Tom Cady, president of XO Communications' Nextlink subsidiary, calls it the “middle mile.” Others, like John Kryzwicki, director of marketing at GigaBeam, refer to it as the “capacity injection layer.” To those with a little less imagination, it is simply the backhaul portion of the network, the circuits — T-1s in most cases, at least in the U.S. market — that carry the traffic created by broadband applications back to the network core.

Backhaul is an important function in any broadband network, but especially so in mobile broadband networks, in which broadband usage is a new, more varied and unpredictable phenomenon than in fixed wireline broadband networks, where the user's location never changes. In 2G mobile networks, which include messaging usage — and can include some Internet access and data application usage — the backhaul requirement is around 5 Mb/s to 7 Mb/s per cell site. Mike O'Malley, group manager of portfolio marketing for Tellabs, said that translates to about five to seven T-1s per cell site for backhaul capacity. “You generally can count on adding a couple of T-1s per site for 2.5G services, and then, when you get to 3G, you want to look at adding more or migrating to something like Ethernet or Gigabit Ethernet to serve that need.”

In the U.S. mobile market, the predominant backhaul method is via T-1s, which a mobile carrier usually leases from an ILEC at such a high cost that backhaul often represents about 30% or more of a given mobile carrier's expenses. Aside from cost, the other frequently voiced disadvantage of this traditional method is that the TDM-based infrastructure is increasingly unfit for the job of backhauling the bandwidth-rich mobile applications on the rise as 3G network coverage and subscription rates continue to increase.

The mobile bandwidth boost that is coming courtesy of 3G should be a surprise to no one at this point, as mobile network capacity has risen several times over the last decade through several technology upgrade cycles. Yet, “even as mobile carriers have been under tremendous pressure over the years to move from analog to digital and to upgrade their networks, we have never seen an upgrade in the backhaul infrastructure through all of these technology upgrades,” said Michael Gallagher, president and CEO of First Avenue Networks, a company that provides high-frequency wireless links for backhaul needs and that recently announced that it will merge with Fiber-Tower, another backhaul vendor.

The deficiency is most glaring in the U.S. market, where there are about 80,000 cell towers, with roughly 300,000 base stations hanging off them. More than 90% of these towers are fed with backhaul based on traditional, copper-based TDM infrastructure. Only about 6% of the towers in the U.S. are fed with fiber-based backhaul. This situation is strikingly different than the backhaul market in Europe, where more than 80% of cell towers count on fixed wireless backhaul. Gallagher said the difference is that when 3G licenses were awarded in Europe, backhaul spectrum was given away for free, encouraging many vendors and other opportunists to aggressively develop wireless-based solutions for the middle mile.

Looking to capitalize on a $3.5 billion market in the U.S. that is expected to double within the next few years, several vendors of backhaul solutions are pouncing on the market, some of them with entirely new feeder infrastructure concepts, such as fiber and fixed wireless, that would replace the copper currently in heavy use. Most of these vendors are looking at these feeder technologies to be the basis for a move to packetized backhaul, either ATM or IP-based Ethernet or Gigabit Ethernet, which would be more cost-effective, flexible and easier to manage than TDM.

Meanwhile, other vendors, convinced that T-1s have a long life ahead of them, are addressing the industry's new capacity needs and cost-consciousness with solutions based on pseudo-wire or circuit-bonding technologies that will help prolong the life of T-1-based backhaul while improving the economic model for it.

“A lot of the backhaul decisions that carriers will be making will come down to the cost and the reliability of the backhaul solutions,” said Dan Murray, vice president of marketing for Kentrox. “Carriers that have used leased-line backhaul will need to take a look at packetized backhaul.” Although to many companies “packetized” means IP, Murray said packet backhaul could very well evolve as a hybrid world using both ATM and IP.

“With 3G, it's going to be all IP at the application layer, but a lot of the deployed radios themselves have ATM interfaces,” he said.

With cost probably being the more decisive factor in the backhaul equation, fixed wireless would seem to have an edge over other types of feeder infrastructure. That's what Nextlink and Cady are counting on. XO recently re-launched Nextlink to use the company's LMDS spectrum in 75 markets across the U.S. to provide backhaul and other services. “Wireless is the easiest technology to set up and upgrade,” Cady said recently. “There are plenty of places where you just can't deploy fiber.”

In addition to Nextlink, other companies supplying backhaul using fixed wireless technology are using a variety of models, such as Fixed WiMAX or high-frequency fixed microwave in point-to-point or point-to-multipoint architectures. Other companies, such as SkyPilot, are using re-routable mesh architectures specifically to provide backhaul capacity for the rapidly increasing number of municipal wireless networks.

Still, critics of wireless technology say that cost benefits may still have a tough time competing with the infrastructure that is already in the ground. Tellabs' O'Malley said: “There is a large deployed base of T-1s that telcos will have a hard time parting with. T-1 prices are coming down, and it is a network that has been paid for.”

“Fiber will cost a lot and take a long time to deploy. Wireless is less costly and easier, but a lot of the incumbents [ILECs] supplying the backhaul capacity have existing contracts in place,” said Dave Stehlin, CEO of Ceterus Networks, a company that provides circuit-bonding solutions to enhance T-1 capacity. “And don't forget that with wireless backhaul, at some point you will still need to convert that back to wireline.”

Fixed wireless technology is beginning to prove itself as a carrier of low-latency traffic such as voice and video, but some providers especially sensitive to reliability issues may balk at putting this kind of traffic over wireless backhaul facilities. Ultimately, voice and richer-bandwidth traffic from mobile 3G networks may end up traveling on different circuits within the same backhaul transport scheme. Data and other applications may travel via IP-based Ethernet, while voice stays on TDM-based T-1s.

Regardless of the favored backhaul method, most of the companies that have a competitive stake in the battle for the middle mile acknowledge that no single technology will dominate the backhaul market. Wireless may be used where it makes the most sense, fiber may be used where it is already deployed and the capacity need is a especially high, and even those copper-based T-1s may still have a long life ahead of them.

“There will be an enormous growth in T-1s over the next decade,” said Gallagher, of First Avenue Networks. “Maybe in the next ten years we'll see the kind of situation there is in Europe, with 80% of the backhaul being wireless and the other 20% being wireline. But we realize that in the carrier world, things tend to stick around for a very long time as the investment in them is being depreciated.”