A challenge for metro WDM?

Just as local service providers begin to seriously evaluate wavelength division multiplexing for their networks, a different idea for optimizing fiber is surfacing.

Semtech Corp., an integrated circuits maker, has established a communications business unit and is marketing a two-chip modem for fiber optic networks. The modem is designed to allow two-way, multiple-channel traffic over one 8.44 Mb/s channel. Semtech acquired the technology when it purchased Acapella of Southampton, England.

The ACS405 modem uses a proprietary architecture called Ping Pong to enable bidirectional traffic over a single channel. An ACS405 is required at each end of the fiber, which contains two chips. The A chip is predominantly analog and contains the laser/LED driver and the PIN receiver circuitry. The B chip contains time compression and decompression logic and performs synchronization and window locking functions.

The modems are synchronized so as one transmits, the other receives. Once the appropriate time has passed, the modems switch roles, and a signal is sent in the opposite direction. This allows the full bandwidth of a single channel to be used for traffic in both directions, as opposed to a channelized solution, which essentially wastes a portion of the total available bandwidth on a fiber.

But despite the fact that transmission occurs over a single channel, the modem can be configured to support eight 1 Mb/s channels, four T-1/E-1 channels or as a single 8.44 Mb/s pipe. This is enabled by multiplexing multiple channels into a single signal using the B chip. The B chip on the other end demultiplexes incoming signals back into separate channels.

The solution delivers a cost-effective alternative to more common solutions for fiber networking problems, said Jeff Drobman, Semtech's telecom market segment manager.

"We reduce cost because we use a single fiber and a single [light source]," Drobman said. "Other solutions use multiple fibers [for duplex traffic] or multiple lasers [for multiple channels]."

Semtech is positioning the ACS405 as an alternative to laying multiple fibers for duplex traffic, but also-and primarily-as an alternative to WDM. Semtech is not only offering to lower costs for fiber networks, but it can also offer the "Holy Grail" of service providers: incremental bandwidth, bandwidth on demand and integrated services, all of which require multiple channels, Drobman said.

It's difficult to gauge how carriers will respond to another WDM alternative. But WDM vendors see little comparison between their systems and those such as Semtech's. Xin Cheng, president of Osicom Technologies Inc., pointed out that although Semtech's system is a maximum 8.44 Mb/s channel over a 25 kilometer span, WDM delivers multiple channels at 2.5 Gb/s and higher over hundreds of miles. The fiber optic modem model might work for specific campus and fiber-to-the-home applications, but it is not viable for broader use in service providers' networks, Cheng said.

"I don't see any appeal," Cheng said. "I don't see equipment providers or end users being able to take advantage" of the solution.

It's hard to see such solutions usurping WDM, said Scott Clavenna, senior analyst at Pioneer Consulting.

The beauty of WDM is that [new metro network WDM systems] are standardized on the ITU grid, and they interoperate with long-haul WDM systems. You can have a managed network from the core of the long-distance network all the way out," he said.

"But if you start doing optical tricks like this at the edge, you're not in sync with what's going on at the core."

Semtech is confident that it can make inroads. The company is looking to new fiber buildouts and existing dark fiber routes as market opportunities for its product, Drobman said. The company believes that market is growing, and it hopes to convince fiber owners that they can deliver needed performance at a much more attractive cost using Semtech's fiber modems. Semtech is working with several carriers on network designs, he said, but cannot yet name those carriers.

"Most [local service providers] are thinking WDM and balking at the cost," Drobman said. "Our plan is to fit right in with [carriers' infrastructure plans]. We want to own both ends of the fiber."