Patent GB-1247586-A (GB)
Title: Time division communications system
IPC: H04L13/06
Filed: 1969-03-12
Abstract: 1,247,586. Multiplex pulse signalling. INTERNATIONAL BUSINESS MACHINES CORP. 12 March, 1969 [14 March, 1968], No. 12908/69. Heading H4L. Means are provided for combining a plurality of low speed asynchronous data channels in a high speed time division multiplex system by the use of intermediate buffer stores. General arrangement, Fig. 1. Sixteen low speed channels at different bit rates, each with its own clock signal, are supplied via interface units 100 to channel buffers 300 under the control of a time slot generator 200 which scans each of the interface units in sequence. Control circuit 400 gates the data bits from buffers 300 in accordance with a predetermined arrangement determined by the bit rates of the sixteen channels, dummy bits being introduced as required. The multiplex bit stream is then scrambled at 50 and transmitted at 60. System details.-Considering channel 1, data and clock pulses are supplied via a matching unit 112 in the interface unit 100, Fig. 2, to an AND gate 114 which gates a bi-stable 116 whenever a data and clock pulse appear together. The clock pulses control a bi-stable 118 and the outputs of both bi-stables are connected to a shift register 302, Fig. 4, in the channel buffer 300. The output of bi-stable 118 is also supplied to the left of a bidirectional counter 304 to cause it to count up, pulses applied to the right causing it to count down. The contents of the counter are decoded in a matrix 306 which controls gates 308 when a full count of 200 is reached and sets a bi-stable 310 when the count is 100. The bi-stable 310 is reset when the transmission is ended. The number of stages required for the shift register &c. is calculated to suit the corresponding channel. During the time that bi-stable 310 is set, channel 1 clock pulses are supplied via AND gate 312 to the gates 308, so that data from the shift register passes via OR gates 314 to gates 402, 404 in the control circuit 400, Fig. 5. The output of gate 312 is supplied to the count down input of counter 304 to ensure that data is always gated from the last information containing position in register 302. Data is read from each of the buffers 300 at varying rates dependent upon the data rate of the corresponding input channel so that the bits for a particular channel will occupy certain positions in the multiplexed bit stream but these positions will differ for each bit rate. For example, assuming that channel 1 data enters at a rate of 64-2 bits per second and data from the 16 channels is transmitted as 16 bit blocks, Fig. 3b (not shown), then 64.2 bits per second can be expressed as a whole number, i.e. 642 bits over a 10 second period. During each second, 70 bit positions are available for channel 1 and the transmission pattern is as follows. In the first second 64 data bits followed by 6 dummy bits are transmitted and this is repeated for the next 8 seconds. In the tenth second 66 data bits followed by 4 dummy bits are transmitted. This is repeated for 10 second intervals, 100 seconds constituting a complete frame. Other bit speeds are accommodated in similar fashion. Control circuit 400 for channel 1, Fig. 5, is arranged to gate 64 bits per second . for the first 9 seconds and 66 bits for the tenth second, AND gate 402 being activated during the first 9 seconds by signal TO from bi-stable 420 via inverter 422 and input 64 which is energized for 64 bits during each of the first nine seconds. During the tenth second AND gate 402 is inhibited and input 10 from bistable 420 activates AND gate 404 to pass 66 bits. The outputs of gates 402, 404 and the corresponding gates for the other channels are supplied via OR gate 424 to the encoder 50. The clock pulse for channel 1 time slot causes the 7 bit counter 408 to count from 1 to 75 each second interval and a decoder 410 energizes a bi-stable 416 during the count from 1 to 64 to enable gate 402. Similarly bi-stable 416 enables gate 404 during the count from 1 to 66. Output 75 of the counter activates a 4-bit counter 412 once every second and every tenth second a decoder 418 thereby enabling gate 404 and inhibiting gate 402. When no data is passed by OR gate 424 during channel 1 time slot dummy bits take the place of the data, the location of data and dummy bits being determined by the particular interconnections used for circuit 400. A similar interconnected control circuit is required at the receiver to separate the data from the dummy bits. The output of gate 424 is shown in Fig. 7A each frame comprising 100 subframes. A subframe, Fig. 7B comprises 75 blocks each containing 16 bits, five blocks containing 80 bits being reserved for synchronization purposes.