ITU-T restructures fiber Recommendations
BY WILLIAM B. GARDNER
Standards organizations face the daunting challenge of keeping their standards documents current in the face of a rapidly evolving technology. International Telecommunication Union (ITU-T) Recommendation G.652 on conventional singlemode fiber illustrates the problem. This document was created in 1984, when the systems du jour were not impacted by polarization-mode dispersion (PMD), L-band transmission, or the nonlinear effects associated with DWDM.
Should G.652 be archived as a basically static document that describes the fiber that was made and installed in the 1980s? Or if G.652 is kept current with singlemode-fiber technology, how do we know to what specifications the various "vintages" of singlemode fibers conform?
At its April 2000 meeting, ITU-T's Working Party 4/15 addressed this problem by restructuring its four fiber Recommendations, G.652, G.653, G.654, and G.655. The result is that each Recommendation now contains several (normative) tables which sort out the various "subcategories" for each type of fiber. In each Recommendation, Table 1 describes the "base subcategory" for which the Recommendation was originally written. Table 1 does not mention PMD or L-band attributes, since these were neither specified nor measured when this fiber subcategory was manufactured.
The subsequent tables describe newer subcategories that have evolved in an era in which PMD, L-band attenuation, and four-wave mixing can be important to system designers. The base subcategory described in Table 1 of G.652 is a "G.652.A" fiber, which is described as being suitable up to STM-16. Table 2 describes the higher-performance "G.652.B" subcategory, which is suitable up to STM-64. The "G.652.C" subcategory described in Table 3 is the extended-range fiber made possible by low-water-peak attenuation.
ITU-T Recommendation G.655 describes nonzero dispersion-shifted singlemode fiber (NZDSF). Table 1 describes the base subcategory G.655.A, for which the "maximum total launch power could be restricted, and the typical minimum channel spacing could be restricted to 200 GHz," and PMD is not specified. Table 2 describes the higher-performance subcategory G.655.B, for which "the typical minimum channel spacing is 100 GHz." Since there are numerous ways of dealing with the tradeoffs inherent in the NZDSF concept, G.655 provides several specific implementation examples in the informative Appendix I. These examples differ primarily in the sign, magnitude, and slope of the chromatic-dispersion coefficient.
This new structure allows technological evolution to be accommodated through the addition of new tables and new examples, while leaving existing tables unchanged. Tom Hanson (Corning USA) is the editor responsible for restructuring G.652 and G.655. The newly restructured versions were "determined" by ITU-T Study Group 15 in April and will be submitted to the World Telecommunication Standardization Assembly for decision this month.
William B. Gardner represents Lucent Technologies (Norcross, GA) on sev eral fiber standards committees. He can be contacted at tel: (770) 798-2674; fax: (770) 798-4654; e-mail: email@example.com.