Статьи 6 семестр / All-optical networks (3)

OPTICAL DEMULTIPLEXER takes a stream of data moving at 40 gigabits per second and breaks it into separate lower-speed transmissions. A device called a nonlinear optical loop mirror receives incoming light pulses (each pulse represents one bit of data) and splits it in two. The signals move through the loop of optical fiber in op­posite directions. A train of control pulses (green) and the blue, clockwise pulses interact with one another because of certain nonlinear physical properties in the fiber. This Interplay of light pulses changes the phase of some of the clockwise pulses (shown as blue turning to red). After the control pulse leaves the loop, the two pulse streams moving in opposite directions merge back at the signal splitter, emitting 10-gigabit-per-second pulses (yellow) from one fiber and 30-gigabit-per-second pulses (purple) from the other.

tory, AT&T has shown that this method of combining signals allows up to 17 different wavelengths, each transporting 20 gigabits per second—a capacity to­taling 340 gigabits per second—to be carried more than 90 miles.

Building Sub networks

Despite the fiber's huge potential, the information-carrying capacity of a network that uses a light-wave broadcasting scheme can soil become exhausted. Adjacent wavelengths of light can transport only a limited num­ber of video transmissions without one signal's interfering with another. To avoid conflicting signals, a "guard band" in an unused portion of the optical spec­trum must be interspersed between each of the wavelengths that conveys information. The presence of the guard bands diminishes the useful bandwidth. Because a network may run out of capacity, it will have to be partitioned into separate segments, each of which is called a sub network. The wavelengths that carry messages within one sub net­work can be reallocated for separate transmissions in another sub network. Once again, existing broadcast media

present an analogy: a radio station in Los Angeles can use the same frequen­cies as a station in New York City with­out interference.

Building an actual network from these concepts poses a number of technolog­ical challenges. In March 1995 a consor­tium that combined the efforts of AT&T, Digital Equipment Corporation and the Massachusetts Institute of Technology demonstrated wavelength-division mul­tiplexing that linked together a number of sub networks. Each fiber on this net­work, located in eastern Massachusetts, can carry 20 wavelengths, each of which can transport up to 10 gigabits per sec­ond of digital data. The network was scheduled to be extended to include an AT&T laboratory in Crawford Hill, N.J., during the summer of 1995.

This All-Optical Networks Program, sponsored by the Department of De­fense's Advanced Research Projects Agency (arpa), with additional invest­ments by AT&T and Digital Equipment Corporation, has tested the critical hardware for wavelength-division mul­tiplexing—the lasers and filtering devic­es needed to send and receive specific wavelengths over the same fiber.

It has also explored an all-optical

means to switch those signals to differ­ent fibers as they move from one sub network to another. A prism like switch­ing device called a router diffracts the light traveling through a fiber into its component wavelengths. Each wave­length can then be "routed" along a dif­ferent pathway in the router's silicon-and-glass structure and into one of up to 20 output fibers that deliver the sig­nals to their destination. The consor­tium has also tested an all-optical de­vice, a wavelength converter, that can change the wavelength—which is useful if two distinct transmissions conflict by attempting to use the same wavelength.

Besides the network in Massachusetts, other projects targeting wavelength-di­vision multiplexing have been launched with funding from arpa: IBM leads one effort; Bell Communications Research (Bellcore) heads another. The European Union's RACE (Research and Develop­ment for Advanced Communications to Europe) Program has also begun to in­vestigate this technology, as has NTT, the Japanese telecommunications giant.

Wavelength-division multiplexing is ideally suited to the growing demand for video communications in which two locations may be connected continu­ously for a matter of hours.

But a different approach to network­ing will be needed for sending data from one computer to another. Comput­er networks, in contrast to video linkag­es, usually send data (such as a digital graphics file) from one point to another

7A Scientific American September 1995