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The Trouble with White Spaces

Like several other engineers, I'm disturbed by the white spaces debate because it focuses on what I regard as the wrong question. The White Space Coalition argues that showing that a system can be constructed that prevents interference between White Space Devices and television broadcast signals compels the Commission to offer up the White Spaces for unlicensed use. This is far from obvious.

I take it for granted that computer-based systems can detect television broadcast signals with reasonable accuracy, either with or without a geo-location database. We do something similar in Wi-Fi systems today with respect to the DFS-II standard for radar avoidance. So the question of non-interference is essentially settled. But the more serious question is whether White Spaces are best deployed as a third set of Wi-Fi frequencies or under some other regulatory model. Proponents of the Wi-Fi model argue, as Tom Evslin did on CircleID, that "If we got a lot of innovation from just a little unlicensed spectrum, it's reasonable to assume that we'll get a lot more innovation if there's a lot more [unlicensed] spectrum available." According to this argument, Wi-Fi has been an unqualified success in every dimension.

Those of us who develop and market Wi-Fi products know this is not the case. In fact, there are serious problems with Wi-Fi deployments. For one thing, Wi-Fi systems are affected by episodic sources of interference they can't detect directly, such as FM Baby Monitors, cordless phones, and wireless security cameras which use spectrum very differently from broadcast TV. Running Wi-Fi on the same channel as one of these devices causes extremely high error rates. If 2.4 and 5.x GHz devices were required to emit a universally detectable frame preamble much of this nonsense could be avoided.

And for another, we have the problem of newer Wi-Fi devices producing frames that aren't detectable by older (esp. 802.11 and 802.11b gear) without an overhead frame that reduces throughput substantially. If we could declare anything older than 802.11a and .11g illegal, we could use the spectrum we have much more efficiently.

For another, we don't have enough adjacent channel spectrum to use the newest version of Wi-Fi, 40 MHz 802.11n, effectively in the 2.4 GHz band. Speed inevitably depends on channel width, and the white spaces offer little dribs and drabs of spectrum all over the place, much of it in non-adjacent frequencies. But most importantly, Wi-Fi is the victim of its own success. As more people use Wi-Fi, we have share the limited number of channels across more Access Points, and they are not required to share channel space with each other in a particularly efficient way. We can certainly expect a lot of collisions, and therefore packet loss, from any uncoordinated channel access scheme, as Wi-Fi is, on a large geographic scale. This is the old "tragedy of the commons" scenario.

The problem of deploying wireless broadband is mainly a tradeoff of propagation, population, and bandwidth. The larger the population a signal covers, the greater the bandwidth needs to be in order to provide good performance. The nice thing about Wi-Fi is its limited propagation, because it permits extensive channel re-use without collisions. if the Wi-Fi signal in your neighbor's house propagated twice as far, it has four times as many chances to collide with other users. So high power and great propagation isn't an unmitigated good without correspondingly wide bandwidth.

The advantage of licensing is that the license holder can apply strict rules that ensure the spectrum is used efficiently. The disadvantage is that the license holder can over-charge for the use of such tightly-managed spectrum, and needs to in order to pay off the cost of his license.

The FCC needs to move into the 21st century and develop some digital rules for the use of unlicensed or lightly-licensed spectrum. The experiment I want to see concerns the development of these modern rules. We don't need another Wi-Fi experiment, we know how it worked out. So let's don't squander the White Spaces opportunity with another knee-jerk response to the specter of capitalism.

I fully believe that the White Space Coalition is sincere in their belief that unlicensed White Spaces would be a boon to democracy, I'm not convinced that their technical grasp of the subject matter is sufficient to dictate policy. Let's face it, many of the members of that coalition have a history of over-promising where wireless networks are concerned. It has not been that long since Wi-Fi was proposed by many of them as a Municipal Network technology, a clearly failed application.

So I would encourage the Commission to accept that White Spaces can be used by computer-based networks in a non-interfering way, and to proceed to raise the much harder question of the regulatory model that needs to be adopted to ensure this computer-based network is deployed in the way that enhances social utility.

A version of this post appears on Broadband Politics, Richard Bennett's blog.

By Richard Bennett, Consultant – Richard is co-creator of the Ethernet and Wi-Fi standards. Visit Page

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