Riding The Wavelengths
And that's where DWDM enters the picture. Instead of pulling new fiber, vendors say, why not upgrade the whole ring with DWDMs? They sit in the basement of the customer premises, just like Sonet ADMs. Two fibers are typically pulled into the DWDM chassis from the metropolitan ring; as with Sonet, one is used as a so-called primary circuit, and the other as the "protect," or backup, circuit (see Figure 2).
The DWDM chassis has multiple slots for cards. Each card allows the unit to drop one or more wavelengths onto the ring; it also contains single-mode and multimode fiber interfaces for delivering services to end-users.
And the number of wavelengths is what helps separate current products. Sycamore's SN8000, for instance,
handles the most: 44. Ericsson's Erion Networker Flexing Bus handles the fewest: just 16.
Distance is another differentiator, and in the metro area it's measured in terms of the ring circumference, or the total distance from the CO to the furthest node and back again. Ericsson and Sycamore claim to support
rings as large as 500 km, thanks to optical amplifiers. Ciena's Multiwave Metro doesn't use amplifiers, and the maximum distance it handles is 80 km. Does that mean amplifiers are the way to go? Not necessarily, says James Rouse, director of market development for optical products at Nortel. His research indicates that 90 percent of rings in the U.S. are under 120 km in circumference, which RHK's Cooperson confirms. Because amplifiers cost a lot of money, both Nortel and Osicom offer them as slot-in options. They can be removed when deployed on shorter rings, keeping down costs. Nortel says its Optera Metro can go to 120 km, and Osicom says its Gigamux reaches 300 km with amplifiers.
Plug and Pay Less
As for provisioning services, carriers can tackle the task two ways. They can bring circuits to customers either directly on top of wavelengths, or they can carry them using current Sonet gear.
In the first scenario, customers just plug their equipment into the DWDM, and each of the services they use travels natively over a 2.5-Gbit/s wavelength, with multiple wavelengths used to provision multiple services. In the second scenario, carriers connect a DWDM to a Sonet ADM. Customers plug their equipment into the ADM instead, using its multiplexing capabilities to break down the 2.5-Gbit/s wavelength into several lower-speed services, like T3s (45 Mbit/s) and OC3s.
Whether one approach is better depends on the service being sold. Splitting wavelengths means "you don't have to waste a whole wavelength for a relatively low-speed signal," explains Ron Mackey, Osicom's executive vice president of technology. But big-bandwidth apps work at such high speeds there's no need to multiplex them down, so they're better handled without additional Sonet equipment, says Steve Chaddick, Ciena's senior vice president of strategy and corporate development. He thinks any service above OC12 is delivered more efficiently on top of wavelengths. Then again, carriers might want to keep their Sonet add-drop multiplexers and use them in conjunction with a DWDM. "Sonet is something CLECs and RBOCs have lived and breathed for a long time, and they love it," Weingarten says. Most of today's DWDM products, he adds, don't offer nearly as much as Sonet in terms of configuration, restoration, or management.
Osicom, however, says its Gigamux may offer the best of both methods. It has so-called EPC (electrical
photonic concentration) cards that split each wavelength into 16 lower-speed pipes supporting such services as DS-3, OC3, fast Ethernet, and FDDI. Baksheesh Ghuman, senior product manager at Electric Lightwave Inc. (Vancouver, Wash.), a CLEC that has been testing the product, says that's made for a 20 percent savings over a Sonet ADM. What's more, the Gigamux bundles everything in a single device, making it easier to manage. "Service providers like solutions," says Deb Mielke, an independent consultant at Treillage Network Strategies Inc. (McKinney, Texas). "They don't like to have to put everything together all the time." But Osicom's multiplexing scheme is different from Sonet's, so carriers may need to retrain their engineers in order to do that management.
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