For each signal from each building, the network management software chooses a pathway through the mesh that passes through one of the system's root nodes. Because node failure can be sensed by the software, affected signals can be instantly directed around the problem. By reserving some unallocated capacity on each optical link, the network designer can ensure that there is sufficient capacity to reroute and recover from single- or multiple-link failures that might occur.

Competing with free-space optics to unclog the last-mile bottleneck is point-to-point microwave radio, a technology that is immune to fog attenuation. On the negative side, licenses are needed to operate in most microwave radio bands, and the spectrum available in most bands is limited, which means that capacity is restricted. Microwave radio is also more costly than FSO systems and may be susceptible to transmission interference. Further, microwave radio is subject to significant signal attenuation in heavy rain, especially at higher frequencies where more spectrum might be available.

If microwave radio were operated at a frequency of 60 gigahertz, however, it could complement free-space optics. The U.S. Federal Communications Commission has allocated some unlicensed spectrum at 60 GHz for high-speed applications. The greater spectrum allocation at 60 GHz implies that more capacity can be provided and a less spectrally efficient (hence, lower-cost) modulation scheme might be used, such as simple on/off signaling. Because severe rain (which might cause a radio link failure) and dense fog (which might cause an FSO link failure) do not exist simultaneously, the opportunity exists to boost network reliability by combining 60-GHz radio with FSO. Linking the two technologies would mean that the resulting system could be highly reliable over significantly greater distances.

Although free-space optics has some distance to go in addressing its remaining concerns, it's still the best bet to reach across the last mile and bring about the long-awaited broadband revolution.

   
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