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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 22.214.171.124 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.
4.5.1 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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- Spring '14