Much has been made of the need for equipment vendors to offer OC192 (10 Gbit/s) interfaces and to be first out of the gate with the next step up in transmission speed, OC768 (40 Gbit/s).

Very little has been said about polarization mode dispersion (PMD), a problem that could prevent a lot of carriers from upgrading their networks with these new technologies.

Now, however, startup Yafo Networks reckons it’s close to offering carriers a way around the PMD problem -- a compensator that will enable them to deploy higher-speed gear on long-distance routes.

Early prototypes of the PMDC-10, as it’s called, are already being evaluated by “four or five” vendors thinking of incorporating it in their equipment, according to Henry H. Yaffe, Yafo Networks’ founder, chairman, and CTO. Commercial prototypes will ship in “a couple of months.”

Developing a PMD compensator is a big deal, mainly because PMD is much tougher to control than other forms of dispersion, due to which signals become increasingly smudged and unreadable as they travel along fiber. This is caused by some parts of the light pulse traveling slightly faster than other parts.

As the name implies, PMD is caused by light traveling faster in one polarization plane than another. Fundamentally, it’s caused by the core of the fiber not being perfectly round in cross-section. As a result, the thickness isn’t absolutely identical on every possible axis.

Up until about five years ago, manufacturers simply couldn’t produce fiber that was circular enough to keep PMD under control at bandwidths exceeding OC48 (2.5 Gbit/s), according to Yaffe. Fiber happens to have a “sweet spot” at OC48 that prevents PMD being an issue, he says. At higher speeds, however it becomes increasingly problematic. It’s “pretty bad” at OC192 and creates a “real mess” of signals at OC768.

The upshot is that carriers with older fiber in their backbones –- like AT&T Corp. (NYSE: T), Sprint Corp. (NYSE: FON), and WorldCom Inc. (Nasdaq: WCOM) -- are now lumbered with infrastructure that can’t really support the latest transmission technologies, Yaffe says.

Other carriers also face problems because otherwise circular cross-section fiber can be squashed out of shape during installation or at any time afterwards, either permanently or temporarily. A lot of fiber is laid alongside railway tracks, for instance, and vibrations from passing trains can flatten it slightly, creating temporary PMD problems, according to Yaffe.

The fact that PMD problems can come and go like this makes it tough to compensate for them. Other dispersion problems don’t change over time and can be dealt with by installing static compensators. PMD compensators, however, have to work dynamically. Yafo's PMDC-10 continuously adjusts the orientation of the light signals and the time difference between the two polarization planes to optimize the signal quality, says Yaffe.

The same concept could be used for automatically adjusting other transmission parameters when traffic is rerouted around a problem in an optical backbone, Yaffe notes. "This is a platform of technologies that lends itself to a whole plethora of applications,” says Yaffe.

Downsides? The PMDC-10 occupies a lot of real estate. Each wavelength requires a whole blade of components, and there are 10 blades per shelf and three shelves per bay. In other words, PMD compensation for a 160-channel DWDM system might occupy as many as six bays of equipment. Yafo Networks isn’t talking about price yet, but it’s a fair bet that it won’t come cheap.

At least one other startup, Phaethon Communications, is thought to be developing a PMD compensator. Phaethon is still in stealth mode, but its Website says that it’s “devoted to delivering OC192 and OC768 data rates over all types of fiber.”

-- Peter Heywood, international editor, Light Reading http://www.lightreading.com