Vendors plot smoother course for CWDM

Th 142915

Almost exclusively, the value proposition for CWDM is based on cost savings to the customer. At the system level, "first-in" cost for CWDM systems is typically on the order of 30–40% less than DWDM systems. There is a technology/cost tradeoff with CWDM, however. Users must ask themselves, "How much do I want to get out of my fiber? And how much am I willing to spend?" Until recently, enterprises were the primary users of the technology, but are carriers jumping on the CWDM bandwagon as well?

CWDM is less expensive thanks in part to the use of uncooled lasers. In a DWDM system, the thermal cooling element holds the laser steady across temperature, allowing the wavelengths to remain constant and tightly packed together. Wavelengths are spaced 1.6 nm apart in 200-GHz DWDM systems, for example, while in 100-GHz systems, the wavelengths are just 0.8-nm apart. The wider wavelength spacing of CWDM systems—typically 20 nm— enables the use of less expensive optics, thereby reducing the overall system cost. (Wider wavelength spacing means fewer wavelengths per system, usually four or eight wavelengths maximum on standard singlemode fiber, compared with 32, 64, or more wavelengths on a DWDM system.)Th 142915

Meriton Networks is one of several vendors to offer a hybrid transmission system; its 7200 OADX, for example, features both CWDM and DWDM interfaces to cost-effectively connect the metro edge to the metro core.

CWDM systems can provide more than just capital-expenditure (capex) savings, however. "The biggest advantage for us has been the small-form-factor-pluggables [transceivers], which add flexibility at the card level," reports Brian McCann, chief marketing and strategy officer at ADVA Optical Networking (Munich, Germany, and Manwah, NJ). "So it's not just the 30% reduced initial capex, but it's the maintenance support and the flexibility that the customer gains going forward. Tunable lasers, for example, are desirable in the DWDM space," he says, "but they come at a huge premium. This isn't tunable, but it's pluggable, which gives you some flexibility without the premium."

The economics of CWDM may be compelling, but they come at the expensive of certain capabilities. Traditional DWDM networks operate in the C-band, which stretches from roughly 1530 to 1560 nm. EDFAs have emerged as the predominant technology for amplifying the optical signal in this wavelength range. The standard eight wavelengths of CWDM extend from 1410 to 1610 nm—far beyond the C-band. EDFAs, therefore, cannot be used to amplify the signal, rendering CWDM a technology for shorter distances. The typical CWDM system spans 20 km or less, although today's systems can handle distances in the 70–80-km range.

While several technologies have been developed to increase the distance of CWDM systems, including semiconductor optical amplifiers (SOAs) and zero water peak fiber, it is still more cost-effective to use DWDM for applications that require longer reach. The sweet spot for CWDM remains the metro access or edge network typified by point-to-point configurations such as the connection between an enterprise and a central office. DWDM is best suited for ring or mesh topologies in metro core networks that require more reliability and greater route diversity.

CWDM is also limited in terms of line rates. The typical system supports 1.25–2.5 Gbits/sec per wavelength. The typical DWDM system, by contrast, supports 2.5–10 Gbits/sec—with the promise of 40 Gbits/sec on the horizon.

Despite its limitations, CWDM seems to be gaining acceptance; the number of systems houses that have incorporated CWDM capabilities into their platforms attests to the popularity of the technology. Among those vendors that have made recent CWDM announcements are Nortel Networks, Alcatel, Siemens, Fujitsu, Cisco Systems, Ciena, NEC, ADVA Optical Networking, MRV Communications, Advanced Fibre Communications, Sorrento Networks, and startups Lumentis, Meriton Networks, White Rock Networks, and Huawei.

Enterprise customers are the obvious target market for CWDM systems; they do not often need the capacity of larger DWDM systems, nor do they want the associated costs. But what about the carriers?

"Some of the RBOCs have been looking at metro WDM in general for quite a while, both from a fiber exhaust perspective as well as from a service perspective," reports Anna Reidy, senior analyst, optical infrastructure, at market researcher Current Analysis (Sterling, VA). "Fiber exhaust has been more of a driver than services for the RBOCs, mainly because there were lots of issues with WDM: the planning, the deployments, adding new nodes. If you're trying to attack the same problem with CWDM as metro WDM, you'll have the same issues, but the RBOCs and other carriers may be looking at CWDM to attack a limited subset of some of those applications for a lot less money."

When asked for a ballpark figure, Rob Gaudet, marketing communications director at Meriton (Ottawa, Ontario), estimates that roughly half of the active metro RFPs he's seen specifically request CWDM technology, 30% specify DWDM, while the remaining 20% are simply seeking the best solution for a certain application. While SBC, BellSouth, and Time Warner are among the carriers rumored to have active RFPs in circulation, it's not known whether they are specifically looking for CWDM or DWDM technology. In some cases, both may apply.

While CWDM may take a small percentage of business away from DWDM, the two technologies are actually very complementary, says Genevieve Beaumier, senior manager of photonics at Nortel (Ottawa, Ontario). Hybrid systems can be used to match the capacity needed for a given application to the right technology. "It makes the overall network more cost-effective," adds Beaumier. "To have DWDM and CWDM deployed on the same platform simplifies your sparing, your operations, and the training of your personnel."

For this reason, many vendors have been selling hybrid systems, including Nortel, Meriton, and ADVA, whose hybrid D/CWDM system is "selling like wild fire," according to McCann. He believes CWDM generally accounts for roughly 10–20% of the total metro market—a percentage that is expected to grow steadily. A recent study from Infonetics Research (San Jose, CA) puts the metro CWDM market at $243 million by 2006. ..

More in Network Design