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In the so-called backbone of the telecommunications system, the fat pipes that pour data across continents, the name of the game is raw speed (see “Building a Better Backbone,”). But the data racing through the telecom backbone can’t fulfill its mission until it is shuttled through the “metropolitan loop,” a complex network of cables and switches that delivers those bits to businesses, factories, schools and homes. It’s there that the information gusher narrows to a relative trickle, because the metro loop is every bit as tangled as downtown rush-hour traffic. If the broadband revolution is ever to be a reality, the metropolitan bottleneck must be broken.

But that’s a tall order. Upgrades in the metro loop have been far slower in coming than advances in the backbone. The reasons range from tougher cost constraints to urban bureaucracy to the presence of a patchwork telecom infrastructure dating back to the 1970s and ’80s. But R&D aimed specifically at the metro loop is slowly pushing a variety of solutions out of the laboratory and under the streets. And if we really want broadband, those fixes had better work.

Weak Link

To grasp the scale of the bottleneck, consider the metro network’s place in the telecom ecology. In the backbone, transmission speeds are measured in trillions of bits per second. On the user end, high-speed networks run at billions of bits per second (gigabits). But the metro systems that link these two high-speed networks poke along at mere millions of bits per second (megabits). “That’s the bottleneck,” laments Steve Schilling, presi-dent of access networks at Nortel Networks. And this metro-loop constriction isn’t just a problem for the companies that run the networks. Regular folk experience it as trunk-line busy signals and stalled Web browsers.

If you’re looking for a culprit here, don’t finger the phone companies that run the metro loop. They planned prudently (at least, so they thought) for steady growth in voice communications, which at the time was their bread and butter. Then they, along with everyone else, were blindsided by the explosion of the Net. “Two to three years ago, we started running into capacity problems” in urban areas, says Stuart Elby, who heads development of Internet-connected networks at Verizon, the phone company serving New York and New England. Speeds of 2.5 gigabits per second, enough to handle heavy Net-generated traffic, are common only in the hearts of big cities like New York or Boston, where Verizon runs fiber-optic cable with 48 strands. More typical metro-loop speeds range from 1.5 to 600 or so megabits per second.

And there’s no letup in sight for the beleaguered metro operations. “More and more applications are emerging as you have more bandwidth,” says Claude Romans, an analyst with the South San Francisco market research firm RHK. If digital television ever gets off the ground, for instance, it could gobble up huge chunks of bandwidth; it takes 1.5 gigabits per second to transmit a single, studio-quality high-definition video channel (although consumers will see only a compressed 20-megabit-per-second version). That kind of data onslaught will bring the metro loop to its knees without significant technological upgrades.

The current and future transmission slowdown afflicts both main components of the metro loop’s hub-and-spoke structure. The “access” portion of the network-the spokes-ferries signals out to residential neighborhoods and individual office buildings. These access lines connect to the “collection ring,” which transports signals around a metropolitan area, linking telephone company service centers and other major traffic centers, such as Internet service providers and large universities.

Technological advances are helping unsnarl both the collection ring and the access lines. Fiber optics, which already dominate the collection ring, are replacing more and more of the residual copper in the access lines as well-in effect, paving over dirt paths with smooth, modern asphalt. And new optical transmission technologies are stuffing more data into the networks that are already in place.

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