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Unformatted text preview: valid data and Fill Data, the Fill Data
must be stripped by the Transfer layer.
The characteristics of the present TC Codeblock structure are such that no more than six
octets of Fill Data can occur. If LESS THAN FIVE trailing octets of Fill Data are present,
then they cannot possibly form a Frame Header, and they will be immediately discarded by
the Transfer layer. If FIVE OR SIX trailing octets of Fill Data exist, the Transfer layer data
handling process might attempt to interpret the Fill Data as a new Transfer Frame Header;
however, the subsequent Frame Validation Check (see 4.3.2) will prevent this from
happening because the Fill pattern of “01010101” appearing in each octet will violate at least
one of the validation tests; in particular, this pattern appearing where the FRAME LENGTH
field might be expected will indicate a frame length that exceeds the number of octets
received from the Coding layer, thus failing a test and causing the trailing five or six octets to
4.3.2 FRAME VALIDATION CHECK PROCEDURE After each TC Transfer Frame is reconstituted from the string of octets provided by the
Coding layer, it will next be subjected to a set of standard tests called a “Frame Validation
Check”. The Frame Validation Check shall be applied to ALL incoming TC Transfer
Frames, regardless of whether they are Type-A or Type-B. Failure to pass any test within the
Frame Validation Check shall cause the Transfer Frame to be rejected (discarded). The
Frame Validation Check consists of the following tests:
CCSDS 202.0-B-2 Page 4-15 CCSDS HISTORICAL DOCUMENT November 1992 CCSDS HISTORICAL DOCUMENT
CCSDS RECOMMENDATION FOR TELECOMMAND: DATA ROUTING SERVICE (1)
(2) The TC Frame must have the expected SPACECRAFT ID. (3) The TC Frame Header must not contain any values which are not consistent with
the implemented features for that spacecraft. (4) The value of the FRAME LENGTH must be consistent with the number of octets
that are present. (5) If implemented, the FRAME ERROR CON...
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This document was uploaded on 03/06/2014.
- Spring '14