CWDM On the Move
Coarse wavelength-division multiplexing (CWDM) is at a crossroads, and some key decisions about its future are in the works.
A flurry of recent announcements demonstrates growing interest in CWDM among component and system suppliers, as well as their carrier customers (see Luminent Intros CWDM, SFP Transceivers, Molex Transceivers Go the Distance, and Finisar Launches CWDM Transceivers for three of the latest). Since CWDM is cheaper to implement than dense Wavelength Division Multiplexing (WDM) – albeit more limited – it presents a welcome alternative for use in metro and access networking gear, particularly as the telecom downturn lengthens. (For more on these matters, see the recent Light Reading report: Metro DWDM.)
But proponents say CWDM needs work to fulfill its potential. They're calling for technical changes that will help overcome its limits of distance and capacity. They're planning new standards. And perhaps most importantly, they're asking for an industry change of attitude. These are the elements they say will make the technology more than merely a stop along the way to DWDM.
Change won't come easily. Standards are part of the problem. Although CWDM products have been available for at least two years, the International Telecommunication Union, Standardization Sector (ITU-T) only finished blessing the wavelength grid used by CWDM gear this past June (see ITU Sets Global CWDM Standard). The group has approved the use of 18 wavelengths from 1270 to 1610 nanometers in CWDM products. Channels are spaced 20 nm apart in order to work with cheaper lasers that don't require expensive cooling mechanisms.
But the job's only half done. The part of the standard that will specify exactly how CWDM wavelengths are to be amplified to extend specific distances over different types of fiber remains to be hammered out by the ITU. Until this is done, products aren't going to interoperate -- a drawback for prospective customers.
The work of completing the CWDM specs is being done by the ITU's Study Group 15, the part of the ITU that's charged with setting standards for "transport networks, systems, and equipment." The specific CWDM project is cryptically known as "Question 16," or G.capp (shorthand for CWDM applications). The group devoted to the G.capp is set to meet October 8–10 in Santa Rosa, Calif.
Sources say it could take a year to finalize specs, especially since several proposals are up for consideration, and opinions seem to vary. "With regard to applications, CWDM is best suited for applications that have lower data capacity requirements and for fiber spans that are 50 km or less," writes Peter Wery, chairman of Study Group 15, in an email today. Wery also works at Nortel Networks Corp. (NYSE/Toronto: NT).
Some vendors already have made inroads on blasting the 50km limit. In an announcement earlier this month, for instance, CWDM system vendor Transmode Systems AB and component maker Genoa Corp. announced a field trial in the U.K. that amplified four CWDM channels of Gigabit Ethernet over 125 km of fiber (see Transmode, Vtesse Trial CWDM).
Distance is just one technical issue that's intriguing vendors right now. There's also talk of systems that blend CWDM and DWDM. Also on the rise are "zero water peak" fibers that reduce the attenuation that can wreak havoc on CWDM setups.
All the discussion points to burgeoning interest in CWDM, which could help accelerate its standardization. At last week's NFOEC tradeshow, a group called the Full Spectrum CWDM Alliance (no Website) reportedly met to discuss ways of speeding up the standards process by reaching consensus before ITU meetings convene. So far, the group hasn't made a public announcement.
Another factor key to CWDM's future is carriers' interest in it. And that, fans say, is helping to shape the technology's future as well. One service provider, Looking Glass Networks, says makers of metro gear need to rethink their position on CWDM.
"We would like to use CWDM, as a number of our fiber rings could take advantage of the fiber gain and cheap optics," emails Steve Plote, director of technology at Looking Glass, "but we need to have the CWDM optics installed on our currently deployed equipment." That means, in this case, that ONS 15454 platforms from Cisco Systems Inc. (Nasdaq: CSCO) would need to be outfitted with CWDM interfaces. Also, Plote would like to see his Sonet grooming gear similarly enhanced.
So far, it hasn't happened. Plote says Looking Glass has asked Cisco to add the interfaces "like they have done for their 12400 router and their Catalyst Ethernet switches."
There's no sign of changes for the higher-end gear. "Cisco is currently examining CWDM integration with the Cisco ONS 15454," writes a spokesperson.
For Plote, a change of attitude on CWDM seems as important as technical and standardization issues to the technology's future. "CWDM is a great technology that has been around longer than DWDM; but until it is available on multiple platforms that a carrier can buy today, it will have limited deployments," he writes. "The Ethernet vendors have done a good job of picking up on this; now it is up to the metro transport vendors to do the same."
— Mary Jander, Senior Editor, Light Reading