To end a cltu a tail sequence is included the tail

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Unformatted text preview: ncluded. The Tail Sequence is a codeblock (of the same length as all other codeblocks in the stream) having a unique pattern constructed in such a way to fail the decoder parity check and cause a CODEBLOCK REJECTION (E4), thereby interrupting the telecommand continuum. No further codeblocks are decoded, no further transfer of codeblock contents to the frame layer is possible, and the decoder logic returns to the SEARCH state (S2). Note that the ONLY function of the Tail Sequence is to force the decoder logic to the SEARCH STATE at the end of a CLTU: the Tail Sequence should not be used to delimit the end of a frame. This is because, in general, 1) there may be more than one frame in the CLTU, and 2) a frame that is not long enough to fill the last codeblock must be followed by fill bits (which are not part of the frame) to complete that codeblock. If, during the decoding process, the physical layer signal is lost (carrier, modulation or bit sync), event E2, CHANNEL INACTIVE, occurs. Upon this event, the decoder returns to the INACTIVE state, S1. Assembling the Frame. If all codeblocks comprising a complete transfer frame are successfully decoded, the frame layer recognizes that the frame it has been assembling from each codeblock is complete when the number of octets received equals one more than the count presented in the "Frame Length" field of the Transfer Frame Header. The frame can then be transferred to the addressee. Other frames in the CLTU are similarly transferred upon completion. If the continuum of codeblocks from the decoder is interrupted (i.e., CODEBLOCK REJECTION has been declared for whatever reason) and the frame length received does NOT yet equal one more than the count in the "Frame Length" field, that frame is rejected by the frame layer. Further action by the frame layer to recover the frame is dependent on the Command Operations Procedure (COP) in use. This process is more fully described in Reference [3]. Other checks on the frame, such as testing for previously undetected errors, may also be made at the frame layer before declaring acceptance or rejection, and before forwarding to the addressee. D-3 FACTORS AFFECTING FRAME REJECTION RATE Each of the factors of performance of frame rejection rate are detailed below. In subsequent sections, the performance for several different CLTU organizations is given. Issue 6 Page D-5 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE D-3.1 Bit Synchronization Factor Initially, the on-board decoder is in the INACTIVE state. When a signal first appears (subsequent to achieving rf carrier lock), it will contain an acquisition pattern consisting of a series of alternating "ones" and "zeros". This pattern has the maximum transition density so as to provide the fastest lock-up time for the on-board bit synchronizer. The preferred length of 128 bits was chosen to provide 0.9999 probability of acquisition of bit sync based on experience with a number of hardware implementations operating at the command threshold level.1 Cl...
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