GB1428050A - Data transmission systems - Google Patents

Abstract

1428050 Data transmission; error correction STANDARD TELEPHONES & CABLES Ltd 14 Dec 1973 58019/73 Heading H4P In a duplex data transmission system in which detection of an error in either direction causes retransmission in both directions, and correct reception (OK) or request for a retransmission (RQ) are included in data blocks so as to be protected by error detection facility, the transmission cycle of each terminal consists of N blocks numbered in a repetitive sequence 1, 2...N+1, which numbers may not be transmitted in normal transmission, but on resumption of error, one or more block numbers may be transmitted temporarily to ensure that blocks are assembled at a receiver in the correct order. The numbers may be held in temporary storage. The arrangement provides indication if a complete cycle of N blocks becomes lost in transit or are faulty. After synchronization, or resynchronization, an initial service or start of data block is transmitted including a sequence number to set the remote sequence counter to correct reading either initially or after error. Fig. 2 shows a modification and relevant parts of a system such as disclosed in Specification 1,366,908. Formation of blocks is under control of TX distributer 1, and sync. from start control 2 in conjunction with retransmission unit 4 determines whether information is fresh data, retransmission data, sync. or start of data blocks. Sequence counter 3 of capacity N+1, by means of inputs from 1, 2, 4, steps only when blocks are transmitted and is reset to appropriate position for retransmission. Another counter 5 records blocks passed to a utilizing device, e.g. a terminal, and reading is applied to control 2. On resynchronization, control 2 receives a signal from 6 indicating that a start of data block including its sequence number has been received which number is compared with counter 5 thus determining when to enable gates G7, G23. For initial synchronization, sequence counter 2 must be in step with that in the remote transmitter hence by dotted lead Figs. 2-5 counter 2 is set to a number contained in start of data block. In the embodiment described, both directions of transmission are slaved to the same clock. The blocks may consist of data bits, service bits (if any) followed by parity bits hence the service bits may be protected by parity.