ng at up to 18MHz (3600 channel 15.3.6.2 Shallow-water cable analogue) and 140Mbit/s digital. The smallest and newest cable, 0.7mm/2.9mm, has had limited application as a 34Mbit/s digital In shallower seas, i.e. less than about 1000m, the risk of damage system bearer. from ships' anchors and fishing activity, particularly trawling, can The performance of these cables is summarised in Appendix 15.1 be high. The solution is to add one or two layers of mild steel to 15.3. In each case the CCITT recommendations make some armouring wires to the lightweight deep-sea cable, packed in a practical distinctions between the performance demanded on a polypropylene bedding. factory length and the results obtainable on a complete and installed Sometimes in vulnerable situations a trench is ploughed on the sea repeater section. For the complete recommendations and explana- floor as the cable is laid and the cable buried for extra protection. tory notes the reader is referred to the CCITT publication. 15.3.5 Coaxial cables for data networks 15.3.6.3 Cable performance Coaxial cable offers better noise immunity and crosstalk than pair This cable design conforms with the outer to inner conductor cable for cabling within a building. It can also be operated at higher diameter ratio of 3.6 to 1 for minimum loss as derived in Section bit rates and has less loss per kilometre. 15.3.1 above. As the dielectric is solid polythene, the characteristic Line plant for copper cables 15/9 A repeatered cable line includes accommodation for equipment Table 15.8 Loss of 37.3mm submarine coaxial cable at e.g. PCM regenerators at 1.8km on pair cable; coaxial line repeaters spaced in the range 1.5km to 9km on large core cable and 2.0 to Parameter Value 4.0km on small core cable. These are robust steel or cast iron housings with a removable lid which when sealed will withstand Frequency several metres of water pressure. Several housings may be used to Loss (dB/nm) 12.54 support one large cable which is separated out into individual pairs at a segregati