Marvelous report from Ofcom detailing how 2.4 GHz is used in England, and how 2.4 GHz is broken: The detailed report contains a lot of interesting observations, raw data, and charts that demonstrate how competing uses of the 2.4 GHz band stack up against each other. One fascinating chart compares the number of Wi-Fi frames used to carry data versus management and beaconing. Only about 10 percent of frames carry actual user data; about half, beaconing. The report doesn't break this out into bytes (the beacon frames are much smaller than a full loaded Wi-Fi packet, of course), but it's part of the report's examination of inefficiencies.
In the most dense areas the report authors tested, namely parts of London, interference among competing networks wasn't the issue, but rather devices of all sorts--seemingly dominated by video cameras and baby monitors--that tromp all over 2.4 GHz without any interest in co-existence. That's not precisely how the band is licensed; devices must not create unnecessary interference and cope with the presence of interference. But in practice, working within the power limits and rules is all you have to do.
The report suggests that better harmony among manufacturers of devices for the band would vastly alleviate the problems seen, even with a lot of legacy devices in the field.
Knowing this, wouldn't it make sense to set aside some frequency space specifically for datacommunications? (Assuming something useful can be found.)
Sure, although that's often tied up with licensing. There's a unique experiment in the U.S. to reserve about 100 MHz at 3.5 GHz for licensed use in which an unlimited number of very very cheap licenses are available. The licenseholders can use WiMax or Wi-Fi (or similar technologies) in the band, and are supposed to register fixed base stations with a central database. However, any conflicts in usage are supposed to be resolved by the licenseholders.
The downside to the band is that it's only available outside of most major U.S. metro areas because it's in use by radar systems in a number of large metro regions. So it hasn't caught fire. I'm not sure who is using it at the moment. It was thought it could be a cheap way to bring WiMax to smaller markets. A few equipment makers modified WiMax gear to work in the band.