Sprint Nextel won't make the April launch it had planned for its Xohm WiMax service, largely because of problems setting up Internet links behind the WiMax part of the network, the head of the project said.

The carrier will miss the target of commercial availability in April, though not by much, said Barry West, CTO of Sprint and head of the Xohm business unit, in an interview at the CTIA Wireless trade show in Las Vegas Tuesday evening. Even in the Chicago and Washington, D.C., areas, where Sprint's WiMax service is in trials, the network isn't as good as West would like it to be, he said.

The national rollout of WiMax, still an emerging technology, is a big bet for Sprint as it loses subscribers and struggles in third place behind AT&T and Verizon Wireless in the U.S. mobile market. The full cost of the project has been estimated at $5 billion, though Sprint has put off discussions of how much it will actually invest until it releases financial results for the first quarter. The company has also backed off earlier projections that the network would reach 100 million U.S. residents by the end of this year. A new forecast is yet to come. The company reportedly has been in talks with cable operators to help pay for the project.

But West is not worried about WiMax itself working. What's holding Sprint back is simply the logistics of building the network, and specifically the problem of provisioning "backhaul" connections to the Internet, he said. Mobile operators typically lease T-1 lines from their cell sites for these backhaul links, but those lines only provide 1.5Mbps. WiMax is designed to deliver more than that to every subscriber.

Fatter leased lines, namely DS-3s (45Mbps), can't be had for any price at many of Sprint's cell towers, West said. So the carrier wants to use point-to-point microwave wireless connections. Though these are used widely in other parts of the world -- about half of all backhaul in Europe is microwave, and more than that in Asia, IDC analyst Godfrey Chua estimates -- Sprint is practically treading on virgin territory by seeking it in the U.S., according to West. There are few engineers well-versed in setting it up, and Sprint has to overcome zoning issues for many installations, he said. The procedures of setting it up are also quite different, with the need to find an unobstructed line of sight through the air and deal with zoning issues, he said.

Getting enough backhaul capacity to achieve the promised speed of WiMax is no doubt a problem, though it's probably not the only one Sprint's facing in the rollout, IDC's Chua said. Even where they are available, leased DS-3 lines are several times as expensive than T-1s, which cost on average $300 in the U.S.

The fact that landline carriers have made T-1s relatively cheap and easy to buy in the U.S. is one reason microwave hasn't been widely adopted here, West said. In fact, the lines always cost just slightly less than deploying microwave, he said.

"They price it very carefully so that it doesn't work out," West said.

The WiMax backhaul issue is symptomatic of a national problem for Sprint, which leases T-1s for almost all of its approximately 45,000 cell sites, said Infonetics analyst Michael Howard. AT&T and Verizon Wireless, affiliated with wireline carriers themselves, suffer less from this issue. Ethernet is another possible solution to the high-capacity backhaul problem, but that would require fiber, which isn't available in many cases, Howard said. It may not be economical for AT&T and Verizon to deploy fiber just to serve a Sprint WiMax base station, he said.