The length of the tc frame data unit may vary from

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Unformatted text preview: nit is the data unit transferred from the Segmentation Layer to the Transfer layer. The length of the TC Frame Data Unit may vary from one TC Frame to the next. Its length shall not exceed the maximum allowed length of the Transfer Frame Data Field. The maximum length of the TC Frame Data Unit depends on whether Frame Error Control is being used (See Section With Frame Error Control Without Frame Error Control 1017 octets 1019 octets The maximum length allowed by a particular spacecraft or ground system implementation on a particular Virtual Channel may be shorter than the maximum specified here. The content of a TC Frame Data Unit is defined in Section 3.3.1. 3.2.2 TELECOMMAND SEGMENT The use of a TC Segment is required for any Virtual Channel which has more than one MAP or which transfers TC User Data Units which are larger than permitted in a single TC Frame. Its use is optional otherwise, except that if it is used in any TC Frame carried on a Virtual Channel, it must be used for all TC Frames on that Virtual Channel. CCSDS 202.0-B-2 Page 3-2 CCSDS HISTORICAL DOCUMENT November 1992 CCSDS HISTORICAL DOCUMENT CCSDS RECOMMENDATION FOR TELECOMMAND: DATA ROUTING SERVICE The format of the TC Segment is shown in Figure 3-2. SEGMENT HEADER SEQUENCE FLAGS MULTIPLEXER ACCESS POINT (MAP) ID 2 SEGMENT DATA FIELD 6 1 OCTET 1018 OCTETS (MAXIMUM) Figure 3-2: Telecommand Segment Format (1) Segment Header. The Segment Header contains the following two fields: Sequence Flags (Bits 0,1) This two bit field delimits the higher layer TC User Data Unit by indicating the sequential position of the TC Segment relative to the TC User Data Unit (e.g., TC Packet) of which the TC Segment is a part. The flags are interpreted as follows: Bit 0 0 0 1 1 (2) Bit 1 1 0 0 1 Interpretation First Segment of TC User Data Unit on one MAP Continuing Segment of TC User Data Unit on one MAP Last Segment of TC User Data Unit on one MAP No segmentation (one TC User Data Unit or multiple complete TC Packets) Multiplexer Access Point (MAP) Identifier (Bits 2 through 7) The MAP facility allows user command data from different sources to be multiplexed together so that they share the communications capacity of one Virtual Channel, thus preventing a...
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This document was uploaded on 03/06/2014.

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