Patent GB-959459-A (GB)
Title: Improvements in or relating to telephone exchanges
Inventor: CURTIS GERALD
IPC: H04Q3/62
Filed: 1961-01-26
Abstract: 959,459. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Jan. 26, 1961, No. 3070/61. Heading H4K. In a system in which different classes of subscribers are differently treated as regards the types of call they are permitted to make, discriminators separate from the conversational switch train are associated with the connection before any digits are dialled and the class of service of the calling subscriber is signalled thereto; the discriminator then responds to one or more dialled digits and allows or bars the call in accordance with the class of service signal received. In certain cases an auxiliary discriminator may also be brought into use by the normal discriminator. The invention is applied to a PABX whose subscribers are divided into four classes as follows : Class A and Class B subscribers can make " O " level assistance calls to a manual board, " 9 " level calls to a public exchange, internal calls commencing with digits 2 ... 7, calls to similar PABX's in a group of such PABX by dialling any of the codes 81... 86, assistance calls to a manual board for the purpose of completing trunk calls by dialling 87, and calls to an enquiry desk by dialling 80. Class A subscribers, but not Class B subscribers can signal to operators that they have priority on O, 87 and 80 calls. Class C subscribers have Class B facilities except for " 9 " level calls. Class D subscribers can make calls commencing with 1.. 7 or 82 or 86 only. All classes are barred 89 calls (unequipped) and only night watchmen can make 88 calls for the purpose of signalling to a watchman's clock facility. On night service all calls 0. 9, 87, 80 are barred except 87 to special night service subscribers, the dialling of 87 by these subscribers then giving access to a public exchange line. In the case of calls to a public exchange further discrimination is necessary on the digits transmitted thereto to bar access to certain public exchange facilities. In the case where O is dialled to the public exchange the discriminators mentioned above are operative. Where the following codes are dialled to the public exchange, viz. 100, INF (463), ENG (364), TEL (835), 99, 557, 559, CON (206), INT (468) an auxiliary discriminator is brought in to bar these calls on day service. On night service however appropriate subscribers can dial all these codes except 99. Operation of conversational switches, Figs. 3, 4, 5. A calling subscriber's loop operates line relay LR to mark the line in the bank PF4 of the first line finder 1LF1 and also brings up (over a chain circuit, not shown) the start relay ST which causes the line finder to hunt FT operating over PF4 to stop the line finder whereupon 1K comes up. ST also marks the first line finder in the bank SF3 of the second line finders such as 2LF1. Relay ST also brings up relay 2ST in a control circuit, Fig. 8, which has preselected an idle 2nd line finder/first group selector link over an allotter switch AL and earths lead STC to bring up 1ST in the link. This energizes the second line finder magnet which hunts for the first line finder, relay 1FT coming up when the first finder is found to stop the switch and bring up 2K. This operates SW in the first line finder, which in turn operates the cut-off relay K. This brings up SA in the allotter, Fig. 8. This releases ST and FT. Relay KR was operated by the allotter when it preselected the link and the line loop is now connected via impulse relay 1A in the allotter to impulse relay A in the link via leads ŒIN, ŒOUT. Relay A thus operates to the line loop and B, HA follow. Relay ST releases 2ST in the allotter and 1ST follows in the link. The impulses of the first digit step the vertical magnet VM of the group selector and at the end of the train HA releases to cause the rotary magnet RMC to step until a free outlet marked by battery on the P lead operates 1FT to stop the switch. HA comes up to switch through and holds to earth reverted over the P-wire in the usual manner and A and B release. If all outlets are busy then at the eleventh step battery is applied over GS4 to operate 1FT followed by HA. Busy tone is passed by the eleventh step springs to the centre winding of A which is held over GS1, GS2 and the eleventh step contacts to the line loop. The discriminator meanwhile has operated as described below and if discrimination has been possible on the first digit and the call is permissible earth is removed from lead KR to release relay KR to complete the talking path and the call proceeds normally. If at this stage it is determined that the call is not permissible the discriminator applies earth to lead BD and D operates and this de-energizes FT which causes the selector to resume rotation to the eleventh step, where operations proceed as above described except that d 2  replaces busy tone with N.U. tone. If discrimination takes place on a subsequent digit and indicates