Patent GB-1520306-A (GB)
Title: Digital tdm telecommunications systems
IPC: H04J3/12
Filed: 1975-07-14
Abstract: 1520306 Loop TDM systems SIEMENS AG 14 July 1975 [8 Aug 1974] 29397/75 Heading H4K In a t.d.m. system in which subscriber stations are connected in sequence in a loop commencing and terminating at an exchange, the exchange transmits a data frame wherein items of telecommunications data for subscriber stations participating in connections are arranged in the same sequence as the stations are arranged around the loop. The loop may be a glass fibre tube on which laser beams are used as data carriers. The subscriber stations may be telephone, video telephone or data stations. At the beginning of each data frame the exchange EVSt sends an enabling pulse group which prepares the decoders D of all the operative stations of a particular type, e.g. telephone stations 1En-1-1En + I etc. for the receipt of a head code. The exchange then a transmits in each of a plurality of time slots a head code, a number of unmodulated bits, and then number of information carrying bits equal in number to the number of unmodulated bits. The head code comprises four bits, the first three bits of which pass on through a repeater by-pass V of the first operative station, the passage of the third bit causing a decoder D to apply a temporary blockage to the repeater V. The occurrence of the fourth bit, which is blocked from onward transmission via repeater V, causes decoder D to operate bi-stable device FF to apply a continuous blockage to repeater V and to operate switches Sch, U. The following unmodulated bits are supplied to a delay element VZ and to a modulator Mod where they are modulated with information to be transmitted and then fed back on to the loop ZM for onward transmission round the loop following the first three bits of the head code. In the following stations the absence of the fourth bit of the head code causes the decoders D to remove the temporary blockage caused by the first three bits and hence the modulated bits sent by the first station pass round the loop to the exchange. In the first operative station the transmission of the last modulated bit coincides with the arrival at the output of delay element VZ of the first delayed unmodulated bit. Decoder D is then caused to operate switch U and the information carrying bits from the exchange are supplied to a demodulator Dem together with the delayed unmodulated bits which are used for decoding purposes. The occurrence of the last delayed unmodulated bit at the delay output causes decoder D to remove the blockage from the repeater V and causes the decoder to be disabled so that it does not react to the occurrence of further head codes during the frame. Hence the information in subsequent time slots by-passes the first active station to cause a similar operation to take place in the-second and subsequent operative stations. The exchange EVSt (Fig. 3, not shown) provides the necessary time slot interchange using a time switching stage to enable two stations to communicate. A special "call set up" code is periodically transmitted to permit calling stations to set up communication with the exchange. Fig. 4 (not shown) illustrates an arrangement wherein information for onward transmission is switched to a return loop, an outward highway from the exchange only extending as far as the final station.