ITU studies submarine cables and systems
WILLIAM B. GARDNER
Two study groups of the International Telecommunication Union, or ITU, are writing Recommendations for underwater optical cables and systems. Study Group 15--Transmission Systems and Equipment--defines submarine cable as "electrically powered underwater optical fiber cable, suitable for shallow and deep water use, which has been extensively tested to show that it can be installed and repaired in situ, even in worst weather conditions, without impairment of optical, electrical or mechanical performance or reliability." Furthermore, nonrepeatered submarine systems are outlined as those that can forego electrical powering, but must employ undersea technologies suitable for rugged environments.
Study Group 6--Outside Plant--defines marinized cable as "an underwater optical fiber cable construction, based on a conventional terrestrial core protected to withstand the marine environment, designed for unrepeatered applications and tested for use in benign shallow waters, with limited repair capability."
A key distinction between submarine and marinized cables is that submarine cables generally interconnect longer distances. In situ repair is necessary in large bodies of water, where cable replacement becomes difficult or expensive. Repair in inclement weather and at water depths exceeding a few hundred meters increases the requirements on cable strength. These factors favor the application of submarine cables. Because submarine systems contain sea-landing points, they are subject to hostile shallow water risks, and the fiber-cable must be armored accordingly.
Question 27 in ITU Study Group 15, co-chaired by Osamu Kawata of Nippon Telegraph & Telephone of Japan and Jean Thiennot of France Telecom, has produced numerous Recommendations dealing with optical submarine systems. These documents include general systems features (G.971), definition of terms (G.972), characteristics of regenerators (G.974), test methods (G.OSS3), submarine systems with optical amplifiers (G.OASS), repeaterless systems (G.RLSS) and forward error correction (G.gifCSS). The last four documents are Draft Recommendations that are expected to be approved during the next few years.
The introduction of optical amplifiers in submarine systems (see Lightwave, April 1994, page 33) presents some new standardization issues. For example, optical amplifiers cause noise, which by itself and through interaction with dispersion and nonlinearities in optical fiber creates operational impairments that accumulate over distance. Consequently, a sophisticated dispersion-mapping scheme is needed for large bit-rate/distance systems to manage these impairments. Schemes for different systems and vendors do not have to be identical, but compatibility among vendor schemes must be achieved in multivendor systems. Such issues are being addressed in Recommendation G.OASS, currently under development in ITU Study Group 15.
Question 11 in ITU Study Group 6, chaired by S. V. Angelidis of the Hellenic Telecommunications Organization SA of Greece, is preparing a draft handbook titled "Construction Jointing, and Protection of Shallow Water Optical Fiber Cables." This handbook includes chapters on survey and route planning, characteristics of vessels, repair methods and testing. The group is also preparing draft recommendations dealing with the depth of fiber-cable burial in hard and soft bottoms, pipeline crossings, route documentation and sheath markings. q