Patent GB-1456063-A (GB)
Title: Time-division multiplex telecommunication system
IPC: H04J3/00
Filed: 1974-05-20
Abstract: 1456063 Automatic exchange systems; multiplex pulse code systems PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 20 May 1974 [23 May 1973] 22421/74 Headings H4K and H4M In an asynchronous TDM exchange system wherein a Data Multiplex is transmitted using a time slot of a 32 channel speech highway, differences of clock rate between the incoming and outgoing highways of an exchange are taken into account for the Data Multiplex by the insertion/removal of stuffing bits in/from time positions of the Data Multiplex frame i.e. the Data Multiplex frame is increased or decreased in length by the number of bits transmitted in one time position. The techniques employed are similar to those used in Specification 1,336,542. Although the following embodiment uses 8 bit PCM on the highways, the invention is applicable to systems using delta modulation. Exchange system Fig. 1.-Three telephone exchanges 100-102 are coupled to each other and to remote telephone concentrators 104-107 by 32 channel PCM speech highways 108 (a-d) and exchanges 100, 101 are additionally connected to a Data Network exchange 109 by similar highways. Remote data concentrators 116-119 are each allocated a time slot on the speech highway to which they are coupled for the transmission of a Data Multiplex comprising four 12,800 b/s data channels and sixteen 800 b/s data channels. Each data multiplex position carries 10 bits of which only 8 can be transmitted over the speech highways in one time slot. It therefore takes 100 frames of speech highway multiplex to transmit one frame of the Data Multiplex, and the deletion or addition of bits to the Data frame as a result of lack of synchronisation between the incoming and outgoing highways of an exchange cannot be tolerated. Fig. 2 shows one of the exchanges 100 of the system which utilizes one non blocking time stage 208-211 in a supermultiplex as the switching stage between incoming highways 201-201 and outgoing highways 203-204, i.e. in sub intervals of time slots incoming information is read via a hopper 208 into positions of a speech store 209 corresponding to the time slot number and incoming highway number. A connection store 210 then governs the read out of the store to the outgoing highways via buffer 211 according to connection information supplied to it by a central control. However in the following arrangement any known type of TDM switching stage between incoming and outgoing highways can be used. Speech connections are made in known manner. Switching of Data.-Information in the 'data' time slot of incoming highways is read into the speech store 209 as for speech information. However each storage position of the store 209 corresponding to a data time slot is designed to be read three times per exchange frame on to a highway 215 leading to circuitry 214 detailed in Fig. 3 (not shown) for correcting for synchronization errors. As is known, because of the asynchronous nature of the system the same piece of information in a storage location of store 209 may be read twice in a frame or not at all in a frame and hence occasionally (about once every 40 secs.) the same piece of data information may be read four times or only twice in a frame on to highway 215. A device 220 or 221 acts on the repeated information from one data channel in a manner similar to that disclosed in Specification 1,336,542 to produce at its output an unrepeated sequence of data information as received on the incoming highway data time slot. To achieve synchronization of the received data multiplex with the bit rate on the outgoing highways stuffing bits are added or removed from the received data multiplex at time positions 5 or 6 of the data multiplex frame by a device 222 or 223. To this end each remote data concentrator uses one of the 800 b/s data channels for transmission of stuffing bits and one for the transmission of synchronization bits, i.e. those data channels occupying positions 5 and 0 of the data multiplex frame. The device 222 or 223 (described in the specification in detail with reference to Fig. 3, not shown) uses ten position shift registers to reconstitute the ten bits of each data multiplex time position from the sequence of eight bit words received in each speech highway time slot and in dependence on the difference in clock rate between the incoming and outgoing highways either removes the stuffing bits from time positions five (decreasing the data frame by one position) or inserts stuffing bits in time position six (increasing the data frame by one position). The data is then returned to eight bit word form and sent to the input side of the TDM switching network via a highway 217 or 218 for connection through the network to the correct outgoing highway e.g. 202 or 203 in the same manner as for speech words, NB the highways 217 or 218 and the outgoing highways are synchronized so that further addition or subtraction of bits cannot occur. Where the data passes through a number of exchanges the situation can occur that a subsequent exchange needs to add or subtract stuffing bits when both positions 5 and 6 of the data multiplex are already used for such bits or no stuffing bits are present at all. The system is therefor made subject to the constraints that only positions 5 and 6 can be used for stuffing bits and that where addition or subtraction of stuffing bits is not possible, the data stream is transmitted as received.