GB1422291A - Data transmission systems - Google Patents

Abstract

1422291 Data transmission; error correction STANDARD TELEPHONES & CABLES Ltd 00248/74 Heading H4P In a data transmission duplex system in which the detection of an error in the transmission in either the GO direction or the RETURN direction causes retransmission cycles in both directions and in which the acknowledgement signals sent from a receiving device to a transmitting device to indicate correct reception OK or a request for the repetition of an incorrectly received data block RQ form part of the data blocks sent from that receiving device to that transmitting device, when a data block is incorrectly ceived or a data block containing an RQ acknowledgement signal is received thus demanding the insertion of an RQ in a transmitted data block, the demand is temporarily stored until this insertion has been effected and no acknowledgement signal is sent in response to any data block that may be received while the demand is temporarily stored. The data transmission system is substantially the same as disclosed in Specification 1,366,908 except that in that Specification the RQ condition is in effect dealt with almost immediately where as in the present case the RQ is retained in the RQ/OK store 17 until it can be acted on. No acknowledgement signals are sent in respect of blocks received while this condition is stored. Each transmitted signal block includes a data component from the data input, an indicator signal from 17 having two states RQ (repeat request) and OK (reception correct) and parity bits from 3. Gates G3, G4, G5 controlled by distributor 1 pass these components to the output channel and a gate G6 passes only data to a store 4. Received signals from the return channel pass through gate G7 to the data output 10, unless an error in the received signal is detected by parity check 5 or else the received indicator bit is found at 6 to be RQ. In these cases until 9 blocks G7 and initiates retransmission cycle control unit 12. This causes gate G1 to retransmit the contents of store 4. When the control 12 receives an RQ from unit 9 it passes it to the TXRQ/OK store 17 but prevents any subsequent change in that condition until after the next enabling pulse has reached G4 from the distributor 1. A retransmission cycle then proceeds as described above, however when the control 11 reaches the end of its cycle and has caused N received blocks to be discarded, it causes control 12, via the connection 20, to restore control of the store 17 to the unit 9 and also if an RQ is then supplied to the store, holds that condition in the same way as for normal data transmission, until the next enabling pulse is supplied to G4. If control 12 has completed its retransmission cycle and is ready to insert an OK/RQ signal determined before unit 11 has indicated via connection 20 that its cycle is complete, control 12 inserts RQ and following its normal operation, repeats its retransmission cycle.