Field Trial of Optical Fibre Cable-TV System Optical Fibre System for ...

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195 Fig. 4 Layout of a typical MD110 exchange MDF LEB LRT Main Distribution Frame London Electricity Board London Regional Transport LIM directly interfaces with a 2Mbit/s PCM digital stream. Originally it was intended to use 2Mbit/s PCM routes throughout the network, but this would have required many regenerators for a reliable system and would not be cost effective. It was therefore proposed to work at 8 Mbit/s between the TSCs and the major exchanges, and then revert back to 2 Mbit/s to serve outlying sites. The 8 Mbit/s PCM systems required a specially designed screened group cable in order to achieve the necessary attenuation and crosstalk performance. On investigating the market in 1980 no such cable was available in the UK, and we found that British Telecom were interested to develop such a cable for their use. We commenced development of a screened group cable, and sample lengths were manufactured and installed on the railway. It soon became evident that the size of the cable was becoming very large in order to achieve the required crosstalk characteristics, and the jointing of this cable was very specialised, particularly in maintaining the structure of the group screen within the joint. From this situation consideration was given to an alternative method of transmission. Optical fibre trial During 1978, when the new telephone system was being designed, it was evident that optical fibre transmission techniques offered attractive advantages in the environment of an electrified underground railway. London Transport therefore decided to install a field trial linking the existing Earls Court and Acton exchanges. This was a route where added capacity was required, and the practical experience of handling an optical cable in tube tunnels as well as on open sections of t he rail way would be invaluable. All installation of cable and terminal equipment was carried out by London Transport communications staff with the cable spliced by contractors. This 7 km link carries four 2 Mbit/s PCM systems which are multiplexed to give one 8 Mbit/s PCM stream without any
196 Fig. 5 Optical fibre cable routes 4(2) 4 tlbres on route, (2) indicates number of fibres spare to Initial requirements KT» Copper conductor cabling Optical repeater (~\ Rickmansworth r\ \ i -f i Neasden intermediate regeneration. After a testing period the trial link was placed into full traffic service in July 1979. It is believed that this was the first operational railway optical fibre link, and certainly the first to be placed in live traffic service in UK. Following the trial a study was implemented to see if optical fibres could be incorporated in the new exchange transmission network. The principal advantages to London Transport would be: - Vast reduction in weight and diameter as compared with the proposed 8 Mbit/s screened copper cable, leading to reduced installation costs (no cable trains reguired as optical cable can be run from a trolley) and saving much reconstruction of cable runs. This saving alone more than cancelled out the higher capital cost of the optical fibre system. - Elimination of repeaters from most routes which would greatly reduce Golders Green East Finchley fault liability. The larger toleranceson distance between remaining repeaters allowed them to be situated at stations with easy access. - The intrinsic immunity of optical fibres from electromagnetic interference allowed better error rates to be achieved on the PCM which was noticeably going to become apparent in the quality of transmission. It was found that the optimum rate for digital transmission for the size and geography of the London Transport network was 34 Mbit/s. It was not economic to extend the optical fibres to the outlying exchanges so a portion of these 2 Mbit/s systems remains on copper cables. Two 140 Mbit/s links between Embankment and Baker Street TSCs were also reconfigured from coaxial to optical cable. The net result would be a total of 120 route km of optical fibre cables against 224 route km on copper cable, fig. 5. The study proved it would be viable to alter our plans at this stage of the Ruislip [^ ^ ^ (~\ Loughton s "Q Hainau " j Hounslow West / / / / / I - 4(2 > Becontree 4(2) Lillie 6(2) Head Bridge Office Embankment -i I 4(2) Stepney Green I Qj i Stockwell
Page 1 and 2: ERICSSON REVIEW 4 1985 Field Trial
Page 3 and 4: Trial of Optical Fibre Cable-TV Sys
Page 5 and 6: Head end (HC] 280 Mbit/s Farsta aut
Page 7 and 8: 158 Switching equipment Fig. 7 Futu
Page 9 and 10: 160 Fig. 10 The teleconference room
Page 11 and 12: GERHARD GOBL Public Telecommunicati
Page 13 and 14: Fig. 5, above left Two-channel A/D
Page 15 and 16: Digital input signals Digital outpu
Page 17 and 18: Power ZAV 280/4 has individual powe
Page 19 and 20: Wavelength Division Multiplexing fo
Page 21 and 22: 172 Subscriber side BP-890 Controll
Page 23 and 24: Fig. 8 Transmission characteristics
Page 25 and 26: Most computer cabinets are 19" wide
Page 27 and 28: 178 Fig. 5 Modems in Series 7 Modem
Page 29 and 30: Computer Aided Production of Plasti
Page 31 and 32: Fig. 4 Product geometry displayed o
Page 33 and 34: Rectifier for Mobile Telephone Syst
Page 35 and 36: I OR | OFU OS I CMUP HlsHi W±±±h
Page 37 and 38: 188 Fig. 6 The rectifier with the o
Page 39 and 40: Fig. 9 Rectifier BMJ A01 001 with t
Page 41 and 42: The Renewal of the London Undergrou
Page 43: 194 Rickmansworth, and the Loughton
Page 47 and 48: 198 In planning the optical fibre n
Page 49 and 50: Fig. 9 The digital telephone DIAVOX
Page 51 and 52: 202 Fig. 11 Transmission equipment
Page 53 and 54: 204 Fig. 2 Automatic testing of mul
Page 55 and 56: 206 Relative occurrence for F Fig.
Page 57 and 58: Fig. 5 Field testing of a 140 Mbit/
Page 59 and 60: 210 Fig. 7 The system down time as
Page 61 and 62: MTBF (years) Equipment 2-5 Transmul

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