Optical amplifiers stretch spacingspaul a. palumbo

Optical amplifiers stretch spacingspaul a. palumbo

paul a. palumbo

In a move to cut equipment costs, reduce installation and maintenance expenses, and increase regeneration distances, Washington, DC-based MCI Telecommunications Corp. is installing the latest optical amplification technology from Pirelli Optical Communication Systems in Lexington, SC.

This progressive technology can boost two optical signals received from opposite directions over a single fiber. In this approach, Pirelli T-31 bidirectional line amplifiers accommodate the two signals by routing them at different wavelengths via narrowband wavelength-division multiplexing.

Consequently, lightwaves are received at 1557 nanometers from one source and 1553 nm from a source in the opposite direction, on the same fiber. After amplification, the signals are re-routed and transmitted out in their respective directions.

These amplifiers can extend the range of optical fiber transmission systems without the need for electronic regeneration. Their primary function is to compensate for fiber attenuation, or deflection, which limits the lightwave transmission distance.

MCI is incorporating Pirelli`s T-31 bidirectional line amplifiers into its fiber-rich backbone transport network. The company eventually expects to cut in half the number of amplifiers installed and to nearly triple the distance--from 140 kilometers to 400 km--between regenerators.

The line amplifiers, which are almost as small as lapto¥computers, are generally deployed in a series of three elements, each 100-km apart. The Pirelli bidirectional system uses telemetry facilities for monitoring the status of all network elements.

Jim Collins, MCI`s senior vice president for public relations, says the carrier`s long-term integration strategy is to leverage the best equipment available against its primary competitor--AT&T, the world`s second-largest network provider. Therefore, the company has shied away from manufacturing components (as AT&T has); it now prefers to sho¥on the open market for the best technology at the best price. That led the carrier to Pirelli.

Among the amplifier`s operational benefits, according to Collins, is "it replaces a hardware set the size of a refrigerator with one the size of a lapto¥computer." The optical amplifiers thus save installation and maintenance time, costs and chores. Via multiplexing, they accept and process huge amounts of voice, video, data and images on a single broadband pipe.

According to MCI, the amplifier technology will be incorporated into all 36,000 route miles (including fiber and digital microwave plant) of its network.

Field kit

MCI has also tested and deployed Pirelli`s emergency restoration optical line amplifiers and transponders, packaged for field use in a "flyaway kit." The kit contains devices that can restore four fiber paths at a regenerator site in the event of an outage.

These kits are the equivalent of fiber network Band-Aids. If part of the network is damaged, the cable can be spliced at the break point, and a restoration amplifier can be connected to boost the optical signal for transmission to the next regeneration station. The transponder converts the lightwave signals to the 1550-nm spectrum to meet the requirements of Pirelli`s T-31 amplifiers.

Collins says the combination of Pirelli amplifiers, synchronous optical network transport and asynchronous transfer mode switching integrated into fiber networks is "non-bit-rate specific." The backbone transport network operates at speeds to 2.5 gigabits per second. MCI is planning to increase network speeds to 10 Gbits/sec over a single fiber by early 1996.

According to Frank J. Kozel Jr., MCI`s senior vice president for network implementation, "It will be possible to upgrade transmission speeds in the future without having to change or add equipment. These amplifiers can handle any speed we want."

By combining wavelength-division multiplexing and dispersion-shifted fiber, MCI forecasts that, in a few years, a pair of fibers will be capable of carrying more than 500,000 simultaneous voice conversations, u¥from today`s 32,000 conversations.

The catch, however, is that present computers cannot handle interface rates at 2.5 Gbits/sec. Consequently, optical amplifier and supporting technologies will be used only to transport massive amounts of data from point-to-point locations. Speed translates into the capacity needed to furnish digital service providers with the bandwidth to offer such services as distance learning, home shopping, Internet access, networked games and video content delivery.

Owned in part (20%) by the United Kingdom`s telecommunications giant British Telecom, MCI has been pouring a majority of its $3 billion-per-year capital expenditure budget into advanced network architectures.

To provide customers with bandwidth-on-demand capabilities--even in advance of mass-market merchandising of digital value-added services--MCI is focusing more on broadening its core long-distance franchise. Toward that end, the carrier is deploying an advanced delivery hardware foundation for point-to-point shipment of data and image-intensive applications (for instance, video conferencing, video-on-demand and near video-on-demand). In addition, MCI is incorporating reliability and restoration (self-healing) capabilities into the network. q

Paul A. Palumbo is a freelance writer based in Seaside, CA.

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