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Can a single fiber carry 56 million calls?

Can A Single Fiber Carry 56 Million Calls? British scientists say they have proved the ability of a single fiber-optic telephone line to carry 56 million simultaneous telephone calls.

Testing a new coherent optical system, researchers transmitted two wavelengths over the same fiber for 200 kilometers (125 miles) with an optical repeater (two optical amplifiers) 70 kilometers (43 miles) from the transmitter.

Each of the two channels carried 622 Mb/s, equal to about 8000 calls per channel.

The British Telecom scientists found that separation of the two wavelengths could be reduced to as little as 7 GHz before they began to interfere with each other. But with 50,000 GHz on one fiber, each fiber could in theory carry 7000 wavelength channels with 8000 telephone calls on each--56 million calls.

In practice, the researchers concede, it is more difficult. The power spectrum of the fiber and the power budget, the difference between the maximum power optical devices can provide and the noise floor, are shared between the many wavelengths used.

Still, the scientists says, it is clear that it will be possible to transmit a vast number of wavelengths on one fiber.

Coherent optical systems, along with optical amplifiers, make it possible to transmit calls and data over long distances. With the number of wavelengths possible, it should be possible to use them to route through the telephone network and avoid the need for electronic switches, whose limited capacity can cause bottlenecks.

Coherent transmission makes electronic regenerators unnecessary because very low noise amplification and distortion-free pulse transmission techniques are involved.

The scientists are also working on another transmission system that shows promise of handling more phone calls in a single channel.

Demonstrating a non-linear high capacity optical transmission system, they simulated a 20 Gb/s data rate over a 100-kilometer (62-mile) fiber span in non-laboratory conditions. This is equivalent to 300,000 phone calls.

Current linear systems transmit the equivalent of 4000 calls over 30 kilometers, while the next generation should handle 30,000 calls over 50 kilometers.

Linear systems are expected to ultimately transmit up to 150,000 phone calls over a distance of 100 kilometers in a single channel.

Beyond that, improvements in performance are unlikely because of a phenomenon that blurs the edges off transmitted data pulses at high transmission rates and makes received data unintelligible.
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Publication:Communications News
Date:Jul 1, 1990
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