If less than five trailing octets of fill data are

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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 be discarded. 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 The Frame Validation Check consists of the following tests: CCSDS 202.0-B-2 Page 4-15 CCSDS HISTORICAL DOCUMENT November 1992 CCSDS HISTORICAL DOCUMENT CCSDS RECOMMENDATION FOR TELECOMMAND: DATA ROUTING SERVICE (1) (2) The TC Frame must have the expected SPACECRAFT ID.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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