CWDM Gets a Boost

The International Telecommunication Union's Standardization Sector (ITU-T) has given the nod to a standard for CWDM gear that will help vendors' devices play nicey-nice.

The standard, ITU-T Recommendation G.695, describes signal attenuation, power levels, distance, interface speed, and other characteristics for coarse wavelength-division multiplexing. CWDM is a cheaper alternative to DWDM (dense Wavelength Division Multiplexing (WDM)) that's increasingly used to transport traffic in metro networks, including Ethernet data and SAN traffic (see CWDM: Low-Cost Capacity).

The significance, say industry sources, is that vendors that want their CWDM-based equipment to work together don't have to engineer their own interfaces to do so.

The new specs also add a 1.25-Gbit/s version for CWDM channels, which formerly were a standard 2.5 Gbit/s. This allows for devices that don't need the top rate -- gigabit Ethernet transponders, for example -- to be made and sold more cheaply.

Until now, there have been no standard interfaces between CWDM-enabled devices, and vendors wishing to create 1.25-Gbit/s CWDM links, usually for Gigabit Ethernet connectivity, haven't had an international standard to point to. Standards are important to carriers worldwide, so not having guaranteed interoperability among interfaces for metro services was seen as a hindrance by equipment suppliers (see CWDM Products Proliferate)."Before, CWDM specs were focused on getting the technology to work," says Andy Bray, principal of technology consultancy Netrius Associates. The new specs talk about what can be done with CWDM, he says.

Bray hosted a Light Reading Webinar on the topic of CWDM today, in time with G.695's announcement. The archived presentation will be posted here within the next few days.

"We've all been in anticipation of this for awhile," says Brian P. McCann, chief marketing and strategy officer for ADVA AG Optical Networking (Frankfurt: ADV). He likens the ratification to the meeting of earnings expectations for stock market investors. "It's a validation."

This isn't to say vendors have been waiting around. ADVA and others have had enough confidence in the standard's eventual approval to go ahead and build products that fit prestandard drafts. ADVA has longstanding resale and interoperability partnerships with Fibre Channel vendor CNT (Nasdaq: CMNT) and Ethernet switch maker Extreme Networks Inc. (Nasdaq: EXTR). In both cases, ADVA maps CWDM wavelengths used in its gear to those used by the other vendors. McCann claims the interfaces ADVA and its allies created presaged the interfaces in G.695.

Another vendor, Transmode Systems AB, hasn't been waiting around, either. Transmode's been selling 1.25-Gbit/s transponders, which were just approved, as an alternative to 2.5-Gbit/s ones for months. According to VP of marketing Michael Crossey, customers looking only to support Gigabit Ethernet that don't require the bandwidth of a 2.5-Gbit/s link can save costs by buying the lower-speed option.While G.695 is a welcome, if expected, addition to CWDM, it doesn't solve everything. Issues continue to surface. According to Bray of Netrius, some suppliers and carriers are talking about 10-Gbit/s CWDM transponders. Right now, to get a full 10-Gbit/s link calls for using four distinct 2.5-Gbit/s interfaces in parallel. In the future, Bray says, vendors may want to have a single, serial 10-Gbit/s one.

There's also no guarantee the new specs will solve disputes about the relative merits of using GBIC interfaces or transponders, a battle that came to a head between Transmode and Cisco Systems Inc. (Nasdaq: CSCO) this summer (see Transmode, Cisco Duel on CWDM).

Mary Jander, Senior Editor, Light Reading