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XOR: A Logical Choice for All-Optical Networks

Photonics Spectra
Nov 2001
Perry J. Greenbaum

A new label-swapping technique promises faster data networks, increasing the commercial potential of videoconferencing, video on demand and video games via the Internet. Researchers at Technical University of Denmark's Research Center COM in Lyngby and at Alcatel Corporate Research Center in Marcoussis, France, have proposed a method based on exclusive-OR (XOR) logic that could increase the efficiency of routers on all-optical networks.

Using XOR, explained Tina Fjelde, the lead researcher on the project and now of Mintera Corp. in Lowell, Mass., reduces the requirements for packet formation and simplifies switch management. XOR describes a logic circuit with two or more inputs and one output, which is high if one input is high. Such circuits rely on truth tables: In binary terms, two 0's or two 1's in the input yield an output of 0, while an input of 1 and 0 (or vice versa) results in 1 at the output.

The researchers used an interferometer to control the waves entering the logic circuit. The instrument caused the interference of two input waves, which produced a phase change between them. At the input of the logic gate, the waves either combined constructively (resulting in an output of 1) or destructively (resulting in 0).

The method is suited to all-optical networks because it simultaneously performs label swapping and wavelength conversion. Consequently, routers that use XOR would not have to convert an optical signal to an electrical one, unlike today's, which hold a packet in queue until the desired outlet is free, thus delaying transmission. An optical XOR system, however, could use wavelength conversion to resolve contention.

Only a few header bits in the label would need to be changed, using short swapping sequences, and the routers could quickly pass on the data packets with a new optical label.

Toward petabit transmission

In trials, XOR has been successful at 10 Gb/s, but companies such as Cisco Systems Inc., Juniper Networks Inc. and Hyperchip Inc. are developing commercial routers to transmit data at speeds of greater than 1 Pb/s, up to 1000 times faster than today's most advanced systems. These next-generation routers will operate in all-optical networks. XOR offers a way, Fjelde said, to implement all-optical label swapping with a limited increase in cost and complexity.


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