Director telephone system

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 because of -

Difficulty in operating the remaining local manual exchanges with all-number calling numbers for all subscribers.

Many (generally more than ten) exchanges are of significant size in large cities.

Only ten or less main exchanges accessible from first selection stage, and:

Every main exchange would need direct junctions to and from all other main exchanges.

All exchanges would require two or more selection stages for a call (and possibly two or more junction required for a call), though in the Director system only one selection stage would be needed for busy routes before the local selection stages for the last four digits (ie two group selectors then the final selector).

In a DSR (distinguishing selector repeater) exchange the DSRs are held for the duration of the call but directors are only held for the call setting-up time of up to 30 seconds. A rack held only two rows of DSRs (ten per row) as against eight rows of ordinary group selectors.

But it would be possible to retain DSR satellite exchanges for small exchanges in a Director area temporarily or permanently if the limitation on outgoing junction routes was accepted and balanced against the need to provide up to eight groups of Directors. Siemens Brothers had mapped a possible system for a large city with either six or seven total digits.

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.