During that possibly extended time other events may

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Unformatted text preview: y the Higher Procedures is governed. When an FDU is on the Wait_Queue, this means that the Higher Procedures have not yet received an ‘Accept Response’ for the corresponding ‘Request to Transfer FDU’. 5.1.5 TO_BE_RETRANSMITTED_FLAG If retransmissions of the Sent_Queue are to be performed because one or more Transfer Frames were not acknowledged within the time allowed, or were negatively acknowledged by a CLCW with the ‘Retransmit Flag’ set, it is not reasonable to shut out all other FOP-1 activity until the last Transfer Frame on the Sent_Queue has been accepted by the Lower Procedures (especially since the Lower Procedures use the ‘Accept Response’ as its flow control mechanism). During that possibly extended time other events may occur, such as the arrival of a CLCW, which must be processed. To handle this situation, each Transfer Frame on the Sent_Queue carries a To_Be_Retransmitted_Flag to distinguish a Transfer Frame that has been transmitted (or retransmitted) and is awaiting acknowledgement (Flag reset) from one that must be retransmitted (Flag set). Upon receipt of an ‘Accept Response’ from the Lower Procedures, these flags will be used to determine which Transfer Frame on the Sent_Queue, if any, to retransmit next. CCSDS 232.1-B-2 Page 5-3 September 2010 RECOMMENDED STANDARD FOR COMMUNICATIONS OPERATION PROCEDURE-1 5.1.6 AD_OUT_FLAG, BC_OUT_FLAG, AND BD_OUT_FLAG FOP-1 records whether or not a ‘Transmit Request for Frame’ is outstanding for each of the three types of Transfer Frames: AD, BC and BD. There are therefore three variables: a) AD_Out_Flag (for Type-AD Transfer Frames); b) BC_Out_Flag (for Type-BC Transfer Frames); c) BD_Out_Flag (for Type-BD Transfer Frames). These variables may take one of two values: a) Ready; b) Not_Ready. When FOP-1 issues a ‘Transmit Request for Frame’, it shall set the appropriate ‘Out’ variable to ‘Not_Ready’. When the ‘Transmit Request’ is accepted by the Lower Procedures, FOP-1 shall set the variable to ‘Ready’. 5.1.7 SENT_QUEUE The Sent_Queue is a Virtual Channel data structure in which the master copy of all Type-AD and Type-BC Transfer Frames on a Virtual Channel is held between t...
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This document was uploaded on 03/06/2014.

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