Lx term has the effect of presetting the shift

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Unformatted text preview: the effect of presetting the shift register to an all "1" state prior to encoding. B-1.2 DECODING PROCEDURE The error detection syndrome, S(X), is given by S(X) = [X16 . C*(X) ⊕ Xn . L(X)] modulo G(X) where C*(X) is the received block in polynomial form and S(X) is the syndrome polynomial which will be zero if no error is detected and non-zero if an error is detected. B - 2 POSSIBLE IMPLEMENTATION A possible implementation of the above-defined encoding/decoding procedure is described below. B-2.1 ENCODING Figure B-1 shows an arrangement for encoding using the shift register. To encode, the storage stages are set to "one", gates A and B are enabled (closed), gate C is inhibited (open), and (n-16) message bits are clocked into the input. They will appear simultaneously at the output. After the bits have been entered, the output of gate A is clamped to "zero", gate B is inhibited, gate C is enabled, and the register is clocked a further 16 counts. During these counts the required check bits will appear in succession at the output. Issue 6 Page B-3 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE GATE A GATE C GATE B OUTPUT INPUT Figure B-1: Encoder B-2.2 DECODING Figure B-2 shows an arrangement for decoding using the shift register. To decode, the storage stages are set to "one" and gate B is enabled. The received n-bits [the (n-16) message bits plus the 16 bits of the FCS] are then clocked into the input. After n-16 counts, gate B is inhibited, the 16 check bits are then clocked into the input, and the contents of the storage stages are then examined. For an error-free block, the contents will be zero. A non-zero content indicates an erroneous block. GATE B OUTPUT INPUT Figure B-2: Decoder Issue 6 Page B-4 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE ANNEX C DATA PROTECTION CONCEPT Purpose: This Annex defines a concept for the protection of Telecommand data sets so that security measures may be implemented if required by a particular mission. Status: This Annex is currently under development by the CCSDS. Issue 6 Page C-1 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE For many missions, there is a firm requirement to prevent intentional or accidental commanding of the spacecraft by an unauthorized party. Some missions may also have user requirements to render the application data private so that they cannot be interpreted by unauthorized users. The methods which are implemented within the Telecommand System to satisfy these requirements are called Data Protection mechanisms. Data Protection may be provided by physical or logical mechanisms. Physical mechanisms involve restricting personnel and terminal access to the networks through which command data flow. Logical mechanisms involve transformations of the command data in a manner which makes unauthorized manipulation or interpretation extremely difficult. The...
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This document was uploaded on 03/06/2014.

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