Patent GB-1462649-A (GB)
Title: Step-by-step telephone central exchanges for use with communi cation networks
IPC: H04M7/12
Filed: 1974-02-14
Abstract: 1462649 Automatic exchange systems GTE AUTOMATIC ELECTRIC LABORATORIES Inc 14 Feb 1974 [30 April 1973] 6840/74 Heading H4K  An electromechanical step-by-step exchange is modified to form a satellite exchange for a stored program controlled PCM switching system. This is achieved by removing all the electromechanical switches except for the line finder and final selector stages and by connecting these stages via modem's to PCM highways extending to a stored program controlled PCM exchange. CLI equipment is also incorporated. The finders and final selectors are of the two motion stronger type. Trunking.-Each group of 100 lines is connected via 12 line finder switches 25 to 12 PCM originating channel units 31 and thence via an interface 35 to a 24 channel PCM highway 37. Highway 37 is likewise connected via an interface to 12 terminating channel units and thence via 12 final selectors 27 to the group of 100 subscribers. The number of originating and terminating channel units may be varied in accordance with traffic flow requirements, and each such unit is allocated a particular time slot on the PCM highway 37. Operation and CLI.-When a subscriber goes off-hook a start signal is applied to a B relay to cause operation of a pre-selected line finder which steps vertically and then horizontally to find the marked terminal. During the vertical stepping a neon tube L1 breaks down on each step to operate a tens counter 65 and a units counter 67 is likewise operated by the breakdown of neon tube L2 during the rotary stepping. Hence the counters 65, 67 give the identity of the calling subscriber within the particular 100's group Concurrently, the starting of the line finder causes the seizure of the associated originating channel unit 31 and this unit sends out an offhook signal to the PCM switching centre via a channel of the PCM highway 37. In the PCM switching centre, a dial pulse receiver and a tone receiver are assigned to the calling channel and dial tone is remitted to the calling party. The subscriber then signals the number of the required party using either loop disconnect or multifrequency tone signalling, the signals being converted to digital form in the interface 35. Assuming a call to a subscriber at the same or a similar step-by-step exchange, the PCM switching centre causes an idle terminating channel unit associated with the required hundred's group to be seized. The CLI information is then demanded by the centre. The digits stored in counters 65, 67 are then read out to decode and dual tone selective circuitry 61 for signalling to the PCM switching centre via the interface 35 over the speech path of the calling subscriber as a digital representation of two consecutive dual frequency signals spaced by about 50 ms. On receipt of the CLI, the switching centre causes the calling subscriber to be provided with talking battery from the originating channel unit and further signals the final two digits of the called subscriber's number to the terminating channel unit. The associated final selector is then stepped to the required position and ringing current is applied. The necessary Analogue to Digital conversion equipment in the interface 35 is not described but such equipment may be a commercially available unit.