Aspate of optical vendors has rolled out modular xWDM platforms enabling migration from coarse to dense WDM. The logic of a pairing between transport mechanisms suited to opposite needs elicits initial pause, but vendors say it fits nicely into several scenarios where service providers are seeking to lower capex in providing metro access services.
Today’s metro DWDM systems typically support 32 protected wavelengths with channel spacing at 0.8nm. Most CWDM systems feature four or eight wavelengths (although 16- wavelength systems are available) with channel spacing at 20nm.
While the number of wavelengths supported is critical, channel spacing has the biggest impact to cost differentials between DWDM and CWDM equipment, notes The Yankee Group analyst Terry Landers in a December 2002 report. “DWDM equipment has tighter wavelength specifications that support a greater number of wavelengths; however, it also requires more costly components. CWDM, with its wider wavelength spacing, allows vendors to incorporate less costly optical components into their systems,” he writes, noting the savings result is a cost advantage for CWDM of about 25 percent. (ADVA Optical Networking and MICROSENS cite savings of up to 50 percent.) In addition to differences in cost, there are differences in distance. While DWDM systems can support distances of more than 640km (400 miles) with amplification, CWDM systems typically are limited to less than 80km (50 miles) without amplification (although some vendors like Movaz Networks claim more). “CWDM is not able to satisfy the distance requirements of larger metro networks,” explains Landers. “While CWDM can be amplified, the breadth of the optical wavelength makes the amplifiers extremely expensive, negating the economics of using CWDM in the first place.
“CWDM’s cost-effectiveness for shorter metro distances makes it an ideal solution for access applications, ring interconnections, and networks that require less than 12 wavelengths. Beyond that, service providers will choose DWDM systems instead.”
When customers’ capacity requirements begin to exceed the channel limits of CWDM, they typically must switch to the more costly DWDM technology and a completely new system, explains MICROSENS. In contrast, the MICROSENS’ modular optical multiplexer introduced late last year offers a soft migration by combining CWDM and DWDM. In its basic stage, the system offers up to eight CWDM channels. When capacities need to be expanded, each individual CWDM channel can be expanded by up to eight DWDM channels, allowing the implementation of up to 64 optical channels in the maximum stage. The optical interfaces for the individual data channels are pluggable transceiver modules.
Due to its modular design, service providers need only use the number of channels actually required, lowering initial costs and also saving in warehousing, maintenance and service, MICROSENS says.
Similarly, ADVA recently announced the addition of a CWDM module set for its Fiber Service Platform (FSP) 2000 product platform with the intent of reducing first installed cost and enabling migration to a hybrid C/DWDM system. The new CWDM design can be configured with two or four channels in a network, and later upgraded to either a full eight-channel CWDM system or 20-channel C/DWDM hybrid system.
The company says this enables carriers and enterprises to implement a more cost-effective WDM solution today, analyze their bandwidth demand and future requirements, and then later commit to additional CWDM or DWDM capabilities. “In the past, the customer has had to make a hard decision [between CWDM and DWDM],” says Brian McCann, chief marketing and strategy officer at ADVA. “Now, they can start with CWDM and get the cost benefit and still migrate to DWDM without a forklift upgrade.”
“In practice the uses for CWDM and DWDM vary dramatically,” says David Gross, senior analyst for CIR Inc., explaining CWDM usually is point-to-point access while DWDM is used in ringed topologies and for aggregating traffic at the core. “No one has a need to pull them together.”
Gross adds CWDM is an enterprise solution and that an eight-channel box is almost always enough. Furthermore, he says while enterprises are solely interested in cost-cutting, service providers have the added variable of revenue generation on the back-end, making DWDM an investment in quality and reliability required to support a commercial offer. Gross says he suspects many vendors have added CWDM modules in answer to the hype over the technology and to be able to say that they support it in answer to RFPs.
Vendors admit that C/DWDM systems’ applications are niche in nature. ADVA’s new CWDM module set was developed particularly in response to small storage projects currently being implemented by small- and medium-sized enterprises that may require two to three wavelengths initially but need to add more channels in the future as storage area network and LAN applications grow.
ADVA’s McCann says an ideal CWDM application is disaster recovery, which typically requires a point-to-point connection over a short distance. Another is server clustering, which can now be done not only within a building, but also within a metro network using a CWDM link.
Sophia Fang, vice president of business development and marketing for Movaz, says another potential application for the hybrid system is providing route diversity. Typically, she says, the point-to-point connection is shorter than the redundant route, so it can make sense to run the former using less expensive CWDM and the latter over DWDM. Movaz, which added CWDM support to its Rayexpress DWDM platform this spring, also offers the advantage of being able to support longer distances up to 120km (75 miles) with CWDM using unamplified forward error correction techniques, which enables optical transport across the metro.
Rob Keats, director of marketing for Nortel Networks, says the available solutions address service delivery vs. infrastructure. “We want to start with the services and then look at the best way to do it,” he says.
Nortel, the optical market leader, introduced its CWDM platform in mid-2002 and announced upgrades in early June 2003. Called the OPTera Metro 5100 Multiservice Platform, it integrates with the OPTera Metro 5200 DWDM platform, with shared hardware, which reduces sparing and inventory.
According to Yankee Group’s Landers, this combination allows networks to support SONET all the way to DS1 granularity and at the same time offer non-traditional services, such as ESCON, FICON and D1 video. Completel, a French carrier, already uses the hybrid system.
