Patent GB-211189-A (GB)
Abstract: 211,189. Automatic Telephone Manufacturing Co., Ltd., (Automatic Electri- Co.). Oct. 17, 1922. Automatic exchange systems.-In an exchange network in which two or more exchanges accessible from a given exchange over different routes are called by dialling the same initial digit or digits, when a subscriber dials a digit which does not determine definitely to which exchange the call is going, a trunk is simultaneously seized to each exchange to which the call might go and as soon as sufficient of the designation has been dialled to establish definitely its eventual destination all the trunks are released except the one actually to be used. The invention is especially applicable to systems in which the exchanges of a network are grouped into a number of geographical districts each comprising a central main exchange and several surrounding branch exchanges. Such a system is shown in Fig. 10, in which four districts are illustrated, Dunham, Lincoln, Blazon, and Medbury being the main exchanges therein. Inter-exchange trunks extend from each main exchange to other nearby main exchanges, whilst direct trunks extend from each exchange to every other exchange in the same district. The subscribers' dials are numbered and lettered as illustrated in Fig. 9 and each subscribers' designation comprises the first three letters of the exchange name and four digits, subscriber No. 1234 in the exchange Market, for example, being listed in the directory as MARket 1234. Assuming that a change of exchange names is undesirable, it will be seen that conflict must of necessity occur between certain exchanges, and the eventual destination of a call cannot be determined until the second and in some cases the third letter is dialled. For example, the first two letters of POLk, NORth and PONtiac all have the same digital equivalents and therefore the direction in which a call is to be routed finally, cannot be established until the third letter is dialled. To meet this condition, when a subscriber in any exchange dials a letter which does not determine definitely to which exchange the call is going, a discriminating switch at the originating exchange is taken into use which seizes a trunk to each of the branch and main exchanges to which or through which the call might go. The discriminating switch successively responds to as many digits as are necessary to determine definitely the ultimate destination of the call, and according to a distinctive setting extends the calling line to the trunk to be used and releases all the others. The invention conveniently lends itself to application to district office systems. Its application to such a system is described in subsequent paragraphs. General operation of system. The manner in which a call from the exchange Lincoln is trunked to any other exchange is shown in the layout diagram, Fig. 11. If a subscriber A in Lincoln calls an exchange, such as Upland, the first letter of which definitely establishes its identity, a discriminating switch is not used and the call is extended over a line switch C, group selector D, a rotary trunk-hunting switch SS4 of the lineswitch type, a repeater R1, a group selector E1 at the intermediate exchange Medbury, and a rotary switch SS16 which selects a repeater DAR2 associated with an idle trunk to Upland. Since the wanted exchange has been selected by the first two letters U, P, the impulses corresponding to the third letter L are arranged to be absorbed by the repeater DAR2. Assuming the subscriber A in Lincoln calls an exchange such as Pontiac the first one or two letters of which correspond digitally with those of other exchanges, such as North and Polk, the group selector D in response to the first digit 6 selects an idle discriminating switch US which causes rotary switches SS3 and SS6 to seize trunks to Dunham and Medbury over repeaters R, R1. In response to the second digit O, a first group selector E in Dunham selects an idle trunk to North by way of a switch SS12 and a digit absorbing repeater DAR, and a first group selector E1 in Medbury extends connection to a second group selector F1. On the dialling of the third letter N the group selector F1 extends connection to an idle rotary switch SS18 which selects an idle trunk to Pontiac. The discriminating switch US has in the meantime successively responded to the first three letters and according to its distinctive setting now releases the trunk to Dunham and connects the calling line to the seized trunk to Medbury. If the third letter L of Polk had been dialled the second selector F1 would have extended the call to Polk over a rotary switch and repeater, such as SS19 and R8, the trunk to Dunham being released as before, but if the third letter had been the R of North, the distinctive setting of the switch US would have caused the release of the connection to Medbury, and the extension of the calling line to the seized trunk to North. Two groups of trunks