Patent GB-1410508-A (GB)
Title: Telephone systems
IPC: H04Q11/04
Filed: 1972-10-12
Abstract: 1410508 Multi-station systems THORNERICSSON TELECOMMUNICATIONS (MFG) Ltd 10 Oct 1973 [12 Oct 1972] 47160/72 Heading H4K In an intercommunication system wherein a plurality of telephone stations have access to an equal or smaller number of speech links, the signalling concerned with setting up connections between stations takes place on a signalling highway operated in T.D.M. and to which each station is connected. In a prepared arrangement the speech links are each constituted by a wire pair, and each station 1-N is connectible to any one of the wire pairs using a respective relay crosspoint array SCP(1-N) situated at the corresponding station. Signalling takes place using a 17 time channel data highway DAH of which one channel is used for station synchronization and each of the remaining 16 channels is associated with a particular speech wire pair. Each channel comprises a 12-bit word; bits 1-5 are used to indicate respectively type of speech link, i.e. local or exchange, acknowledge/ring, busy and hold whereas bits 6-8 and 9-12 are respectively used for signalling in binary the first and second digits of a telephone station number. Of the two types of speech link, local links are used only for calls between stations and are indicated by a binary 0 in bit 1 of their corresponding data channel whereas exchange links are used only for calls to the public network and are represented by a permanent binary 1 in bit 1 of their time channel. Spare time channels having no corresponding speech link are marked busy by a binary 1 in bit 4. Each subscriber station, Fig. 3, is identical apart from a plug-in printed card which causes a unique station identity to be stored in store ONS and may utilize integrated circuitry. A phase locked oscillator PLO driven over wire SL3 by a master oscillator OSC at a central control CN drives a clock pulse generator CPG . for providing clockpulses CK1 and CK2 respectively in the centre of and at the beginning of each sub line slot and further drives a 12 stage counting chain CN1 at the bit rate, the final stage of chain CN1 being used to drive a 17 stage counting chain CN2 at the time channel rate. A further clock pulse train CK3 is produced at the beginning of each 12th sub time slot by chain CN1. At the required rate and position of occurrence of the 17th time channel a pattern generator SPG at central control CN injects a fixed binary code on to the data highway which causes a detector SPD to reset counting chains CN1, CN2. Internal call.-The subscriber keys an initial digit 8 and logic element LEM, which reads bits 1-5 from a receive register SR1 during the 12th sub time slot of each channel, examines the bits read for a free local link indication, i.e. all bits zero, busy signal being applied to the hand set TTE if after a complete frame no free local link is detected. On detection of a free local link, the corresponding channel identity is read from counting chain CN2 into a store CNS1. During sub slot 1 of the next occurrence of this channel, comparator Comp. 2 detects a coincidence between the contents of store CNS1 and counting chain CN2 and supplies a "yes" indicator to logic element LEM to cause the stations own identity to be fed from store ONS into bit positions 6-12 of send register SRL and to cause a busy bit to be inserted into position 4. The register contents are then fed out during sub slots 2-12, the calling station's identity being received by a system monitor SYM at central control where it may be used for metering purposes. Assuming the two digits of the required station have been keyed, these are fed out in sub slots 6-12 of the seized channel on its next occurrence and if necessary for the four following occurrences of this channel. If after four repetitions an acknowledge bit has not appeared in sub slot 3 of the channel, the called station is assumed busy. The seized channel is released by removing the "busy" bit and store CNS1 is cleared. Called station free.-A comparator comp. 1 monitors bits 6-12 of data received and compares them with the station's own identity. On registering an agreement a pulse is supplied to logic element LEM which, if the station is free, causes an acknowledge bit to be inserted in sub slot 3. The called station's tone sounder is operated and the channel identity is stored in store CNS. At the calling station receipt of the bit in sub slot 3 causes ringing tone to be supplied. When the called station goes off-hook an answer bit is inserted in sub slot 2. In both stations the stored channel code is fed to the crosspoint control to connect the stations to the corresponding speech wire pair and during the call the bits in sub slots 2-4 are maintained. Hence when either station goes on-hook one or more of the bits are moved enabling the logic elements at both stations to release the connection. Conferences.-The calling station can add any number of subscribers to an existing connection in turn simply by in turn keying 8 followed by their station number. However as bits are already present in sub slots 2 and 3 of the channel a verbal reply must be awaited. Any party except the calling station may withdraw from the conference as all called stations supply bits for sub slots 2 and 3 but only the calling station supplies the busy bit for sub slot 4. Outgoing external call.-The subscriber keys an initial digit 9; a free exchange link is searched for by logic element LEM, i.e. bit combination 10,000 and if a free exchange link is found the calling station's identity and the busy bit are transmitted as for a local call. The channel identity is immediately fed to operate the station's crosspoints to complete a connection to the exchange link. The appropriate exchange line circuit detects the busy bit in sub slot 4 of the seized channel and acts to connect the exchange line to the internal exchange link. The subscriber receives public exchange dial tone over the speech link and proceeds to key the required external number, the digits of which are transfered one at a time using sub slots 9-12 of the seized channel to the exchange link circuit where they are converted to a form suitable for the public network. The call proceeds under the control of the public exchange until the caller hangs up thereby releasing the seized channel and exchange link. Incoming external call.-A calling exchange line causes its exchange line circuit to insert a bit in sub slot 3 of the respective channel enabling a control bell BR to be rung which alerts all the subscribers. To accept the call a subscriber keys "0" to cause the station logic element to look for the bit combination 10100 and on detecting the combination the station seizes the channel by inserting the "busy" bit and is connected to the corresponding exchange link. Detection of the busy bit at the exchange line circuit causes the removal of the bit in sub slot 3 so terminating the ringing and also causes the exchange link and exchange line to be interconnected. Interim internal and external calls; call transfer.-To make an interim internal call whilst engaged on a first external call, incoming or outgoing, a subscriber presses a non-latching hold button HB which causes the logic element to remove the "busy" bit and insert a "hold" bit in sub slot 5, the detection of this hold bit causing the exchange line circuit to hold the external line. A special dial tone is provided and the subscriber may now make an internal call using the exchange lines channel and the exchange link, which call is set up as for a normal internal call. As the interim internal call uses the exchange link, call transfer is effected by the calling internal subscriber going onhook which causes removal of the hold bit. The called internal station responds by inserting a busy bit in sub slot 4 and the exchange line circuit removes the line split. If however a call transfer is not to be effected the called station goes on-hook and the calling station subscriber depresses the hold button to cause removal of the hold bit and reinsertion of the busy bit, thereby causing reconnection of the held exchange line. For an interim external call the first external line is held as above but the dialling of the prefix 9 causes a second external link channel to be seized and the identity of the seized channel is fed into store CNS2, this causing the crosspoints to discriminate the first exchange link and connect the second exchange link to the station. The second external call then proceeds to be set up using a second exchange link as for a normal external call and whilst engaged on this second external call an interim internal call can be initiated as above. When the subscriber terminates the second external call he goes onhook. As a further external line is still held the station's tone ringer is sounded and the subscriber returns to the original external call by going off-hook, and depressing the hold button. Modifications.-By increasing the number of sub slots per channel it is possible to provide a called station engaged bit and for acknowledge signals when setting up a conference. Further all tones, e.g. dialling busy, &c. may be produced at a central station for feeding to the stations.