Статьи 6 семестр / All-optical networks (4)

in short, high-speed bursts. To accom­modate this type of traffic, computers often communicate through a network in which the data move from a sender to a receiver in discrete units called pack­ets. A message may be broken into many discrete packets and then switched onto one of several open pathways that lead to a receiving computer.

Packet-switched data can be trans­ferred more rapidly and with less ex­pense because there is no need to spend time or network resources to establish a dedicated path between a sender and receiver. Each packet contains an ad­dress that denotes both its destination and how it fits together with other pack­ets that are part of the same message. The packets can travel to their destina­tion over any of several pathways through the network. The packets are then reassembled, like the pieces of a puzzle, at the receiving site to form a coherent message.

The Future of the Internet

Perhaps the best-known electronic version of a packet network is the internet. The technological future of the Internet may be gleaned by examining advanced research in optical-packet networks. In such a network, a single wavelength would carry pulses of light from a laser that can switch off and on in a trillionth of a second. The light pulses are generated by lasers quickly enough to transmit as many as 100 gi­gabits each second, with each pulse rep­resenting one bit. Each pulse would be combined with perhaps 10,000 or so other pulses from the same laser to cre­ate a data packet.

As with wavelength-division multi­plexing, one of the principal challenges in building such an optical superhigh­way is to multiplex and switch these packets of data without having to con-муке them to an electronic signal. A fi­ber may transmit a total of 100 gigabits each second but is divided into designat­ed time intervals so that 10 users, say,

can each send 10 gigabits per second.

Each sender of data can be assigned a slot of time in which to place packets with 10 gigabits per second of data onto the network, or it may transmit them in any unused time slot. The packets from one sender get interspersed with pack­ets from other senders, each of them having a different time interval in which to transmit a message. Because com­munications capacity is apportioned by time, not wavelength, the technique is called time-division multiplexing.

At M.I.T., we have been working on the technologies and designs for a 100-gigabit-per-second, all-optical network using this multiplexing technique. The network hardware under development ranges from high-speed lasers that can transmit 100 billion pulses per second to optical buffers for storing pulses.

In one project, we have shown how to extract data from a fiber transporting a total of 40 gigabits of data using time-division multiplexing. A device known as a nonlinear optical loop mirror is ca­pable of processing the optical signal to multiplex, demultiplex, switch or even store information. For demultiplexing, it receives light pulses from a fiber trans­porting a 40-gigabit-per-second stream of data. In the loop mirror, which is a circular strand of fiber with special ma­terial properties, the optical signal inter­acts with another series of light pulses that have been injected into the device by a laser. The interaction of these dif­ferent trains of light pulses causes a signal to emerge, and it transports 10 gigabits of the data into a new fiber.

At the same time, the original signal-now carrying the remaining 30 gigabits per second of data—returns to the fi­ber from which it entered the mirror. If demultiplexing is not desired, the light returns to the original fiber unaltered, still transporting the full 40 gigabits per second of data [see illustration on page 74]. M.I.T. is not alone in its re­search. Recently NTT performed a sim­ilar experiment with a 100-gigabit-per-second transmission.

The optical loop mirror can also serve as a digital-processing device. In demul­tiplexing the signal, the loop mirror ei­ther modifies the signal or leaves it un­changed—an on-or-off state identical to the 0 or 1 of digital logic. By chaining together several loop mirrors, a logic de­vice can be fashioned that "reads" the addresses in packets of data.

Network designers could combine op­tical logic and demultiplexing to build a rudimentary communications switch. The logic devices would determine whether some of the data should be un­loaded from the incoming signal by reading the address of a packet—and, if so, the demultiplexer would switch the desired data to one of several out­put fibers for routing to the appropri­ate computer or sub network.

M.I.T. researchers have also used an optical fiber to construct an optical stor­age device, a buffer that can retain light pulses temporarily before they are rout­ed to their destination. In the prototype, a signal that enters the loop is chan­neled through a series of devices that restore and condition the signal. The ability to "buffer" information in these devices is enhanced by a type of light pulse called a soliton, which retains its original shape almost indefinitely. The soliton's ability to resist degradation also makes it well suited for long-dis­tance transmission on optical networks.

All-optical communications will, for years to come, face competition from electronic communications, whose price continues to drop while its performance improves. Nevertheless, an all-optical network offers compelling advantages. It would provide so much capacity that the exchange of video and large comput­er files would become routine. A video camcorder owner could plug the cam­era into a cable wall outlet and have rel­atives across the country participate in a child's birthday through video linkag­es. In fact, one can only begin to imagine the uses for a network in which bandwidth becomes as inexpensive as electricity, gas or water.

The Author

VINCENT W. S. CHAN is associate head of the Communications Division of the Massachusetts Institute of Technology's Lincoln Laboratory. He oversees projects in optical communications, space communications and wireless net­works. Chan received a bachelor's, master's and Ph.D. in electrical engineering from MIX On receiving his doctorate in 1974, he became an assistant profes­sor in the school of electrical engineering at Cornell University. He joined Lin­coln Laboratory in 1977. Since 1992 he has served as director of the Consor­tium on Wideband All-Optical Networks, which is funded by the Advanced Re­search Projects Agency as well as AT&T and Digital Equipment Corporation.

Further Reading

A Precompetitive Consortium on Wide-band all-Op-tical Networks. Vincent W. S. Chan et al. in Journal of Lightwave Technology, Vol. 11, No. 5-6; May 1993.

MUtTI-WAVELENGTH TRANSPORT NETWORKS. Peter J.

Chidgey in IEEE Communications, Vol. 32, No. 12, pages 28-35; December 1994.

Toward Customer-Usable All-Optical networks. Paul E. Green, Jr., in iEEE Communications, Vol. 32, No. 12, pages 44-49; December 1994.

76 Scientific American September 1995