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The director telephone system was a development of the Strowger or step-by-step (SXS) switching system used in London and five other large cities in the UK from the 1920s to the 1980s.
A large proportion (c. 70% to 80%) of telephone traffic in large metropolitan areas is outgoing traffic, and it is distributed over many exchanges. A non-director SXS exchange system is not suitable for these areas.[1]
As the translation facility incorporated was similar to the register in common control systems, the director system incorporates two features of the Panel system, which was introduced in large American cities, and which were required regardless of the type of exchange system for these large areas, which would have a mixture of manual and automatic exchanges for some years. Customer stations were assigned seven-digit numbers, with the first three digits spelling out the local exchange name; this expedited call handling particularly to and from manual exchanges. Direct or tandem junction routes to other exchanges could be allocated as required, with routing independent of the telephone number and able to be altered at any time to cater for traffic growth or the introduction of new local or tandem exchanges.
Each local exchange incorporated up to eight groups of directors which translated the first three digits (ABC digits) comprising the exchange name into a pulse train of one to six digits, as required for each exchange and unique to that exchange. The translated digits were sent to the code selectors, and then the four numeric digits were sent to three switching stages in the terminating exchange (two group selectors and a final selector). Hence local calls within the exchange and busy direct junction routes to exchanges with high traffic from that exchange could be trunked via one code selection stage, which reduced both the setting-up time and the total number of selectors required in the network. Distant exchanges which did not justify direct junction routes could be called via one or more tandem exchanges; being routed via one, two or three local code selectors in the originating exchange, one or more selectors in the tandem exchange(s), and finally the numeric selection stages in the terminating exchange for the last four digits, which were stored and forwarded without translation.
- ^ Atkinson, Telephony, volume II pp 373–74