mer degradation, magnetic hydrodynamic effects and other generally sufficient to ensure compatibility between conforming causes. By careful choice of fibre and cable materials, and by cables and manufacturers' optical repeater equipment. Key par- appropriate design the hydrogen effect can be limited to a tolerable ameters from these recommendations are given in Appendix 15.5 level (Worthington, 1986). and 15.6. 15.7.3 Optical cable for information systems 15.10 Future developments Much attention has been devoted to using optical fibre technology The current standard single mode telecommunication fibre provides in a 10Mbit/s ethernet. Another current project is FDDI-II (fibre remarkably low attenuations of 0.5dB/km in the 1300nm window distributed data interface) which is a 100Mbit/s dual optical fibre and, optionally, 0.2dB/km at 1500nm. These values, in conjunction ring (Flatman, 1989). with a variety of transmitters and receivers of different powers, Clearly the wideband transmission characteristics of optical fibre, sensitivities and costs, enable system designers to produce econ- omic and technically attractive solutions to most telecommunica- and its immunity to interference make it an ideal medium for this work. The inhibiting factors are the relatively high cost of optical tion needs. This fibre is the culmination of many years development devices and connectorisation in the context of short distance, high in laboratories throughout the world, and it is unlikely that it will be superseded as a general transmission bearer for some years. Optical density links. An elegant solution to this problem has been reported device development will certainly continue and new system designs by workers at STC Technology Limited (Cannell, 1988). Access to offering advantages in transmission bit rates and costs will continue a single mode fibre is accomplished via a non-intrusive tap and a to emerge. Cable designs incorporating standard fibre will also 10Mbit/s ring with a capacity of 100 nodes is described. continue to ev