Digital coaxial cable systems

Abstract

To produce digital transmission systems, the same types of coaxial pairs as are already used for analogue systems are needed, namely, the small size pair (1.2/4.4 mm) and the standard size pair (2.6/9.5 mm). For a long time the coexistence of analogue and digital systems in the same cable will be necessary: this constrains the latter systems to use common infrastructures. The most important constraint concerns the regenerative section which will have to be consistent with the amplifier section of analogue systems. As regards the quality of the coaxial pairs, it seems almost certain that the manufacturing quality achieved to meet the requirements of high capacity analogue systems would also be sufficient for digital systems having comparable capacity; nevertheless it seems sensible to investigate the electrical characteristics strictly relevant to digital transmission in order to be able to submit newly manufactured coaxial cables to tests concerning this new type of transmission; studies in this area have developed quickly and will perhaps be completed soon. As regards new types of coaxial pairs to be used only for digital transmission, in the last few years a very small size pair (0.7/2.9 mm) has been produced, and has been called the ‘microcoaxial pair’. It is devoted to medium capacity, but nevertheless trends do not seem to be favouring production of further types of coaxial pairs for digital transmission only. The choice of information bit rates to be assigned to the different systems has been and still is one of the main problems; both the multiplexing hierarchy recommended for the terminal equipment, and the economic optimization of transmission by coaxial pairs have to be taken into account, as also does the need for maintaining a certain competitiveness with the analogue systems. At present, the 34 Mbit/s system on the microcoaxial pair and the 140 Mbit/s system on the small size pair (1.2/4.4 mm) seem to be the optimum ones. On the other hand, the optimum bit rate for the systems to be used on the 2.6/9.S mm coaxial cable has not yet been defined: it is likely to be allocated between 500 Mbit/s and 1000 Mbit/s, probably nearer the latter. Nowadays several types of digital coaxial systems are either in service or undergoing field trials, from 34 Mbit/s to a maximum of 400 Mbit/s. For such systems, international recommendations do not yet exist; however, it is certain that international standardization studies will be concentrated on a few systems, probably with priority given to the 34 and 140 Mbit/s systems. The problems that still have to be faced concern (1) the definition of the quality-error rate, jitter and bit-sequence independence; (2) the potentialities of completely regenerative and hybrid solutions; (3) the line code and signal characteristics; (4) the alarm conditions and supervision criteria.