Patent GB-1429508-A (GB)
Title: Time division multiplex communication systems
IPC: H04L5/22
Filed: 1973-07-04
Abstract: 1429508 Looped communication systems WESTERN ELECTRIC CO Inc 4 July 1973 [7 July 1972] 31786/73 Heading H4K In a TDM system utilizing a unidirectional closed loop highway, each substation is allocated a particular time slot which is always used for data destined for him and all data on the loop is stored and regenerated by each station. The system is rapidly expandable by simply providing extra time slots for additional subscribers without the need for changing any existing subscribers equipment. The data may be speech, telegraph, TV, data or signalling information. The data format is preferably singlebit companded delta modulation but PCM may be used provided each substation has storage capability for a full PCM word. The storage and possibly the supervisory logic for each substation may be located at a central position in which case the storage may consist of one stage of a recirculating shift register. LSI constructional techniques are used. The general picture is conveyed by Fig. 1 wherein a master terminal 107 injects a negative level framing bit on to the loop highway 100 and this bit, after a delay of one time slot in each telephone terminal 101 &c. and exchange line interface terminal 104 &c., returns to the master terminal so that it may be regenerated. Data is transmitted as zero or positive level bits. Substation Fig. 3 comprises a tristable repeater 301 which regenerates negative, zero or positive level bits circulating on the highway 100a, 100b after a storage period lasting just one time slot. At the station's own time slot, the repeater is disabled and any incoming bit is decoded 305. Any outgoing bit, as obtained from coder 304, is injected on to the highway during the time slot of the other party to the connection. A quartz crystal clock is provided for timing purposes. Setting up a call (Fig. 7, not shown).-When a calling subscriber goes off-hook, his supervisory logic immediately inserts a busy code of alternate "1"s and "0"s into his own time slot so as to dissuade other stations from calling home. He actuates his keyout in accordance with the wanted party so that a string of 65 bits is sent during successive frames in the time slot of the wanted party. This string alerts the latter's supervisory logic to the subsequent transmission of the calling party's time slot identity which is conveyed as a string of pulses equal in number to the caller's position along the loop relative to the master terminal. The caller's logic meanwhile examines the wanted party's time slot to determine his free/busy state. If the party is free, his logic activates a tone caller whose output is also fed back as a ringing tone to the caller. Outgoing call is set up as just described to one of the exchange line terminals 104-106 which converts subsequently "dialled" digits into MF format for transmission to a not shown exchange. Accidental operation of the key set during a call in progress is disregarded by a station logic (Fig. 9, not shown). Incoming call to an exchange terminal is forwarded to the appropriate substation in an obvious manner. If the caller is unaware of the particular extension he requires the terminal circuit sets up a connection to an operator terminal (Fig. 12, not shown). Call transfer (Fig. 15, not shown).-The operator on learning the requirements of a calling party depresses a transfer button. This has the effect of causing the exchange line terminal to stand by and the operator's terminal to adopt a dialling mode in which she assumes the time slot identity of the exchange line terminal. Calling of the wanted terminal proceeds as usual. The terminal replies using the exchange line terminal's time slot but the exchange lineterminal can not intercept the reply since it is in its standby mode. The operator depresses the transfer button again so as to restore the exchange line terminal and permit it to receive the wanted party's time slot identity from a store in the operator's terminal. Release of the transfer button permits the operator to withdraw and the communication via the exchange line terminal to proceed. A release button is provided in the operator terminal for relinquishing the call transfer operation at any time prior to its actual completion.