ITU makes optical system recommendations
LOUISE ALTERMAN
Experts of the International Telecommunication Union (ITU) on Synchronous Digital Hierarchy (SDH) optical-line systems and optical devices concluded the 1993 to 1996 Study Period by completing work on three Recommendations. These Recommendations were "determined" at the Study Group 15 meeting in Geneva recently, and are scheduled for approval at the World Telecommunication Standardization Conference this month.
The Recommendations are: (1) G.scs, "Optical Interfaces for Single Channel Systems with Optical Amplifiers, and STM-64 Systems," (2) G.lon, "Functional Characteristics of Interoffice and Long-Haul Line Systems Using Optical Amplifiers, Including Optical Multiplexing," and (3) G.663, "Application Related Aspects of Optical Fibre Amplifier Devices and Sub-Systems." Recommendation G.mcs, "Optical Interfaces for Multichannel Systems With Optical Amplifiers" was not considered sufficiently mature yet; work on it will continue into the next Study Period.
The final version of Recommendation G.scs contains the optical loss budget parameters for SDH systems with optical booster amplifiers and optical preamplifiers, as well as for STM-64 systems without amplifiers. Although the document was considered stable, the optical loss budgets for STM-64 systems are incomplete, and many parameters have been left for further study. Additionally, systems with inline optical amplifiers, referred to as "Part B" systems, have been held over for a future version of the Recommendation.
The first open item on "Part B" systems to be addressed in the next Study Period will be application codes. This item refers to the number of inline amplifiers installed between regeneration points, and the target spacing (in km) between them. Other technical issues that need to be addressed include developing a model from which power levels can be calculated, further study of eye-mask specifications, and issues relating to the optical supervisory channel.
A session on optical safety for optical amplifiers led to a procedure in G.lon for automatic power shutdown, including a set of numerical values to go with the procedure. Delegates agreed that since the purpose of this Recommendation is to provide functional models, it would not be useful to include examples of specific implementations. Management of optical and SDH network elements was not discussed at this meeting; it will be pursued in a future version of this Recommendation in conjunction with the management experts. In accordance with this decision, the topic of management of optical amplifiers was removed from Recommendation G.663 in anticipation of the future work on Recommendation G.lon.
The primary reason that Recommendation G.mcs was not sufficiently mature was that the delegates were still discussing which channels to use with each application code. The document currently includes a table containing more than 40 frequencies that may be used for the channels in multichannel systems. Most of the delegates believe that a smaller number of channels is required to achieve transverse compatibility. Other work items for Recommendation G.mcs include the defining of parameters for gain variation and amplifier tilt, continuing work on optical crosstalk and specifying power levels for each application code. All this work will proceed by correspondence until the next meeting.
Added to this future work for the next Study Period will be the topic of all-optical networks. It is expected that this work will be pursued separately by experts from many disciplines, coordinated by a single ITU Special Rapporteur.
The next meeting of the optical experts of Study Group 15 will be in Kyoto, Japan, this month. q