13 bit numbering convention and nomenclature in this

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Unformatted text preview: BERING CONVENTION AND NOMENCLATURE In this document, the following convention is used to identify each bit in an N-bit field: The first bit in the field to be transferred (i.e., the most left justified when drawing a figure) is defined to be “Bit 0”; the following bit is defined to be “Bit 1” and so on up to “Bit N-1”. When the field is used to express a binary value (such as a counter), the Most Significant Bit (MSB) shall be the first bit of the field, i.e., “Bit 0”. BIT 0 BIT N-1 N-BIT DATA FIELD FIRST BIT TRANSFERRED = MSB In accordance with modern data communications practice, spacecraft data fields are often grouped into 8-bit “words” which conform to the above convention. Throughout this Recommendation, the following nomenclature is used to describe this grouping: 8-BIT WORD = "OCTET" By CCSDS convention, all “spare” bits shall be permanently set to value “zero”. Note that throughout this document, the word “Telecommand” may be abbreviated as “TC”. CCSDS 202.0-B-2 Page 1-2 CCSDS HISTORICAL DOCUMENT November 1992 CCSDS HISTORICAL DOCUMENT CCSDS RECOMMENDATION FOR TELECOMMAND: DATA ROUTING SERVICE 2 TELECOMMAND DATA ROUTING SERVICE OVERVIEW A complete summary of the acronyms and terminology used internal to this document is presented in Annex A. A detailed specification of the services provided by each layer is presented in Annex B. Figure 2-1 illustrates the significance of the Telecommand (TC) Data Routing Service in the overall Telecommand System, which contains three principal service elements: Telecommand Data Management Service; Telecommand Data Routing Service; and Telecommand Channel Service. Each service is documented in its own Recommendation. A more thorough discussion of the layered services, including expansion of Figure 2-1 in greater detail, is contained in Reference [2]. The Telecommand System is related to the Telemetry System as documented in Reference [5] and Reference [6]. The TC Data Routing Service enables user telecommand data units associated with higher layers to be reliably transferred to the spacecraft, drawing upon the Channe...
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