GB1363357A - Telephone system - Google Patents

Abstract

1363357 Multi-station systems STANDARD TELEPHONES & CABLES Ltd 10 Feb 1972 6234/72 Heading H4K In a "ring main" p.c.m. system a plurality of substations are connected into a closed loop unidirectional transmission line which line starts and terminates at a control station. The system described differs from that described in Specification 1,187,488 in the provision of a separate signalling channel so that substations only need to monitor this channel as the control station transfers any signals from a speech channel on to the signalling channel after retiming and regeneration. The control station also provides all synchronizing and operating tones, the latter being sent on speech channels as a predetermined digital code with one digit modulated. To make a call the calling substation seizes a speech channel which is marked as free by the control station and receives dialling tone on this channel. The number of the called substation is sent and this is put, together with the identity of the speech channel, on to the signalling channel. The called substation recognizes its identity code and locks on to the respective speech channel. Ringing tone is sent around the loop on this speech channel until the called party answers when speech between the parties can occur. When the called substation is engaged it does not send the special code (inviting ringing) back to the control station and an engaged tone is sent to the calling substation. The calling data is continuously sent on the signalling channel since the transmitting store in the control station is reset only on receipt of the special code. Thus the system automatically provides a camp-on-busy facility. Each substation may be provided with a plug-in identity card so that the subscriber can be reached on any substation where his card is inserted.