CWDM moves toward standardization


At the February 2001 meeting of the International Telecommunication Union's ITU-T Study Group 15, a definition of the term "coarse wavelength-division multiplexing" (CWDM) was considered for Recommendation G.671. Discussion also began on a definition for a grid of wavelengths for CWDM. The role of a CWDM grid parallels the roles of the DWDM grids that appear in Annex A, "Nominal Central Frequencies," within ITU-T Recommendation G.692. In each case, the grid provides the centers of allowed optical channels to be used as reference points when defining applications. The creation of a grid is fundamental to system design, since it is necessary (not sufficient) for compatibility between systems. The DWDM grid has been referenced time and again by DWDM system and component developers. A CWDM grid should draw similar interest. For example, it would serve as guidance to laser manufacturers in their choice of wavelengths to develop.

Two proposed grids for CWDM systems submitted to the October 2001 meeting of ITU-T Study Group 15 received strong support for development into a Recommendation. The plan was to convene a correspondence group within Q.16/15 and hold an experts group meeting to fully develop a CWDM grid to submit for consent at this month's meeting. Consent at this meeting should result in approval shortly thereafter.

The correspondence group, led by Yu Wang (Swisscom), actively discussed the contents of the CWDM grid contributions through the autumn of 2001. Swisscom then hosted an experts group meeting in Bern, Switzerland, in February, where a CWDM grid contribution was made ready for submission to the May ITU-T meeting in Geneva.

The CWDM grid agreed upon has channel centers spaced every 20 nm at "odd tens of nanometers" wavelengths throughout the optical spectrum. This set of wavelengths extends indefinitely, but the channels with practical applications reside within the singlemode optical transmission spectrum.

A companion contribution focusing on CWDM applications was also created, but it is in a much earlier state of development and needs input from CWDM system developers. The new Associate Membership category of the ITU-T with reduced dues should encourage broad participation.

Bernard R. Eichenbaum is a systems engineer representing OFS (Whippany, NJ) in standards work. He can be reached at tel: 973-386-5483; fax: 973-386-3127; e-mail:

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