Patent GB-1470442-A (GB)
Title: Switching systems
IPC: H04Q11/04
Filed: 1974-06-11
Abstract: 1470442 Automatic exchange systems WESTERN ELECTRIC CO Inc 11 June 1974 [14 June 1973] 25806/74 Heading H4K In a multistage switching network utilizing an in-slot signalling method, 4-wire paths therethrough are established on a step-by-step basis and within each stage one path is selected and then maintained exclusively for a same connection based on the in-slot signalling data. Preferably signals in the two directions occupy physically separate paths and preferably also occur in different time slots. Delta modulation is used for encoding speech and dialling signals. A system clock provides 5 successive phases # 1 -# 5 , each of which is devoted to a specific activity, e.g. loop monitoring, forward speech, digit transmission, switch setting. The phases are used for different activities at different stages of the cell. A connection between a caller TS266 and a called party TS299 is set up via the caller's line circuit 2, a line finder 3, a group selector 4,'a final selector 5 and the called line circuit LC299. In response to TS266 going off hook, a loop detector 207 in his line circuit enables a common start unit 232 which causes a chain of line finders to start hunting for the calling line. Eventually a finder such as the one shown at 3 receives a start signal over lead RQ thus triggering a counter 302. This steps a so-called multiplexor/ demultiplexor pair 300/301 over the lines L266 L299 having an appearance on this finder. During a phase # 3 , the calling line L266 will be found whereupon the counter is stopped, this line finder is marked busy and a register 350 is apprised of the situation. Dialling tone in the form of a delta modulation code is reverted during phase # 4  from the register to a decoder 203 in the calling line circuit and thence via hybrid 201 to the caller. Three dialled digits are transmitted, over the speech path (204, L266,304) and recorded in the register during successive phases # 2 . (The link LK which will be subsequently used for speech transmission between the finder and group selector is not utilized at this stage of call setting-up). The register then marks lead 352 so as to prepare a local memory 402 in the group selector and transfers the first dialled digit thereto over LK using a 4 bit code during 4 successive appearances of phase # 2 . The memory selects one of the group of outlets 407- 1 to 407-10 in accordance with the stored digit and then enables a counter 404 to hunt for a multiplexor/demultiplexor pair 408/409 which extend to an idle outlet in the selected group. The register 350 then forwards the next digit to a memory 500 in the final selector so as to set this to the relevant tens group and finally transmits the last digit to memory 501 so as to set the selector to the required line circuit. With the connection set-up, speech is transmitted through the switches (integrated circuit AND gates) from calling party to called party during phase # 1  and in the reverse direction during phase # 4 . Connections to other exchanges may be set-up, the dialled digits being forwarded through the path during phase # 2 . Release of the connection by either party is recognized during phase 95 and a release signal is then propagated through the stages during phase # 3 .