Patent GB-1302597-A (GB)
IPC: H03M3/02
Filed: 1970-03-13
Abstract: 1302597 Automatic exchange systems TELEFONAKTIEBOLAGET L M ERICSSON 13 March 1970 [18 March 1969] 12286/70 Heading H4K In a time division system each connection is allocated a unique digital call number which is transmitted with each data item i.e. speech sample, so as to ensure that only that subscriber (or those subscribers in a conference call) who is entitled to receive the item actually does so. Although PAM or PCM may be used, the preferred modulation is asynchronous, ternary delta modulation. With this arrangement, the call number alone or the call number plus a signal pulse are transmitted in dependence on whether the amplitude of a sample is greater in a decreasing or increasing sense respectively than the quantization strip, from the amplitude of the preceding sample. If the change is less than the quantization step no signal is sent. Each station thus transmits as and when it detects a sufficient change in amplitude and since it is expected that such changes occur in a random manner it is assumed that simultaneous transmission of signals from a plurality of stations will not normally occur. Separate Go and Return highways are provided. In a same group these highways are interconnected (c.f. "relay" R in Fig. 2b) in alternate time slots so as to allow signal transfer. In order to connect subscribers in different groups (Fig. 5, not shown) signals are stored in one group and then transferred to the other group during a defined time slot, means being provided in this case for translating the different call numbers allotted to the two conversing subscribers. The other alternate time slots are used for control purposes i.e. line scanning, digit reception, ringing called party, insertion of call number in each party's substation and clearing. Setting up a call.-When a subscriber Sb (Fig. 2b) goes off-hook, his line relay LRb is effective to mark lead cb. Each subscriber's address is produced in turn by an address generator LC in a control circuit Cu (Fig. 3) so that eventually with the appearance of this address on wires r2-r9 of the receive highway during one of the alternate control time slots, a comparator E1b in the substation is enabled by coincidence with the station's address stored in LNb so as to open gate A3b. Since wire r10 of the receive highway would also be marked to indicate that a normal scan is taking place, gate A8b is opened to indicate over wire t1 of the transmit highway that the station is calling. In the control circuit this is effective to stop the normal scan. A call number circuit CC is stepped-on one step and after zeroizing LC prior to stepping it through every position, each station is interrogated to see if its call number store CNb contains the number in CC. If coincidence occurs, as detected by the gates E2 of the control unit, CC is stepped-on and LC is again zeroized. The procedure repeats until a unique call number is found. This is transferred via wires r1-r9 of the receive highway during an alternate control time slot to the store CNb of the calling subscriber whose identity has been retained in store LNA in the control unit. A free digit receiver KM is allotted and the caller dials in over the transmit highway, each "digit" being accompanied by the call number. (The call number is ineffective in the calling station in so far as demodulation of any signals outgoing therefrom is concerned.) The identity of the called party is recorded in store LNB. The latter's output is used to interrogate the called party so that if he is free a ringing code from register RR in the control unit can be inserted in his call number register. The ringing code, as subsequently supplied during each alternate information time slot from a ringing code generator, is demodulated so as to provide an audible output via a loudspeaker. Ringing tone may be reverted by the digit receiver to the caller. When the called party replies, a pulse is transmitted over the t1 wire together with the ringing code on wires t2-t9 so as to alert the control unit. The latter now transmits the call number to the wanted party so as to replace the ringing code number whereupon a two way connection between the two parties is set up. The control unit clears its stores and continues interrogating the subscribers.