identify or otherwise deal with non-Wi-Fi sources. But note the energy-based approach is also useful in managing Wi-Fi installations, for example, in picking the best radio channel based on its relative lack of any form of traffic. I always carefully monitor the airwaves when running benchmarks, just to make sure interference doesn't skew the results.

Enter the assurance tool

While not yet common, a new class of WLAN assurance tool is now available for doing Wi-Fi-oriented real-time spectrum assurance (SA) - dealing with energy and not just protocols. Based on the concept of a spectrum analyzer, an expensive piece of test equipment used primarily by engineers for product design work, these new SA tools are inexpensive and suitable for use by non-engineers - just what the network manager ordered. SA tools are available as stand-alone products or integrated into some WLAN assurance tools already on the market.

So we have the technology we need for managing interference, but wait, it gets even better. Eventually, tools dealing with the energy domain will be integrated directly into WLAN systems, making the RF spectrum management functionality already present in many products even more effective. So, while interference is likely to get worse, most of us will never even notice. And, in the interim, SA tools will be used to implement manual work-arounds whenever the challenge of interference becomes noticeable.

For more on this topic, see our latest white paper (download PDF) and Tech Note (download PDF). I'm now finishing up a series of empirical studies on the effects of interference of various forms of WLAN traffic, as well as a Tech Note on interference and Wi-Fi meshes. I'll have all of these for you soon.

Craig J. Mathias is a principal at Farpoint Group, an advisory firm specialising in wireless networking and mobile computing. This article appeared in Computerworld.


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