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The Renewal of the London Underground
Telecommunications Network
Roger Linton
The London Underground railway covers 668 miles of surface and tube track,
served by 272 stations, and carries over 2 million passengers every working day.
In 1979 this busy undertaking decided to replace its telephone system, which is
vital to the efficient operation of the railway and administrative organisation. The
old one was a Strowger system of the 40s.
The author gives the background of the project and describes the various parts
of the provision of MD110 exchanges, optical fibre transmission systems, the
cut-over and the organisation required to complete the task.
office wiring changes was also a new
feature. However, there were a number
of disadvantages when applied to railway
use since:
- it was not a fully solid state switch,
relays were used as an interface on
exchange and tie line boards.
- it consumed a large amount of power:
10 kVA for a 248-line exchange.
- it had no battery backup.
UDC 621.395.2
535.394
private telephone exchanges
telephone networks
optical links
railways
installation
In 1974 the major part of London Transport's
Strowger PABX had been in service
for 35 years, and preliminary planning
commenced to replace it, fig. 1.
Generally, Strowger equipment was expected
to have a service life of between
25 and 30 years but with the development
of crossbar, followed by reed electronic
exchanges in the late 1960s, it
seemed prudent to wait for a fully electronic
exchange to appear.
IBM manufactured their 3750 Stored
Programme Controlled (SPC) analogue
exchange, and London Transport purchased
one for evaluation in 1975. It
proved very popular with users, especially
the facilities; and the ability to
change numbers from the maintenance
terminal which did not normally involve
The IBM 3750 was marketed at a time
which caught the UK Telecommunication
Industry without an equivalent
product, but by the late 1970s other
SPC exchanges became available and
with battery backup.
In 1975 we commissioned our first 30-
channel PCM system to Hounslow West
exchange for the extension of the Piccadilly
Line to Heathrow Central. Other
PCM systems were quickly incorporated
into the network to improve overall
transmission quality, and having
adopted time division multiplexing for
junctions it became clearthere would be
considerable advantages to do likewise
with the new exchanges.
Traffic studies of current and future
growth led to a requirement for 18 telephone
exchanges to cover the London
East Finchley
41XX
Finsbury Park
6XXX
Barking
29XX
Fig. 1
The old Strowger telephone system
Tandem exchange
Hounslow
West
22XX
^P
Main exchange
Stockwell
42XX