Interference: Attack of the metro-scale Wi-Fi system

In my last column, I talked about the dramatic effects that radio-frequency interference can have on different forms of wireless LAN traffic I also talked about interference mitigation techniques, and why I believe the new crop of spectrum assurance (SA) tools will provide the solution to this challenge, especially once they are integrated with the wireless LAN assurance (WLA) and RF spectrum management (RFSM) capabilities already available. Again, a networkwide, systems-level approach to integration and management is ultimately the core requirement. Even if we do put that in place, interference will still represent a challenge -- albeit one that we can deal with effectively.

But there is one more issue related to interference that needs to be covered: whether metro-scale Wi-Fi systems being deployed around the world will be subject to interference from unrelated WLAN systems operating in the same geographic locations.

And, more important, there is the question of whether large-scale Wi-Fi meshes will cause interference to these other installed Wi-Fi systems, which we expect to become the default connectivity medium for both commercial and residential systems. The potential for interference in the unlicensed bands remains a concern, especially as we continue to add more systems with more users, and these users continue to demand bandwidth.

Interference both from and to the mesh needs to be considered. It's not at all unusual to find 50, 100 and even more WLANs running simultaneously in a given location in major metropolitan areas. Most of these use channels 1, 6 and 11 in the 2.4 GHz band because that's what most people have been conditioned to do, even though these are seldom the best choices in single-AP residential deployments with small numbers of users and (usually) limited throughput regardless -- the throughput limitation being determined by the speed of the connection to the Internet, perhaps no more than a few megabits per second. But even if the vast majority of these WLANs have limited duty cycles, the combined effect might be detrimental to WLAN mesh nodes located nearby.

The reverse, however, is even more of a concern. Mesh nodes operate at far higher duty cycles, with far greater numbers of users, and with increasing traffic requirements in terms of both raw and time-bounded throughput. And even more important, mesh nodes operate at much higher transmission power, and with greater antenna gain, than their residential and commercial brethren. So an interim conclusion is that meshes will indeed cause detrimental interference to nearby unrelated WLANs.

One way to deal with this in residential environments is to use customer-premises equipment (CPE) functioning as part of the mesh in place of a traditional residential AP. This technique can dramatically cut down on interference, at least in settings where the residence is getting access via the mesh. You can see an example of a CPE at he Ruckus Wireless Inc. Web site.

I think the long-term solution is to integrate spectrum assurance tools into all WLAN products. No matter what we do, interference never goes away entirely, but we have some good approaches to applying a high degree of automation to minimize its effects. You can read more about this topic here. I'll continue to explore this area as I look into the next big challenge in metro-scale Wi-Fi meshes -- how to evaluate performance. That's a topic for later this year.

Craig J. Mathias is a principal at Farpoint Group, an advisory firm specializing in wireless networking and mobile computing. He can be reached at

Copyright © 2007 IDG Communications, Inc.

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