Patent GB-1070506-A (GB)
Title: A communication switching system
IPC: H04Q3/545
Filed: 1965-05-12
Abstract: 1,070,506. Automatic exchange systems. AUTOMATIC ELECTRIC LABORATORIES Inc. May 12, 1965 [May 15, 1964; April 23, 1965], No. 20109/65. Heading H4K. A non-blocking reed-relay cross-point switching network provides terminations for local lines and trunks and for registers and senders; a calling line or trunk being initially connected over the network to a register by a common marker on instruction by an identifier, the marker later responding to registered dialling information as processed by a common translator to set up a connection over the network: from a sender to a wanted trunk or line, the marker subsequently being conditioned by a common control circuit instructed from the register to set up a connection over the network from the calling line to the called line and to clear down the connections involving the register and sender. Network.-The network is part of a system designed for military use with dialling on a world-wide basis. Each network of such a system has a size from 100 to 2000 lines provided by a 3- or 5-stage cross-point array built from card assemblies of 50 cross-points each with 5 to 10 terminal switching. Each crosspoint is designed to transmit frequencies up to 50 kcs. and switches 6 wires comprising two pairs of speech wires, a hold wire and a supervisory wire. Unswitched pull wires are employed for making a connection over the network. Each terminal circuit is given an appearance on both sides of the network which is such that if the marker finds no free path in one direction across the network it is sure of a path set up in the other direction. Inter-exchange working:-Transmission may be over land lines or employ HF, VHF, UHF, tropo-scatter or microwave facilities. Address digits required for a connection are a priority digit, two route code digits, a three digit area code, an exchange code of three digits and a four digit line member. The route digits, apart from eliciting alternative route data from the translator, also provides identification for abbreviated dialling service, hot-line facilities in which off-hook condition by itself demands a predetermined connection, and data grade calls. Inter-exchange signalling employs 2/6 tones with control of tandem switching vested in the originating exchange so as to require end-to-end signalling. Seizure of a distant trunk is effected by D.C. signal, the fact of seizure being signalled back in a 2/6 code (termed the KP code) which is returned forward in confirmation. The distant exchange now reverts a 2/6 code (termed ID as it is mainly employed as an interdigital code) to demand the first digit of the wanted address. On receipt of a digit the distant exchange reverts the digit received and receives confirmation by a forwarded ID code. To gain the next digit the distant exchange reverts ID code and so on. Should an all trunks busy condition be encountered a TB code is reverted acknowledged by a forward TB code, and ID code is reverted and the originating exchange explores alternative routes. Tandem exchanges receive all digits except the line number which is forwarded to the terminating exchange when this is reached. When a tandem exchange has switched the connection through it reverts an ST code, acknowledged by a forward ST code and confirmed by an ID code reverted. The ST code is also used for a connection completed. If a wanted line is busy LB code is reverted with the same type of acknowledgement and confirmation. Some connections are established without the use of confirmation signalling. Subatations.-Local subscribers to a network are served by four-wire lines and have pushbutton number senders with four buttons, each indicating a priority of service requested for a call, a button for an abbreviated dialling facility, and an " end-of-send " button. The pushbutton number senders transmit in 2/8 audiofrequency code termed dual tone multifrequency (DTMF). Such subscribers have direct access to the network but the priority demanded is subject to correction if it exceeds the maximum priority authorized and recorded with subscribers' data in a memory section of the common control circuits. PABX subscribers are served by two-wire lines and have pushbutton sets or dial impulsing sets if entitled to dial direct and not confined to assistance from the PABX attendant operator. Hot-line subsets are four-wire sets local to the network and by their nature require no push-buttons, questions of priority, routing, and destination-being a common control function on detection of off-hook condition within a special group of line circuits serving hot-line subscribers. Line and trunk circuits; line identification; special ringing signals.-Four-wire line circuits, both-way trunk circuits, register-sender junctors, and PBX trunk circuits are supervised over a common data highway to the markercommon control complex. A scanner-identifier has the function of searching for a calling condition on a one-at-a-time basis as directed by the marker. The line group is identified by scans over thousands and hundreds markings while the member of the group is identified by scanning tens and units markings. As each digit of the identity is found, from thousands downwards, all other calling conditions are blocked by a masking potential. Simultaneous service demands are thereby separated by choice of the first identity found. The scanner is employed reciprocally by being set up on command from common control with the identity of a wanted circuit so as to gain access to that circuit over the common data highway for the loading of data or the conditioning of the circuit. The line circuits employ E and M signalling according to customary American practice. PBX trunk circuits may be of a sort to provide access to PBX attendants alone or of a sort giving access to both attendant and subscribers. Since a PBX call may use impulse dialling or tone sending the trunk circuit must call for both until it can discriminate. For calls going beyond the local network the PBX trunk switches in echo suppressors. An idle line circuit transmits tones in both directions, the tones being removed to indicate a calling condition. Answer and disconnect is delayed in the line and trunk circuits to avoid response to spurious signals. As well as connecting ordinary ringing signals a line circuit can connect a special ringing signal to call up a subscriber who has been disconnected pre-emptively, or disconnects himself, after a warning pre-empt release tone. Inter-exchange trunk circuits serving transmission channels governed by a pilot signal are kept busy automatically should the pilot signal fail as otherwise the consequent release of the circuits together would result in an unacceptable surge in demands upon the control system. For the same reason a time delay guards against release by fading of the pilot signal. An interexchange trunk seizes the distant exchange before the marker completes its connecting cycle, such early seizure reducing the time during which the distant exchange can seize the trunk for an incoming call. If such simultaneous seizure occurs nevertheless, the trunk is released for seizure by the incoming call and a new trunk is chosen by the common control to set up the outgoing, call. A group of trunk circuits is provided to give access to operators' positions, these.are termed dial assistance or DAS trunks and are connected to link circuits, serving the operators, over a DAS matrix of cross point reed relays, such connections being governed by an independent DAS marker. Each DAS trunk is given a time slot in which it can be set into any of 9 possible states which allow for out-in by the operator, line splitting forwards and backwards, release of the front or rear connections of the trunk, three states indicating classes of service attaching to a call, and a state permitting release of the operator's link circuit. The register sender junctors comprise two units separately terminated on the network, one unit receiving and the other sending. The units are grouped separately but both have the same trunk number. Register-sender group and receiver pool.-The register-sender junctors provide an interface between the space divided network and the time division multiplex of the common control system. A pool of DTMF receivers for 2/8 codes, and a pool of transceivers for 2/6 codes are available over a cross-point matrix for association with any register sender as assigned by the common control. Priority and class-of-service.-Priority, demanded for a call by an initial digit, enables a call of higher priority to seize inter-exchange trunks, operator trunks, and to break in on a connection of lower priority. The orders of prioroty are routine, priority, immediate, flash and flash-override. A trunk will be pre-empted only if no other trunk is free and no alternate route exists; a first search is made for an idle trunk and a second search is made for a busy trunk of lower priority if the first search is not successful. The maximum priority available to a line is recorded in the memory section of the common control. All dial pulse subscribers, including all PBX subscribers, have no more than routine priority and in conditions of overload of the exchange the PBX subscribers are taken out of service by prohibiting call origination and pre-emption of existing off-hook demands. Overload working may be introduced from a manual position or by automatic control. Hot-line connections are of two kinds. The permanent connection for a hot-line is always set up in readiness for the end stations. Should the connection be interrupted it is immediately re-established in accordance with data stored in the common control memory section. The demand connection for a hot-line is set up in response to the off-hook condition of one of the parties in accordance with common control data, hot lines being grouped together so that the scanner can