TDM vs. WDM-again: While the impact of WDM spreads, some still argue the best way to grow a network

When vendors first began talking about being able to take Sonet bandwidth up to a bit rate of OC-192, the only response they heard was, "Why?"

Even as the bandwidth beast of Sonet applications-the Internet-began to boom, many industry watchers questioned the need for such wholesale boosts in bit rate, especially when wave division multiplexing (WDM), an up-and-coming technology, allowed carriers to increase capacity on existing Sonet systems by four, eight or 16 times.

The criticism contributed to a rift between two industry factions. Stalwarts of time division multiplexing (TDM) supported capacity growth through jumps to higher Sonet bit rates in combination with lower-channel WDM, while the trend-backers favored moves to higher-channel WDM as an alternative to larger bit rates.

However, three years later, supporters of OC-192 are seeing a payoff, even as WDM shines brighter than ever. At the recent National Fiber Optic Engineers Conference '97, Northern Telecom announced that it had signed Williams Communications Group's Vyvx business unit as its eighth customer for OC-192 systems.

Although Nortel is, by far, the leading collector of OC-192 contracts, the growing commercial success of OC-192 strengthens bit-rate acceleration as a solution to bandwidth needs.

"Bandwidth needs appear to be insatiable," said Mike Unger, group vice president of transport networks at Nortel.

Meanwhile, Nortel also views dense WDM as a necessary element of OC-192 networking. Its current OC-192 offering includes eight-channel WDM for a collective 80 Gb/s of capacity. The company wants to migrate to OC-192 with 16-channel WDM by about the middle of next year, said Unger.

Some suggest that OC-192 will hit a ceiling of carrier interest soon. After all, several of the carriers committed to OC-192 deployment, such as MCI, Qwest Communications and Vyvx, have somewhat unique applications for it. MCI is on a short list of network operators supporting nationwide data networking needs, Qwest's primary strategy is to become a carrier's carrier and Vyvx moves extremely high-bandwidth video traffic.

Still, the timing for industry migration to OC-192 is upon us, Nortel claims. "OC-192 didn't just show up. It really took about four years to develop it and do volume business," said Brian McFadden, assistant vice president of brand management for Sonet networks at Nortel.

However, many industry sources maintain that higher-channel WDM at lower Sonet bit rates, such as OC-12 and OC-48, will continue to be the best way to address changing capacity needs. This becomes especially true as the capacity needs of carriers' larger customers become harder to predict.

"Bit rate growth can be cheaper in some situations if you are talking about moving from OC-3 to OC-12. But as you go from OC-12 to OC-48 and further, if you can't predict what you need, it's better to just add wavelengths for incremental growth," said Jesus Leon, vice president of access products at Ciena Corp., a winner of many recent WDM contracts.

Nortel's McFadden, however, counters, "We're committed to both because we want to give customers the choice, but it is easier to manage OC-192 with less wavelengths than OC-48 with more. It's less complex. You don't have to keep thinking about swapping out wavelengths or how to find a problem that occurs on a single wavelength."

NEC SEES NEW VISTA NEC America announced its new Vista low-speed Sonet access multiplexer at the recent National Fiber Optic Engineers Conference '97. The system, capable of OC-3 and OC-12 networking, incorporates full access grooming and ring protection functionality into its backplane. This reduces deployment expenses and keeps the system's price under $10,000, NEC said.

5ESS-2000 GETS ADSL BOOST Lucent Technologies has added a Broadband-Access Interface Unit to its 5ESS-2000 switch that will enable carriers to offer switch-integrated ADSL service. The B-AIU's broadband access capabilities supplement the switch's existing analog and ISDN services.The ADSL application offers up to 8 Mb/s downstream and 1 Mb/s upstream, depending on line conditions.