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Unformatted text preview: s for repeated retransmissions for telecommand Page 12 4. Conclusion
4.1 Choice of option
Two of the options look better than the rest:
− The option described in section 3.2.2, for systematic retransmission in FOP-1, keeps the frame
retransmission in the Transfer Sublayer, which is the layer with knowledge of the frame type and
with responsibility for end-to-end delivery of the frame. The systematic retransmission can respond
efficiently to a CLCW that acknowledges a frame. − The option described in section 3.3, for systematic retransmission in the Synchronization and
Channel Coding Sublayer, is simpler to specify and probably needs less changes to the standards
documents. If the systematic retransmission is used as intended for a link with long light delay, the
disadvantages discussed above can be handled. However, if the systematic retransmission is used
on an unsuitable link, then the inefficiencies discussed in section 3.3.1 could be significant. 4.2 Other issues
4.2.1 One frame per CLTU
The Recommended Standard CCSDS 231.0-B-1, TC Synchronization and Channel Coding, allows
multiple TC Transfer Frames to be carried in a single CLTU. In CCSDS 232.0-B-1, TC Space Data Link
Protocol, the maximum number of frames per CLTU is a managed parameter for a Physical Channel.
At the receiving end, when a decoding failure occurs in the processing of a CLTU, the rest of the CLTU
is discarded. So, if a link error causes a frame to be lost, then the same error will also cause the loss of
any copies of the frame carried later in the same CLTU. It therefore seems reasonable to recommend
that, whenever the systematic retransmission option is in use, then the option for multiple frames in a
CLTU should not be in use. (See also section 3.4.4 above.)
Note that ECSS-E-ST-50-04, Space data links – Telecommand protocols, synchronization and channel
coding, places a limit of one TC Transfer Frame in a CLTU. 4.2.2 Upper limit on the number of systematic retransmissions
In practice, the nu...
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This document was uploaded on 03/06/2014.
- Spring '14