Patent GB-1413690-A (GB)
Title: Closed-loop telecommunication system
IPC: H04J3/00
Filed: 1972-12-18
Abstract: 1413690 Automatic exchange systems PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 18 Dec 1972 [21 Dec 1971] 58288/72 Heading H4K In a P.C.M. communication system comprising a unidirectional closed loop transmission line interconnecting a plurality of subscriber connection units and a synchronizing unit, subscribers wishing to make a call select any free channel, separate speech and signalling time slots are used for each connection, the signalling time slots for a connection occurring less frequently than the speech time slots for that connection and each connection unit breaks the loop during time slots associated with its subscribers. As described each subscriber connection unit 3 (only one of which is shown) serves a plurality of subscriber units 4 of which only one is shown. The synchronizing unit (Fig. 4, not shown) comprises two sets of shift registers which under control of input and output timing units intermediately store and time redistribute the various slots to compensate for delays &c. in the connection units. The bits on the transmission line are transmitted in phase shift code and each unit includes means to convert to N.R.Z. code and means to convert an N.R.Z. output phase shift code. The means are shown at 7 and 11 respectively in connection unit 3. The transmission sequence of the line comprises 120 speech time slots repeated 1024 times and preceded by a signalling block comprising 224 signalling time slots headed by a synchronizing block and separated into blocks of seven time slots by secondary synchronizing blocks. A signalling channel comprises four signalling blocks each of 8 binary bits. The first 120 signalling channels are each associated with a speech channel. The first two signalling bits indicate the busy free state of the channel, the next 14 bits indicate the wanted subscriber address, the next 8 bits for answering, ringing codes &c and the remaining 8 bits are not used. Each connection unit 3 includes a register 24 to receive a signalling channel code, a unit 25 which generates subscriber identities sequentially and when the address corresponds to a calling subscriber unit tests the first two bits to see if that channel is free and if so allocates it to associated subscriber units 4, synchronizing circuits 20, 21, speech and signalling channel counters 22 and 23, and a gating arrangement 10. The subscriber unit 4 comprises a decoder 27 which decodes subscriber addresses in the signalling channel, a register 30 in which is stored a signalling channel identity associated with an incoming call or seized for an outgoing call, and AND gates 37 and 46 which compare any stored channel identity with the currently scanned speech or signalling channel as given by the corresponding counters 22 and 23 in the connection unit. Each subscriber unit further includes means 43 to encode speech from transmitter 44, means 33 to generate codes related to the state of the substation, e.g. ringing but not answered, answered, &c., means 45 to generate wanted subscriber addresses and the two busy/ free signalling bits, means 36 to recognize incoming busy code, means 39 to decode incoming speech and a bell 32 coupled via a ringer 31 to the decoder 27. All signals involved in a call pass between the substations involved in P.C.M. form. To insert a code on the line a substation blocks the line during the appropriate time slot by removing the enabling signal to AND gate 12 and inserts the code via OR gate 16.