The farm only accepts frames if they are received

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Unformatted text preview: t detected error. As soon as an error is encountered, the FARM enters a lockout condition and rejects all subsequent frames until reset by a Control Command from the FOP. The FOP monitors a CLCW counter, generated by the FARM, that indicates how many good frames were accepted. When it observes the FARM in lockout, it uses this counter to compute how far to back up, sends an unlock command, and begins retransmission. (2) COP-1 COP-1 operates on the principle of sequential frame acceptance and retransmission, with frame sequence numbering. The FOP initiates the transmission of TC Frames whose sequence numbers are arranged in upcounting sequential order. The FARM only accepts frames if their sequence numbers match the expected upcounting order. As soon as a sequence error is encountered, the FARM rejects all subsequent frames whose sequence numbers do not match the expected order. The FOP monitors the CLCW to determine if frames are being rejected, and if so backs up and retransmits the series of frames, beginning with the frame whose sequence number matches the number which the FARM is expecting. Issue 6 Page 3-10 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE (3) COP-2 COP-2 operates on the principle of sequence-independent frame acceptance and selective retransmission, with frame sequence numbering. The FOP initiates the transmission of TC Frames whose sequence numbers are originally arranged in upcounting sequential order. Although nominally expecting the received frames to be in sequential order, the FARM will accept any frame whose sequence number falls within certain allowable windows. Any discontinuities in the received sequence are noted by the FARM, and the sequence numbers of detected "missing" frames are reported to the FOP via telemetered CLCWs. The FOP then schedules the retransmission of these missing frames at an opportune time. Since the numbering of frames is a key feature of COP-1 and COP-2, it is important to understand the terminology which is used within the CCSDS Recommendations: (1) The FOP, which is the numbering authority, maintains a master counter which assigns the frame sequence number. The current value of this master counter, i.e., the number which will be assigned to the NEXT TC Frame, is called V(S). (2) The frame sequence number which is contained within any particular transmitted TC Frame is called N(S). (3) The FARM maintains a counter which contains the value of the next TC Frame sequence number which it expects to receive. The current value of this counter is called V(R). (4) The telemetered CLCW contains reports of the observed values of V(R). The observed value of V(R), received by the FOP in a particular CLCW, is called N(R). This relationship is summarized in Figure 3-6. "N (S)" SENDING END RECEIVING END TC FRAME FOP "V (S)" FARM "V (R)" CLCW "N (R)" COP Figure 3-6: Transfer Layer Numbering Relationships Issue 6 Page 3-11 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE 3 . 3 TC CHANNEL SERVICE Operation of...
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This document was uploaded on 03/06/2014.

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