Packing Bits and Wavelengths

The heaviest lifting in a metro system is typically done by the collection ring, which runs all the way around the region, providing local access as it goes. To burst through the bandwidth bottleneck here, engineers have two basic choices: they can crank up the bit rate on a single beam of light traveling through a fiber, or they can multiply capacity by using several wavelengths as information carriers. In the second alternative, known as “wavelength division multiplexing,” each fiber carries multiple light beams of different colors-with a different digital signal encoded in each beam. The more wavelengths you can pack in, the more information you move. (These “colors” are actually different shades of infrared and are invisible to the eye.)

Both approaches are now being tried by the companies that run the metro loop. Various technical problems make it tough to raise the bit rate. But in encouraging recent developments, two optical-networking leaders-Ciena and Nortel Networks-have demonstrated single-wavelength transmission of 40 gigabits per second over lengths of fiber typical of a metro network. That’s a big leap over the 2.5 gigabits per second at which today’s fastest metro networks operate. Taking this research feat out of the lab and under the streets, however, will require advances in the electronics that manipulate the signals, since standard chips don’t yet operate that fast.