June 2001

Breaking the Metro Bottleneck

Continued from page 2

By Jeff Hecht

Burrowing In

A trip around the metro collection ring shows that it is stuffed full of fiber; copper has been almost banished. But in the access lines at the fringe of the network-the links that connect the ring to homes and businesses-fiber still coexists with its old-fashioned counterpart. "Fiber is getting further into the access network every day, but it's got a long way to go," says Brian McFadden, president of photonics networks at Nortel Networks.

That's understandable. Even though fiber is cheaper to operate and more stable than copper, established companies can't afford to rip out all of their installed cables at once. "The amount of infrastructure is enormous; even changing out a few percent a year is a tremendous investment," says Verizon's Elby. That's why a consortium of telecom equipment makers and service providers is pushing to develop evolutionary pathways to bring fiber ever closer to the homes and offices that use the network.

The key technology in this evolution, called a passive optical network, extends the reach of fiber optics further out to the fringes. In order for this technique to work, at least some fiber service must already be in place; but passive optical brings fiber to parts of the network now served only by copper.

Here's how passive optical works. A transmitter at a central facility generates an optical signal at one of two standard telephone-system data rates-155 or 622 megabits per second. This signal is a composite, which includes information for as many as 32 users. A "passive" optical coupler-which requires no electrical power-then divides this signal among fibers that link directly to end users or to other branching points. Equipment at the end of each of those fibers sorts out the signals, relaying only the ones in-tended for the local user. The central transmitter can reallocate bandwidth among customers almost instantaneously.

For a telephone company, passive optical networking offers an attractive way to extend the reach of optical fiber with minimal fuss. The passive design keeps hardware, operation and installation costs down. Moreover, the sensitive equipment needed to transmit, receive and reroute optical signals is kept safe inside buildings at the ends of the system. And since the passive optical network requires no electrical power between its end points, it generally needs less maintenance than networks based on active components.

A dark-horse technology that has recently joined the metro network, called Gigabit Ethernet, ups the speed ante even further. These systems use fibers to transmit information in the Ethernet format commonly used for office computer networks. Their data rates of one gigabit per second leave other access-line technologies in the dust. A gigabit is 1,000 megabits; gigabit-per-second transmission would, for instance, whisk away the entire contents of a CD in less than a second.

In a Gigabit Ethernet, a single fiber pipeline goes to a central switching point. This Ethernet aggregator, as it is called, distributes signals out to as many as 200 fibers. Each output fiber-like the input fiber-can carry up to one gigabit per second for short bursts, but the total output speed cannot exceed the input. An aggregator box the size of a telephone booth can serve more than 200 homes within a radius of up to 10 kilometers. That's well beyond the reach of digital subscriber lines, or DSL-the phone company service that provides broadband connections through copper cable.

Gigabit Ethernet can work as a cheap end run around telephone companies for delivery of broadband access. That's why an Ottawa, Ontario-based nonprofit consortium of companies and universities called Canarie is promoting the technology for broadband connections to cash-strapped schools. In the United States, Veradale, WA-based startup World Wide Packets has developed its own version of the technology for rural telecommunications. It is field-testing a system in Ephrata, WA, for the Grant County Public Utility District.