Patent GB-1045521-A (GB)
Title: Improvements in or relating to communication systems
IPC: H04Q11/04
Filed: 1964-01-16
Abstract: 1,045,521. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. Jan. 16, 1964 [Jan. 21, 1963], No. 1916/64. Heading H4K. A communication system comprises a plurality of switching units 10 ... n each connected to a call processing section 22, containing a stored programme, via an input-output section 21 which acts as an intermediate store for information -and commands. The invention is described in relation to a set of PBX's which form the switching units which are all controlled from a central point containing the sections 21, 22. As shown, a PBX comprises a T.D.M. highway over which subscribers may be variously connected locally to other subscribers or tone sources and externally to outgoing trunks or digit trunks terminating at the input-output section 21. The switching unit is also connected to the section 21 by send and receive data trunks. The section 21 operates under control of the circuit 203 which serves to control the feeding of information to or from the digit and data store 204, (a) to and from the data trunks, (b) from the digit receivers 209, and (c) to and from the sender control 211 for controlling out-pulsing in the case of calls between PBX's or to a main exchange. The data control 203 comprises scanning means which in any cycle scans all the data trunks and three of the digit receivers, different groups of three being scanned in successive cycles. The scanning may be interrupted on receipt of commands from the call processing section 22. Information transfer between the switch units and the section 21 is in serial binary form with the usual parity checks. The call processing section contains a stored programme 221; a store 222 comprising a sect on for each PBX unit, containing subsections which hold the state of every call in storage areas allocated to each TDM channel of the PBX; and a line information control circuit containing information such as class of service for each line. The programme control 220 operates in accordance with the call status 221 the line information 223 and information received from the digit and data store 204 to extract the programme from the store 221 appropriate to the occasion, transmits the necessary commands to the digit and data store and updates the call status control 222. Information passes between sections 21 and 22 in parallel form. Set-up of call.-A calling subscriber is detected by scanner 110 which causes the data transmitter to transmit the calling subscriber's identity and off-hook condition, which information is in due course scanned and inserted into store 204. The call processing section 22 scans the call status stores sequentially. Each time the relevant sector is scanned the corresponding section of store 204 is interrogated. If at the end of the sector scan no correspondence in the identity of the lines stored in the sector and in 204 is obtained a new call, is assumed and a programme is extracted from the store which causes allocation of an idle T.D.M. channel, allocation of a digit trunk, and sending of the relevant information via 204 to the outgoing data trunk to cause the network control 120 to connect the calling subscriber to the digit trunk the appropriate information being stored in the call status subsection relevant to the T.D.M. channel. The programme also causes the connecting up of a free digit receiver 209 by issuing the necessary instructions to the connector 209, which instructions are also stored. The digits are received at 209 and when sufficient of them have been received the digit control enables them to be transferred to the digit and data store 204. As the call processing equipment scans the subsector containing the receiver number in question it extracts the called number from the digit and data store 204. The call processing section has now enough information to be able to determine the setting up of the call with the help of the programme store instructions being issued to the switch units via store 204 and the data trunks. The switch units monitor any changes in conditions, signalling the call processing store via section 21 to enable the former to issue instructions via the latter as appropriate. Intra-PBX calls, inter-PBX calls and exchange calls are distinguished by their first digit, which is used by the digit control circuit 210 to decide how many digits are to be transferred to the call processing section 22 and how many must be retained in store 204 for outpulsing by the sender control circuit 211. In the case of an intra-PBX call, all the digits are transferred to section 22 and the store 204 is cleared. In the call processing section the call status control 222, the programme control 220 and programme store 221 are duplicated, arrangements being made for transfer of information from one status control to a stand-by which is normally only provided with skeleton information.