Visions of license-free, monopoly-shattering, high-bandwidth networks are certainly dancing through the heads of some business-minded individuals these days. On the surface, it looks like sound reasoning: if people are conditioned into believing that 6Mb DSL costs $250 per month to provide, then they'll certainly be willing to pay at least that much for an 11Mb wireless connection that costs pennies to operate, particularly if it's cleverly packaged as an upgrade to a brand name they already know. The temptation of high profits and low operating costs seems to have once again allowed marketing to give way to good sense. Thus, the "Wireless DSL" phenomenon was born. (Who needs an actual technology when you can market an acronym, anyway?)
In practice, many WISPs are finding out that it's not as simple as throwing some antennas up and raking in the cash. To start with, true DSL provides a full-duplex, switched line. Most DSL lines are asymmetric, meaning that they allow for a higher download speed at the expense of slower upload speed. This difference is hardly noticeable when most of the network traffic is incoming (i.e., when users are browsing the web), but it is present. Even with the low-speed upload limitation, a full-duplex line can still upload and download data simultaneously. Would-be wireless providers that build on consumer-grade wireless technology are limited to half-duplex, shared bandwidth connections. That means that to actually provide the same quality of service as a wired DSL line, they would need four radios for each customer: two at each end, using one for upstream and one for downstream service. If the network infrastructure plan is to provide a few (or even a few dozen) wireless access sites throughout a city, these would need to be shared between all of the users, further degrading network performance, much like the cable modem nightmare. Additional access sites could help, but adding equipment also adds to hardware and operating costs.
 Wireless Internet Service Providers. No, I didn't make that one up.
Speaking of access points, where exactly should they be placed? Naturally, the antennas should be located wherever the greatest expected customer base can see them. Unless you've tried it, I guarantee this is trickier than it sounds. Trees, metal buildings, chain-link fences, and the natural lay of the land make antenna placement an interesting challenge for a hobbyist, but a nightmare for a network engineer. As we'll see later, an antenna site at least needs power and a sturdy mast to mount equipment on, and, preferably, it also has access to a wired backbone. Otherwise, even more radio gear is needed to provide network service to the tower.
Suppose that marketing has sufficiently duped would-be customers and claims to have enough tower sites to make offering network services at least a possibility. Now imagine that a prospective customer actually calls, requesting service. How does the WISP know if service is possible? With DSL, it's straightforward: look up the customer's phone number in the central database, figure out about how far they are from the CO, and give them an estimate. Unfortunately, no known database can tell you for certain what a given address has line of sight to.
As we'll see later, topographical software can help perform some preliminary work to help rule out the definite impossibilities. Some topographical packages even include tree and ground clutter data, although these tend to be considerably more expensive (and of dubious real value, unless they are up-to-date). Using such software, we might even be able to upgrade the potential customer to a "maybe." Ultimately, however, the only way to know if a particular customer can reach the WISP's backbone over wireless is to send out a tech with test gear, and try it.
So now the poor WISP must be prepared to "roll a truck" for new installations, making on-site calls to people who aren't even customers yet. If they're lucky, technicians might even get a test shot to work. At this point, finally, equipment can be installed, contracts signed, and the customer can get online at something almost resembling DSL. That is, be online until a bird perches on the antenna, or a new building goes up in the link path, or the leaves come out in the spring and block most of the signal (at which point, I imagine the customer would be referred to the fine print on that contract).
I think you can begin to see exactly where the bottom line is going in this sort of arrangement. The private WISP approach is filled with unanticipated (and expensive) challenges it comes to solving the problem of ubiquitous access on a large scale. What hope does our "wireless everywhere" vision have in light of all of the previously mentioned problems? Perhaps a massively parallel approach would help . . .