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In order to keep from declaring that the link has

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Unformatted text preview: RD FOR COMMUNICATIONS OPERATION PROCEDURE-1 5.1.10 TRANSMISSION COUNT VARIABLES General When a Type-AD or Type-BC Transfer Frame is lost, the normal recovery procedure is to retransmit it. If, however, there is a serious problem on the underlying communications link, no amount of retransmissions will permit an acknowledgement for the Transfer Frame to appear in the CLCW for the Virtual Channel. If nothing were done, there would be no way for COP-1 to detect the error. Therefore, all FDUs containing user data passed from the Higher Procedures at the sending end (as well as all directives from the Higher Procedures) have associated with them, at least implicitly, a limit to the number of times the corresponding Transfer Frame is to be transmitted. In order to keep from declaring that the link has failed when it is in fact getting Transfer Frames into the receiving end, the transmission count limit shall apply only to the first Transfer Frame on the Sent_Queue. Once that Transfer Frame is acknowledged, the count is reset, even though the remaining Transfer Frames on the Sent_Queue have already been transmitted, possibly more than once. The effect is that the transmission count can be considered to be associated with the Sent_Queue, rather than with each Transfer Frame; therefore, each Transfer Frame will be transmitted ‘at least’ the number of times corresponding to the value given by Transmission_Limit, unless this value is set to 1. In this last case, no retransmission will be tried and each Transfer Frame will be transmitted only once. Three FOP-1 variables are used for controlling the retransmissions: a) Transmission_Limit; b) Timeout_Type (TT); c) Transmission_Count. Transmission_Limit The Transmission_Limit holds a value which represents the maximum number of times the first Transfer Frame on the Sent_Queue may be transmitted. This includes the first ‘transmission’ and any subsequent ‘retransmissions’ of the Transfer Frame. A Transfer Frame in the Sent_Queue that moves from...
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