Patent GB-1444585-A (GB)
Title: Time-multiplex telecommunication method and system
IPC: H04Q11/04
Filed: 1973-06-27
Abstract: 1444585 Telecommunication systems; closed loop systems HASLER AG 27 June 1973 [4 July 1972 8 June 1973] 30498/73 Headings H4K H4P and H4L In a closed loop, unidirectional TDM communication system, each substation incorporates a register which is connected in shunt with the loop when the station is in its rest position but which is connected in series with the loop when the station is in its transmitting condition. Successive bytes circulate on the loop without any pause therebetween so that it is necessary for a transmitting station to break the loop in order to inject a byte thereon and meanwhile to buffer the bytes already existing on the loop in the register. Bytes are removed from the loop either by the original transmitting station, by a receiving station or by a monitoring station. The bytes may relate to PCM or delta modulation speech, teleprinter, data, standard clock or fire alarm signals.  Fig. 1 depicts the normal rest position of a substation 2 wherein every byte circulating anticlockwise on the loop 1 is injected into a shunted register ES and at the same time is forwarded to the next station by virtue of switch 6 being in its position a. If the station desires to transmit, it composes a message in store SS, awaits the end of the currently occurring byte loop as detected by decoder 9 and then actuates switch 6 to position b so that the stored byte is passed out on to the loop while the next following byte on the loop is buffered into ES and is not simultaneously forwarded. When store SS is emptied, switch 6 is turned to position c so that the buffered byte may then be forwarded. Subsequently the station receives back the transmitted byte and removes it from the loop by reoperating switch 6 to position a. A message incoming to the station is recognized by the address which invariably accompanies it. If the station is in mode a it acknowledges the message by changing a marking bit thereof during its normal passage through switch 6. If the station is in mode c or b it eliminates the message by switching to mode a so that the next following byte is forwarded directly instead of via the register. Substation, Fig. 2. In order to call another station, the latter's identity is keyed into part 8c of a sending store, a calling indication is written into part 8b and the station's own address is entered into part 8a. When decoder 9b detects the end of the current byte in register 5, it enables control curcuit 7 to close gate 6a thereby buffering subsequent already circulating bytes and to open gate 6b so as to read out the contents of store 8 on to the loop. At the destination station the latter's own address now stored in part 5c of the receiving register 5 is recognized by decoder 9a. The control circuit is mmediately advised to close gate 12 which thereby alters the marking bit in the currently forwarded word. This gate reopens for the succeeding bit. The caller's address in part 5a is inserted into store 22 so that subsequent messages therefrom may be recognized. At the originating station, the appearance of his own address in part 5a of his receiving register permits the control circuit to close gate 6c, which was opened as soon as the sending store 8 had been emptied, in order to delete this calling byte from the loop. Gate 6a is opened to permit normal forwarding of subsequent bytes. At any station which is in the buffering mode, i.e. gates 6b or 6c open, the recognition of an inverted marking bit in part 5d of its receiving register permits the station to delete the byte and to revert to its rest mode, i.e. gate 6a open. The called station replies, if it is idle, by inserting an appropriate code in part 8b of the sending store, the caller's address in part 8c and its own address in part 8a. Subsequent off-hook and final on-hook indications are sent in similar manner. Coded speech is exchanged between the stations in an obvious manner. It should be noted that each station after a transmission must be restored to its rest position, i.e. gate 6a open, before it may transmit a further byte. Monitoring station 3 (Fig. 1) produces synch bytes at periodic intervals for resetting the stations' clocks 4 (Fig. 2). In addition it eliminates bytes that have already circulated by marking a bit in each byte when it first appears and then removing any bytes which carry this mark. It inserts filler bytes into such vacated positions, the substations being capable of recognizing a filler so that if they wish to transmit they do not forward a buffered filler as they would a normal byte, i.e. they open gate 6a instead of 6c immediately after transmitting a byte. Bytes need not be of the same length, in which case an end of byte or an indication of byte length is inserted in each message byte. Receive only stations may be provided. Conference connections are set up in an obvious manner with the proviso that the marking bit must not be altered by any of the conferees. A transmitting station that cannot be restored to rest due to the lack of an own station address, or of an inverted marker bit in the passing bytes may do so in response to the appearance of two successive synch bytes. The removal of one of these from the loop by such a restoring station does not normally cause problems.