Traffic on fiber-optic networks hits a speed bump

Traffic on fiber-optic networks hits a speed bump

By William Jackson

GCN Staff

Network managers who want to get more speed out of their existing fiber-optic infrastructure are about to run into a technical speed bump.

An optical phenomenon called polarization mode dispersion distorts signals on old fiber and can produce unacceptably high bit error rates at OC-192 speeds of 10 Gbps or faster.

'In the beginning it's fine'you can run high bit rates over fiber,' said Mark Graf, technical sales manager for Yafo Networks Inc. of Hanover, Md.

OC-48 speeds of 2.5 Gbps are not seriously affected by dispersion, and the now limited use of higher rates has made it easy to pick and choose the best fiber route and tweak the signals. But OC-192 use is growing.

'Dispersion is an arguable limitation at 10 Gbps,' Graf said. 'It's indisputable at 40.'

Yafo was one of several companies at last month's SuperComm trade show in Atlanta that demonstrated products to get around or compensate for polarization mode dispersion. Yafo will release its PMD Compensator for optical receivers late this year.

Troublesome delays

Kestrel Solutions Inc. of Mountain View, Calif., expects to make its TalonMX add/drop multiplexer available this month. The TalonMX does optical frequency division multiplexing to avoid PMD.

Light travels over single-mode fiber in two states of polarization. At high speeds over long distances, light in one state can be delayed relative to the other by fiber imperfections, mechanical stresses, vibration or temperature changes.

The delay stretches out each digital pulse. Measured in picoseconds, or trillionths of a second, it introduces transmission errors.

Communications carriers began worrying about dispersion as a practical problem in the mid-1990s. A 1997 study by Bellcore, now Telcordia Technologies Inc. of Morristown, N.J., found that about 20 percent of the tested fiber in the United States had polarization mode dispersion that would limit transmission speeds to 2.5 Gbps.

The industry has since adopted standards specifying 10 picoseconds of dispersion as the upper limit for 10-Gbps transmissions. Fiber manufacturers have responded by making fiber cable that will support higher speeds.

NEC America Inc. of Herndon, Va., announced in May its dense-wavelength-division-multiplexing SpectralWave 160. When available late this year, it will transport up to 160 OC-192 and OC-48 wavelengths in any combination over a single fiber.

Broad deployment of the higher-speed backbones is only beginning. Sprint Corp. does not yet have OC-192 in its network, although 'we do have plans to roll it out over the next 12 months,' spokesman Brad Bass said.

The standard for AT&T Corp.'s network is OC-48. The company in November opened its first OC-192 link, between New York and Boston.

'We're in the process of upgrading a significant number of routes across the country,' said Dave Johnson, spokesman for AT&T network services.

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