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The information acknowledges the receipt of frames

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Unformatted text preview: d of COP-1). The information acknowledges the receipt of frames and indicates problems, such as a gap in the received sequence. The information is contained in a communications link control word (CLCW), transmitted in a telemetry transfer frame. FOP-1 starts the automatic retransmission of a sequence of frames when: − FOP-1 receives a CLCW indicating that FARM-1 has detected a gap in the sequence, or − FOP-1 does not receive acknowledgement of receipt of a frame within a given time. In the automatic retransmission, FOP-1 applies the “go-back-n”: it retransmits the first missing frame (or the earliest unacknowledged frame) and all subsequent frames. 2.2 Interface for sequence-controlled service The interface to FOP-1 for the sequence-controlled service can be considered as two parts: − The interface for management of the sequence-controlled service. − The interface for sending data units on the sequence-controlled service. This view implies the existence of two user entities: − The management entity. − The user that is sending the telecommand data. In a typical application, the immediate source of the data is the Segmentation Sublayer, which in turn is sending data on behalf of layers above. The management entity uses the management interface to start and stop the sequence-controlled service and to set its parameters, such as the length of the timeout period (called T1_Initial) and the limit on the number of automatic retransmissions (called Transmission_Limit). It also receives failure messages from FOP-1 and manages the consequences. The management of the normal case can easily be automated but safe recovery from some failures is more complex. TCretrans7 2008Sep09 I.B. + M.A. DE LANDE LONG SOFTWARE + CONSULTANCY Options for repeated retransmissions for telecommand Page 5 2.3 Forcing FOP-1 retransmissions with the currently-defined COP-1 W hen COP-1 was developed more than twenty years ago, the needs of deep-space missions were considered. The timeout period and transmission limit can be set to cause early automatic retransmissions and the Suspend / Resume mechanism can maintain the FOP-1 status, such as its internal queues, during a l...
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