For its networks in Baltimore, Washington, D.C., and Chicago, the first three markets for its WiMax service, Sprint will use equipment from DragonWave, an Ottawa company that makes wireless Ethernet nodes that can be arranged in a mesh.
The backhaul networks will start to be rolled out this month and will be completed by the end of the year, said Alan Solheim, vice president of product management at DragonWave. Sprint has said its service will first be commercially available in Baltimore in September.
Backhaul has been a challenge for the WiMax network because Sprint will need high capacity to support the fast service it's promising, which the carrier estimates will deliver between 2M bps (bits per second) and 4M bps to each customer. That service will come from a WiMax radio serving part of a city, but Sprint needs to find a way to carry the traffic of all the customers in that area to the Internet.
U.S. cellular networks are typically backhauled over T-1 lines, which deliver just 1.5M bps. Faster leased connections such as DS-3 lines (45M bps) aren't available at many of the sites Sprint wants to use, Sprint CTO Barry West said in April. Setting up backhaul was one of the biggest hurdles holding up commercial release of WiMax, he said. Sprint was working on using wireless but had difficulty getting unobstructed line-of-sight paths, finding qualified engineers and dealing with zoning issues, he said.
DragonWave makes Ethernet equipment that uses point-to-point microwave links instead of cables or fibers for transmission, Solheim said. The company's mesh technology improves upon traditional microwave backhaul so carriers and enterprises can deploy more resilient backhaul networks while paying less for antennas, he said.
In a mesh of radios, if one base station fails or has to be taken offline, traffic can take a different path. This is especially important for point-to-point microwave radios because of "churn" among radios caused by problems with zoning or property-owner permissions, according to Solheim. It also allows for shorter paths between nodes, so smaller antennas can be used, he said.
Sprint will use a combination of DragonWave's Horizon Compact and Horizon Duo units, for the edge and the core of networks, respectively, Solheim said. These are much fatter pipes than typical leased lines: 800M bps on one link for the Compact and 1.6G bps per link for the Duo. Multiple links can be set up on each. Rain can affect the speed of a link, but distance is not a factor in DragonWave networks, where the nodes are typically placed less than five kilometers apart, Solheim said.
Sprint struggled with wireless backhaul at first because it has traditionally used leased lines and lacked in-house expertise in this type of technology, Solheim said. DragonWave doesn't have trouble finding qualified engineers, according to him. But he acknowledged that microwave backhaul is much more widely used outside the U.S., which he estimated is less than 10 percent of the global market for it. Relatively abundant and inexpensive T-1 lines have stifled the technology here, according to Sprint's West.
Sprint announced earlier this year it would form a joint venture with ClearWire to offer the WiMax service, a deal that is still pending regulatory approval. DragonWave already provides wireless backhaul equipment to ClearWire, according to Solheim.