GB1224388A - Improvements in or relating to apparatus for causing repetition of incorrectly received messages in electrical digital signalling systems. - Google Patents

Abstract

1,224,388. Digital transmission systems. PLESSEY TELECOMMUNICATIONS RESEARCH Ltd. 20 Aug., 1969 [30 Aug., 1968], No. 41614/68. Heading H4P. In a synchronous electrical signalling system means are provided for causing repetition of incorrectly received messages, comprising at the transmitter a counter allotting numbers to successive messages and a store for storing a number of the most recently transmitted messages together with their allotted numbers. The receiver determines whether any received message contains an error and sends an error message to the transmitter when an error is detected, the error message including the number of the message containing an error. The transmitter extracts from the store the message corresponding to the number in the error message and retransmits it. No acknowledgement is sent of a correctly received message. Data to be transmitted is applied at 10 to queueing unit 12 which stores a queue of messages to be transmitted. The message counter 14 reads out the stored messages in turn, generating a number for each. If the store is empty, the counter transmits an idle message consisting of the next message number. The message to be transmitted is passed to a parity digit insertion unit 16 and to a sent message file 18. The unit 16 inserts a digit to give the message the required parity and the message is then transmitted. Incoming signals are applied to a parity check unit 30. If no message is received at the time one is expected, this is treated as an error. The checking unit 30 determines whether a received message is a data message, an idle message, or an error message. Data messages pass to output store 32. An idle message passes to message counter 34 so that the number in the idle message can be compared with that indicated by counter 34. Corrections are made if the numbers differ. If the parity checker detects error or if no message is received when one is expected, a signal is sent to an error message compiler 35 which produces an error message including the message number indicated by counter 34 and inserts it in the queueing unit 12. If an error message having the correct parity is detected by unit 30 a signal is passed to error message control 36 which passes the number of the erroneously received message to the sent message file 18 causing the corresponding message to be re-inserted into the unit 12 for retransmission. Unit 12 has three queues, a first for original data messages, a second for messages to be retransmitted following receipt of an error message, and a third for error messages. The third queue has the highest priority and the first queue the lowest. If there is a number of receivers, receiving messages from a single transmitter, any one receiver may send an error message, such message including the identity of the receiver. The transmitter then retransmits the required message only if it is relevant to that receiver.