4422 sequence controlled service the sequence

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Unformatted text preview: type with sequence-control mechanisms of both sending and receiving ends and a standard report returned from the receiving end to the sending end using CLCWs. For the Sequence-Controlled Service, COP-1 ensures with a high probability of success that a) no SDU is lost; b) no SDU is duplicated; c) no SDU is delivered out of sequence. For the Sequence-Controlled Service, TC-SDLP uses a type of Transfer Frames called Type-A Transfer Frames. Control of sequentiality is maintained using the Frame Sequence Number that must be present in each Type-A Transfer Frame. Type-A Transfer Frames are transmitted by the FOP-1 with their Frame Sequence Numbers arranged in strict up-counting order. The FARM-1 permits Type-A Transfer Frames to be accepted only if they are received bearing Frame Sequence Numbers in the proper up-counting sequential order. Upon detection of the first sequence error, the FARM-1 will reject all subsequent Type-A Transfer Frames, on that Virtual Channel, which do not contain the expected Frame Sequence Number. FOP-1 initiates retransmission either in response to a ‘Retransmit’ Flag in the CLCW or by detecting a timeout. FOP-1 performs retransmission by re-sending all unacknowledged Type-A Transfer Frames on that Virtual Channel. FOP-1 and FARM-1 also have a window mechanism to prevent a new Transfer Frame with the same Frame Sequence Number as that of a missing Transfer Frame from being transmitted or accepted. CCSDS 130.2-G-1 Page 4-6 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS 4.4.2.3 Expedited Service The Expedited Service (or the Type-B Service) is normally used either in exceptional operational circumstances, typically during spacecraft recovery operations, or when a higherlayer protocol provides a retransmission capability. For the Expedited Service, TC-SDLP uses a type of Transfer Frames called Type-B Transfer Frames. Type-B Transfer Frames are also used to send Control Commands from the FOP-1 to the FARM-1. At the sending end, Type-B Transfer Frames are transmitted as close to immediately as possible, and at the receiving end, the SDUs carried by Type-B Transfer Frames will be delivered to the receiving user immediately. Type-B Transfer Frames are transmitted only once (i.e., no retransmission), and there is no guarantee that all SDUs transmitted with Type-B Transfer Frames are delivered to the receiving user. Valid Type-B Transfer Frames will always be accepted by FARM-1, and processed only to the extent of incrementing a counter for Type-B Transfer Frames in the CLCW. Type-B Transfer Frames are protected by the error correction/detection capability of the underlying Synchronization and Channel Coding Sublayer. Therefore, Type-B Transfer Frames accepted by FARM-1 have a very high probability that they are error-free, but their sequentiality is not controlled by COP-1. 4.5 4.5.1 TRANSFER OF PACKETS GENERAL How Packets are transferred with Transfer Frames differs depending on the Space Data Link Protocol. The following subsections provide an infor...
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