Patent GB-1402506-A (GB)
Title: Pcm tdm telecommunications systems
IPC: H04Q11/08
Filed: 1973-03-22
Abstract: 1402506 Automatic exchange systems; digital transmission; TDM systems SIEMENS AG 22 March 1973 [23 March 1972] 13810/73 Headings H4K and H4P In a PCM, TDM, exchange system including a plurality of exchanges wherein some pieces of information require the use of a plurality of time slots in the same frame for transmission, there is provided at the receiving end a buffer store having a number of rows equal to the number of time slots used for the piece of information and a number of columns equal to the maximum number of frame displacements that can occur during transmission, the store being utilized so as to reproduce the transmitted data in the correct sequence. To transmit a data signal, e.g. a video telephone or facsimile signal which is sampled four times per 125Á frame and which is coded into 12- bits per sample, six time slots per frame are utilized each capable of handling 8-bits, each set of 8-bits P1-P6 being termed a piece of subsidiary information. They are allocated six free slots for transmission and are transmitted in the order P1-P6 but are received in an order, e.g. P5, P2, P4, P6, P1, P3 per frame, which remains invariable during the connection, but each item may suffer a different number of frame displacements, e.g. P2 and P4 no displacement, P6 one frame displacement, P1, P5 two frames displacement and P3 four frame displacements. As is known two intermediate stores are used at both the transmitting and receiving end, one being written into as the other is being read-out of. In a first embodiment it is assumed that the frame displacements remain constant during the whole transmission. Once the connection has been set up in known manner the channel numbers relating to the 6 channels as initially transmitted are transmitted in the respective time slots for a number of frames. These 6 channel numbers are recorded in a circulating or rotary store U1 at the receiving end according to the time slot in which they were received, e.g. the channel number P5 was received in slot 2 &c., and the store U1 and decoder D1 are used to gate the incoming items of subsidiary information into a row of a store SP corresponding to the channel number of the transmitted information. At the end of each frame the store contents are stepped by one column until at the end of five frames the information relating to the first frame as originally transmitted is stored as shown within the black squares. To enable the information for each frame to be assimilated there is provided a second rotary store U2 and a counter Z1. The counter is stepped once per frame and the count LOL at the time of arrival of the first piece of subsidiary information, i.e. P2, first frame, is read into a corresponding stage of the rotary store which is not circulated during the read-in procedure. This proceeds until the pieces of information transmitted in the first frame have all been received over five frames and store U2 has a count in each of its six stages corresponding to the frame in which the first of each piece of subsidiary information P1-P6 was received. Hence by addressing the buffer store SP from the counter U2 via a decoder D2 in conjunction with a switch Scha1 the six pieces of subsidiary information can be read-out in the correct order for each transmitted frame. To overcome random changes in frame displacement that can occur during a connection, e.g. in asynchronously operated networks, each piece of subsidiary information is repeated in an auxiliary time slot once every six frames in the order P1-P6 and at the receiving end they are read into a seventh row of buffer store SP as items Ph1-Ph6. By use of a comparator V, switches Schv1-Schv6, a1-a6 counter Z2 and counter decoder DZ2 a comparison is performed each frame between the output of row seven which is not controlled by store U2 and the output of one of the other rows as determined by counter Z2. After an initial synchronization to allow for the information P1 being displaced by a different number of frames in its own and the auxiliary time slot, the comparison should prove positive unless a frame jump occurs in one of the channels. For the items P1-P6 transmitted in their own channels this shows up as a negative comparison in the corresponding channel and the store U2 count for that channel is increased by one. If the next comparison for that channel is again negative, the count is decremented by 2, it being assumed that a jump forwards or backwards of only one frame can occur. A frame jump in the auxiliary channel manifests itself as a negative comparison for all six channels over six frames, and a resynchronization process is carried out. A prior art arrangement is mentioned wherein equidistant time slots are used throughout a connection involving several exchanges. A modification is also referred to in which equidistant time slots are used only in the first and last exchanges with any six time slots being used on the other intermediate links.