Richard Bennett

Like several other engineers, I’m disturbed by the white spaces debate. The White Space Coalition, and its para-technical boosters, argue something like this: “The NAB is a tiger, therefore the White Spaces must be unlicensed.” And they go on to offer the comparison with Wi-Fi and Bluetooth, arguing as Tom Evslin does on CircleID today 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. People who make this argument haven’t worked with Wi-Fi or Bluetooth systems in a serious way, or they would be aware that there are in fact problems, serious problems, with Wi-Fi deployments.

For one thing, Wi-Fi systems are affected by sources of interference they can’t detect directly, such as FM Baby Monitors, cordless phones, and wireless security cameras. 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 your 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.

The advantage of licensing is that the license holder can apply authoritarian 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, we know how it worked out.

So let’s don’t squander the White Spaces opportunity with another knee-jerk response to the spectre of capitalism. I fully believe that people like Evslin, the White Space Coalition, and Susan Crawford are sincere in their belief that unlicensed White Spaces would be a boon to democracy, it’s just that their technical grasp of the subject matter is insufficient for their beliefs to amount to serious policy.