12 protocol features the tm space data link protocol

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Unformatted text preview: RONIZATION AND CHANNEL CODING DATA LINK LAYER PHYSICAL LAYER PHYSICAL LAYER Figure 2-1: Relationship with OSI Layers 2.1.2 PROTOCOL FEATURES The TM Space Data Link Protocol provides the users with several services to transfer service data units over a space link. To facilitate simple, reliable, and robust synchronization procedures, fixed-length protocol data units are used to transfer data through the weak-signal, CCSDS 132.0-B-1 Page 2-1 September 2003 CCSDS RECOMMENDATION FOR TM SPACE DATA LINK PROTOCOL noisy space links: their length is established for a particular Physical Channel (a single stream of bits transferred over a space link in a single direction) during a particular Mission Phase by management. These protocol data units are known as TM Transfer Frames (unless otherwise stated, the terms ‘Transfer Frame’ and ‘Frame’ in this document refer to the TM Transfer Frame). Each Transfer Frame contains a header which provides protocol control information, and a fixed-length data field within which higher-layer service data units are carried. A key feature of the TM Space Data Link Protocol is the concept of ‘Virtual Channels’ (VC). The Virtual Channel facility allows one Physical Channel to be shared among multiple higher-layer data streams, each of which may have different service requirements. A single Physical Channel may therefore be divided into several separate logical data channels, each known as a ‘Virtual Channel’. Each Transfer Frame transferred over a Physical Channel belongs to one of the Virtual Channels of the Physical Channel. 2.1.3 ADDRESSING There are three identifier fields in the header of Transfer Frames: Transfer Frame Version Number (TFVN), Spacecraft Identifier (SCID), and Virtual Channel Identifier (VCID). The concatenation of a TFVN and a SCID is known as a Master Channel Identifier (MCID), and the concatenation of an MCID and a VCID is called a Global Virtual Channel Identifier (GVCID). Therefore, MCID = TFVN + SCID; GVCID = MCID + VCID = TFVN + SCID + VCID. Each Virtu...
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