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518 expectedacknowledgementframesequencenumber nnr the

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Unformatted text preview: he time a copy of the Transfer Frame is first passed to the Lower Procedures for transmission, and the time the FOP-1 has finished processing the Transfer Frame. FOP-1 has finished processing a Type-AD or Type-BC Transfer Frame when: a) it receives (via the CLCW) a positive acknowledgement of receipt of the Transfer Frame (perhaps after retransmission); or b) an event causes FOP-1 to purge the Sent_Queue (i.e., an exception or a ‘Terminate AD Service’ Directive). Once the processing is finished, the master copy of the Transfer Frame shall be removed from the queue and discarded and the (successful or not successful) transfer of the FDU shall be reported to the Higher Procedures by means of a ‘Confirm Response’ (Positive or Negative). 5.1.8 EXPECTED_ACKNOWLEDGEMENT_FRAME_SEQUENCE_NUMBER (NN(R)) The Expected_Acknowledgement_Frame_Sequence_Number, NN(R), contains the Frame Sequence Number of the oldest unacknowledged AD Frame, which is on the Sent_Queue. This value is often equal to the value of N(R) from the previous CLCW on that Virtual Channel. Certain Directives set the value of NN(R). ‘NN(R)-1’ is the value of the Frame CCSDS 232.1-B-2 Page 5-4 September 2010 RECOMMENDED STANDARD FOR COMMUNICATIONS OPERATION PROCEDURE-1 Sequence Number of the latest Type-AD Transfer Frame which FOP-1 can guarantee has arrived safely. Because of the loop delay in the communications link, this value may lag behind the value of the Receiver_Frame_Sequence_Number, V(R) at the receiving end. 5.1.9 TIMER_INITIAL_VALUE (T1_INITIAL) General Whenever a Type-AD or Type-BC Transfer Frame is transmitted, the Timer shall be started or restarted with an initial value of Timer_Initial_Value (T1_Initial). If a Transfer Frame is lost on the communications link, no positive acknowledgement for that Transfer Frame shall be seen in the CLCW. If no later Type-AD or Type-BC Transfer Frame were transmitted on that Virtual Channel, there would be no way for FOP-1 to discover that the Transfer Frame had not arrived. Therefore, each Virtual Channel has a Timer whi...
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