Feature Article

November 21, 2008

Affordable Backhaul

(This article originally appeared in the September issue of Internet Telephony magazine.)
The mobile Internet is coming. Today’s 3G radio technology delivers more than 1 Mbps per user and, in competitive markets like the UK (five 3G operators), open access to the Internet is becoming quite affordable. For operators however, a major bottleneck is backhauling the traffic from cell towers to the core network. This is true for traditional cellular mobile operators and for new wireless ISPs using WiFi or WiMAX.
Until very recently, voice telephony was the bulk of the traffic. Given the compression already in use over the air, you can fit a lot of voice calls on a T1 or E1 link, so most cell sites were connected via a single T1/E1 link. In fact, in rural areas, one E1 link might serve a whole string of cell towers. Suddenly with 3G HSPA (High Speed Packet Access), one cell tower serves ~14 Mbps to each of three sectors; i.e., 30-50 Mbps, with higher data rates coming. Fiber to the cell towers could accommodate these much higher data rates. But, not only is fiber construction expensive, fiber requires access to physical rights of way that are typically bound up in layers of permits and regulation, including regulations tied to traditional regulated monopoly phone service. This exposes a communications policy issue that hasn’t been widely discussed. An effective policy for broadband access should also promote the availability of dark fiber to competing operators to help foster mobile broadband access as well as fixed!
Meanwhile, where fiber is too expensive, point-to-point wireless backhaul is a practical alternative. Indeed, many cellular networks already use microwave radios capable of carrying 2-4 T1/E1 links and mobile licenses in many developing countries include both GSM spectrum and spectrum for microwave backhaul links. But four E1 links (6 Mbps) is woefully inadequate for 3G data.
Luckily, the performance of point-to-point radio links has been improving at Moore’s law rates or better. Wireless links can’t match the throughput of fiber over long distances, but today they can provide 100 Mbps to over 1 Gbps for distances of a few miles. Obviously there’s a capital cost, for the equipment, its installation and for spectrum. And there are reliability issues. High capacity point-to-point radio links require tens of MHz of spectrum — something only available above 10 GHz where rain attenuates radio signals. Luckily, adaptive modulation can keep some bits flowing even during rain, providing for example, 400 Mbps 99.9 percent of the time and 70 Mbps 99.999 percent of the time. With priority, during rain, voice still works while TCP/IP slows down a bit.
Ideally we’d have dark fiber available to all comers, but until that day arrives, wireless backhaul will allow mobile Internet access to flourish.

Brough Turner, co-founder and CTO of NMS Communications, writes the Next Wave Redux column for MobilityTechzone. To read more of Brough�s articles, please visit his columnist page.

Edited by Greg Galitzine

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