itu completes multichannel recommendation
By KLAUS RAHN
At the International Telecommunication Union`s (itu) Study Group 15 meeting in Geneva in April, chaired by John Eaves, Synchronous Optical Network project manager at Bellcore, the group completed work on Recommendation G.mcs, "Optical Interfaces for Multichannel Systems with Optical Amplifiers." Final approval is expected at the next itu Study Group 15 Plenary, to be held in February 1998. A number of other itu optical system recommendations (see Lightwave, November 1996, page 28) received final approval at the same meeting.
Recommendation G.mcs provides application codes for systems with and without inline optical amplifiers. Longitudinal compatibility is provided, which ensures that the codes can cope with the related physical transmission-path characteristics. Because multichannel system issues were too complex, full transverse compatibility had to be postponed to a future version of the recommendation. Despite this, it was agreed that there is a strong market need for an immediate standard for point-to-point systems to provide guidance to both network operators and manufacturers.
The application codes in G.mcs define the target distances of the transmission spans between terminals or amplifier stages, the maximum number of channels, the maximum number of inline amplifiers, and the maximum bit rate per channel. The application codes are defined for long-haul and very-long-haul spans with target distances of 80 and 120 km, respectively. The attenuation, dispersion, polarization-mode dispersion, and reflections are defined for the spans.
Channel transmission rates of stm-4 and stm-16 (622 Mbits/sec and 2.5 Gbits/sec, respectively) are covered in the main text of the recommendation, while some considerations concerning stm-64 transport are covered in an appendix. The maximum number of channels covered in the main text is four and eight. Systems with 16 or 32 channels are contained in an additional appendix.
In addition to the parameters ensuring longitudinal compatibility, definitions of parameters to be measured at the reference points of the related reference configuration are given in the main body as a step toward transverse compatibility. In addition, parameters describing the behavior of optical inline amplifiers in a multichannel application are defined. A table template of those parameters is provided. The parameter values in the template must eventually be filled in, if transverse compatibility is to be achieved.
A normative annex contains the grid of optical frequencies with 100-GHz spacing to be used for multichannel systems, while the main body contains tolerances for the channel frequencies. An appendix lists examples of frequency grids with even spacing for dispersion-unshifted and non-zero dispersion-shifted fibers containing four and eight channels. Examples provided for dispersion-shifted fibers use uneven spacing in order to reduce distortions caused by four-wave mixing. Other appendixes contain a methodology for the derivation of the unequally spaced channel frequencies, and for using the number of channels, amplifier spans, optical signal-to-noise ratio, and nonlinear distortions to derive power levels.
The wavelength of the optical supervisory channel for systems with inline amplifiers is defined in the recommendation. An annex contains wavelength options for alternative optical supervisory channel implementations. q