In such cases one instance of the space data link

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Unformatted text preview: both directions on a single space link. In such cases, one instance of the Space Data Link Protocol is used on a Physical Channel in one direction and another instance on another Physical Channel in the other direction. On a given Physical Channel, only one Space Data Link Protocol must be used (i.e., multiple Space Data Link Protocols must not be mixed on a single Physical Channel). The TC-SDLP is usually used for (but not limited to) sending commands from a ground system to a spacecraft (or from a spacecraft to another spacecraft) on a forward Physical Link. The TM-SDLP is usually used for (but not limited to) sending telemetry from a spacecraft to a ground system (or from a spacecraft to another spacecraft) on a return Physical Link. The AOS-SDLP is also used for sending telemetry from a spacecraft to a ground system (or from a spacecraft to another spacecraft) on a return Physical Link, but it may be used on a forward Physical Link as well if there is a need for two-way on-line communications (e.g., audio and video) between a spacecraft and a ground system (or between two spacecraft). If we suppose that a space link consists of one forward Physical Link and one return Physical Link, there are three combinations of Space Data Link Protocols that are frequently used by space missions (see figure 2-2). Combination (a) of figure 2-2 has been used by most missions, while combination (b) is used by missions that require extended features provided by AOS-SDLP for the return Physical Link. Combination (c) is used by missions such as piloted missions that need two-way on-line communications between the spacecraft and the ground system. CCSDS 130.2-G-1 Page 2-2 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS TC-SDLP (forward) Ground System or Spacecraft TM-SDLP (return) Ground System or Spacecraft (a) TC-SDLP and TM-SDLP TC-SDLP (forward) Ground System or Spacecraft AOS-SDLP (return) Ground System or Spacecraft (b) TC-SDLP and AOS-SDLP AOS-SDLP (forward) Ground System or Spacecraft AOS-SDLP (return) Ground System or Spacecraft (c) AOS-SDLP for both directions Figure 2-2: Frequently Used Combinations of Space Data Link Protocols 2.3 LAYER ARCHITECTURE Figure 2-3 shows how the Space Data Link Protocols are used with other protocols of other layers and how they are related with the OSI Basic Reference Model (reference [6]). CCSDS defines two Sublayers in the Data Link Layer of the OSI Model: the Data Link Protocol Sublayer and the Synchronization and Channel Coding Sublayer. The Space Data Link Protocols belong to the Data Link Protocol Sublayer, which specifies methods of transferring data supplied by the higher layer over a space link using data units known as Transfer Frames. The Synchronization and Channel Coding Sublayer specifies methods for reliably transferring Transfer Frames over noisy space links, which include methods of delimiting and synchronization of Transfer Frames and methods of error correction and/or detection. CCSDS 130.2-G-1 Page 2-3 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS CCSDS has developed two standards for the Synchronization and Channel Coding Sublayer: 1 the TC Synchronization and Channel Coding Recommended Standard (reference [9]), and the TM Synchronization and Channel Coding Recommended Standard (...
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