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The Director telephone system was a development of the Strowger step-by-step (SXS) switching system used in London and five other large cities in the U.K. from the 1920s to the 1960s. A large proportion (ca. 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.〔Atkinson, ''Telephony'' volume II pp 373–74〕 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 numbers 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. ==Numbering plan== Subscriber telephone numbers were seven digit numbers, with the first three (ABC) digits corresponded to the first three letters of the local exchange name. The translation map of letters to digits was displayed directly on the telephone rotary dial, by grouping the letters around the corresponding digits. The British letter arrangement was similar to American dials, except that the letters “O” (and "Q") mapped to digit 0, not 6: 1 (unmapped), 2 ABC, 3 DEF, 4 GHI, 5 JKL, 6 MN, 7 PRS, 8 TUV, 9 WXY, 0 OQ. The mapping of O and Q to 0 was to eliminate the possibility of a subscriber misdialling as a result of misreading a number. A subscriber in Wimbledon (say) would be assigned the number ; the first three letters, written in capitals, indicated the exchange code to be dialled. The actual trains of pulses from the subscriber's dial were 9461234. The exchange code digits dialled by the calling subscriber were the same from any telephone in the London director area, which has a linked numbering scheme. Subscribers on manual exchanges asked for a number in the format , whether the called number was on a manual or automatic exchange. The three-letter code was written in bold capitals if the caller should dial all seven digits. If written merely in capitals it indicated that the desired number was on an exchange which had not yet been converted to automatic working, and that the caller should dial only the initial three code digits, and wait to be connected by an operator. Later some of the remaining manual exchanges were equipped with Coded-Call Indicators (CCI) which displayed the local digits dialled by the caller to the operator, The number would be listed as for an automatic subscriber with the first three letters in bold, and automatic subscribers would dial all seven digits. If an exchange was outside the Director linked-number area, the exchange name and number was in small type, e.g. Laindon 2263. In a Director area there were 8 digits available for the A-digit letter of the exchange name (excluding ‘1’and ‘0’), and 9 digits each for the B-digit and C-digit letters; hence there were a maximum of 648 exchange names (8 x 9 x 9), though in practice some ABC codes (e.g. 555) did not have a usable name equivalent, and by 1966 London with 350 exchanges was running out of exchange names. Special Service numbers apart from “0” for operator used a three-letter exchange name; e.g. TOL (toll), TRU (trunk), DIR (directory enquiries), TIM (time, the “speaking clock”), ENG (engineering i.e. Faults), UMP (for the Test Match cricket scores); or 999 (emergency). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Director telephone system」の詳細全文を読む スポンサード リンク
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