that the call is barred then D is operated as before and this releases HA to allow A to come up to the line loop and B follows. Stepping to the eleventh position then occurs as before. Release is initiated by removal of earth from the succeeding selectors to release HA, 2K following to operate the release magnet ZM and also to release SW in the first line finder which is thereupon released. The discriminator may also time out the call, in the event of no dialling after 30-60 sec., by earthing lead K which shunts down relay 2K. This releases A, B, the former stepping VM up once but HA is released by B and this restores the switch by operating the release magnet. If the call is a priority one then battery is signalled over lead PF and thence over GS3 to light the operator's priority lamp. Operation of facility discriminator control circuit, Figs. 6, 7, 8. This circuit controls the allotting of the 2nd line finder/1st group selector link the seizure of the discriminator and the repeating of impulses thereto. Whilst there is a free link relay B2 is operated over lead 12. This energizes the latch-magnet LM of the allotter switch AL which hunts for a free link indicated by battery on lead N. When this is found 2FT operates to stop the switch and brings up 3K which earths lead KR to bring up relay KR in the thus preselected link. Relay 3K also connects lead STB to relay 2ST which thus operates when STB is earthed by the line finder start relay ST. This initiates the operation of the second line finder as described above and when this is complete earth over lead K brings up SA which connects impulse relay 1A in series with A in the link. SA also extends earth over lead D1 via the link and the line finders to the subscriber's line circuits for discrimination purposes as described below. Relay 1A operating to the line loop operates AA, 1B and BC, BB follow. Relay 1B extends earth to the discriminator FD which then returns earth over lead BA1 to bring up BA, which releases 2FT and 3K. Relay BA extends the stout lead STB to the next facility discriminator control circuit for the group. These are taken into use in turn and when all the BA relays have become operated, the normally operated relay GB releases to open locking circuits for the BA relays and the cycle is repeated. Dialled impulses are repeated by 1A, AA to the facility discriminator FD over lead PU. The facility discriminator earths leads PFA if a priority call is indicated, RL if normal permission for the call is given and BD if the call is barred. Relay RL operates in all cases to release the discriminator control circuit, resulting in removal of earth from lead KR to the link, but meantime in the case of earth or leads BD or PFA, relay BD or relays PF, PFR are operated, to put earth on lead BD or battery on lead PF to the link with results . as already described. If the call is abandoned before discrimination takes place 1A, 1B release to release the rest of the relays in the control circuit. If in the group selector returns all outlets are busy the eleventh step springs return battery over PF to operate PFR and RL follows to release the circuit. Operation of discriminator circuit, Figs. 9, 10, 11. The terminals A, AW ... D, DW are common to the group of line circuits served by a set of discriminators one of which is shown in Figs. 9, 10, 11. Strappings from terminal D, in the line circuit to one of these terminals indicates the class of service, terminals A . . . D being normally used, but terminals AW ... DW being used if the line is also to be used by the night-watchman. When the discriminator is taken into use, relays 1BB, 1BA are operated from earth over lead BBA and BR is operated over lead 1B. The normally operated relay 1BC is thereby released. Earth extended over wire D, as previously described, operates one of the relays ZA. . . ZD indicative of the subscriber's class of service. Release of 1BC applies earth to BA1 and this results in removal of earth from lead BBA by the discriminator control circuit and 1BB, 1BA release. The latter extends leads A, AW . . . D, DW to the succeeding discriminator in a chain circuit. Impulses received over PU set switch AS, CD operating for the duration, the train after which EA operates to transfer the stepping circuit to the magnet of switch BS. Operation of discriminator circuit.-For calls within the PBX the first digit is any of 1-7 and for any of these digits earth is applied over one ASC to lead RL leading to the control circuit FDC to permit establishment of the connection. In digit O earth over AS3 is applied to PFA if 2A is operated, RL if 2B or 2C is operated and BD if 2D is operated. If the night service relay NS is operated however this earth is applied to BD for in all classes. In digit 9 earth is applied to BD for classes C and D. For night service earth is applied to BD for classes A and B as well. For classes A and B the digit 9 followed by public exchange codes indicating the various services listed above are also barred. In this case AS3 on re