Transparent as viewed by the user the invisible and

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: NSPARENT: As viewed by the user, the invisible and seemingly direct (virtual) transfer of command data from the command originating point to the controlled process. USER: A human or machine-intelligent process which directs the progress of a space mission by sending commands to a space system. VALIDATION: A process performed at the receiving end of the Transfer layer to check the integrity of a TC Transfer Frame. VIRTUAL CHANNEL: Within the TC Data Routing Service, an identifier which permits all Transfer Frames who are members of a given sequence to be uniquely identified. It permits multiple user data types to be multiplexed together so that they may share the finite capacity of the single physical space data channel. Issue 6 Page A-7 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE ANNEX B TELECOMMAND TRANSFER FRAME ERROR DETECTION ENCODING/DECODING GUIDELINE Purpose: This Annex provides a description of the error detection encoding and decoding procedures which may be used in association with the optional Frame Error Control field of the Telecommand Transfer Frame. Issue 6 Page B-1 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE B - 1 CODING FOR ERROR DETECTION IN TRANSFER FRAMES This Annex describes the error detection encoding/decoding procedure that is recommended for Transfer Frame coding. The code specifies the same generator polynomial used by HDLC (ISO), ADCCP (ANSI), V.41 (CCITT), etc. It has the following capabilities when applied to an encoded block of less than 32,768 (215) bits: (1) All error sequences composed of an odd number of bit errors are detected. (2) All error sequences containing at most two bit errors anywhere in the encoded block will be detected. (3) If a random error sequence containing an even number of bit errors (greater than or equal to 4) occurs within the block, the probability that the error will be undetected is approximately 2-15 (or approximately 3 x 10-5). (4) All single error bursts spanning 16 bits or less will be detected provided no other errors occur within the block. B-1.1 ENCODING PROCEDURE The encoding procedure accepts an (n-16)-bit data block and generates a systematic binary (n,n-16) block code by appending a 16-bit Frame Check Sequence (FCS) as the final 16 bits of the codeblock. This FCS is inserted into the Frame Error Control Word of the Transfer Frame Trailer. The equation for the FCS is: FCS = [X16 . M(X) ⊕ X(n-16) . L(X)] modulo G(X) where M(X) is the (n-16)-bit message to be encoded expressed as a polynomial with binary coefficients L(X) is the presetting polynomial given by: 15 L(X) = ∑ xi (all "1" polynomial of order 15) i=0 Issue 6 Page B-2 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE G(X) is the generating polynomial given by: G(X) = X16 + X12 + X5 + 1 n is the number of bits in the encoded message ⊕ is the modulo 2 addition operator (Exclusive OR) Note that the encoding procedure differs from that of a conventional cyclic block encoding operation in that: The X(n-16) . L(X) term has...
View Full Document

Ask a homework question - tutors are online