from the rotary switches SS17, SS18 in Medbury are combined in a common group of trunks to Pontiac terminating at Medbury in bi-level repeaters BLR, and at Pontiac in traffic separators TS. According to whether a bi-level repeater BLR is seized by a rotary switch SS17 or SS18 at Medbury, a traffic separator TS extends the call to a rotary switch SS24 or SS25, respectively, at Pontiac. In cases where a rotary switch, such as SS9, accessible from a discriminating switch, such as US3, has direct access to exchange trunks, it is not connected up unless the exchange to which it has access is the one required, and then only after the dialling of the third letter. In a similar manner, when local calls are made, the rotary switch SS26 having access to the local group selectors G1, is not connected up by the discriminating switch US1 until the third letter has been dialled and the connections over US1, SS2, R, E, F, SS14, R6 and SS5, R1, E1, F1, SS17, BLR, TS, SS24, R13, G5 to Maburn and Market respectively, released. A development of the invention is described as applied to a network of 200 exchanges divided into 20 districts of 10 exchanges. Direct trunks are provided between each exchange and every other exchange in the same district, and a main exchange in each district is provided with trunks to each of the main exchanges in the other 19 districts. These inter-district trunks are accessible from second selectors in the main exchange and terminate in, exchange selectors in the distant main exchanges, separate levels in these exchange selectors being allotted to the different exchanges in the district. It is therefore necessary that the digital equivalent of the third letter of an exchange should not be the same for any two exchanges in the same district. Fig. 15 shows two main exchanges, Ghent and Akard, of the network. The exchanges Mahope and Macey belong to a main exchange Topaz, but owing to the geographical position of Macey direct trunks are provided between it and the main exchange Akard. It is therefore necessary when calling Macey from Akard that trunks to Topaz and Ghent should be taken into use when the first two letters are dialled, since Main and Mahope also begin with these letters. This and other similar contingencies are met by the provision of discriminating switches US6, US7, &c., which operate as already described with reference to the lay-out shown in Fig. 11. District offices of 100 and 1,000 lines are also assigned to the Akard exchange, direct trunks being provided between the 1,000-line district office 6,000, and the 100- line district office 9,700. The arrangement is described in relation to calls from the district office 6,000. Normally when a call is originated by a subscriber A2 the line is extended over a. rotary switch C2, discriminating switch US5, rotary switch D1, and repeater R15, to a group selector E3 at the parent exchange Akard, and thence to its destination. If the call is one local to the district office 6,000, in response to the fourth digit 6 the selector HI at Akard seizes a trunk to the district office 6,000 over a rotary switch D15 and a repeater R17. The discriminating switch US5 having in the meantime recorded the first four digits, frees the trunk to the main exchange and causes a rotary switch D2 to select an idle local selector I1. If, however, the call is for a subscriber in the district office 9,700, in response to the fourth digit 9, the call is extended to a fifth selector I in Akard, and thence in response to the fifth digit 7 over a rotary switch D11 and repeater R20 to the district office 9,700. The discriminating switch US5, which has recorded the first five digits, now disconnect the connection to the main exchange and causes a rotary switch D16 to select a direct trunk to the wanted district office 9,700. To illustrate the operation of the bi-level repeater BLR and traffic separator TS, and of discriminating switches, such as US1, the operation of the switching apparatus when a subscriber in Lincoln calls a subscriber in Market will first be described. Subsequently the operation of a digit absorbing repeater DAR2 will be described with reference to a call from Lincoln to Upland. The discriminating switches vary slightly in arrangement according to the number of routes between which they have to discriminate. Slightly modified forms of discriminating switches will be described in relation to calls from Lincoln to Cedar and Lincoln to South and with reference a district office calls. Call from Lincoln to Market: selection of an idle discriminating switch US1. When a subscriber A in Lincoln removes his receiver to make a call, a line switch C, Fig. 1, operates in known manner to extend connection to an idle first selector D, which in response to the dialling of the first letter M raises its wipers to the fifth level and rotates them in search of an idle trunk, such as the trunk 32 - - 34, extending to a discriminating switch US1. Operat