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Unformatted text preview: between the first and second Transfer Frames of VC 1 that carry the low-priority Packet. To use this kind of algorithm, a buffer memory with a sufficient capacity to store SDUs and/or Transfer Frames temporarily must be implemented in the service provider. High Priority Packet Low Priority Packet Transfer Frame of VC 1: Low Priority Low Priority Packet Transfer Frame of VC 0: High Priority High Priority Packet Transfer Frame of VC 1: Low Priority Low Priority Packet Figure 4-2: An Example of How to Use Virtual Channels CCSDS 130.2-G-1 Page 4-2 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS By using a proper algorithm for multiplexing Transfer Frames of different Virtual Channels, the QoS requirements of SDUs can be met to some extent. If the SLE Forward Space Packet Protocol Service (reference [20]) is used on the ground to forward Space Packets to be transmitted to a spacecraft using the TC-SDLP, the algorithm specified in reference [20] must be used for multiplexing Virtual Channels. Otherwise, CCSDS does not recommend any standard algorithm for multiplexing Virtual Channels, and it is the responsibility of the project to ensure that the multiplexing algorithm used for that project actually satisfies the QoS requirements. 4.3 4.3.1 ADDRESSING AND MULTIPLEXING GENERAL The Space Data Link Protocols use some addresses or identifiers to identify data streams. All the Space Data Link Protocols use the following identifiers: the Transfer Frame Version Number (TFVN), the Spacecraft Identifier (SCID), the Virtual Channel Identifier (VCID) (explained in 4.2), the Master Channel Identifier (MCID), and the Physical Channel Name. In addition to these identifiers, the TC-SDLP optionally uses an identifier called the Multiplexer Access Point Identifier (MAP ID). Figure 4-3 shows the hierarchy of these identifiers and the channels identified by them. MAP Channels (TC-SDLP only, Optional): Identified by MAP ID Virtual Channels (VC): Identified by VCID Master Channels (MC): Identified by MCID=TFVN+SCID Physical Channel: Identified by Physical Channel Name Figure 4-3: Hierarchy of the Identifiers and Channels Used by the Space Data Link Protocols CCSDS 130.2-G-1 Page 4-3 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS 4.3.2 TRANSFER FRAME VERSION NUMBER The Transfer Frame Version Number (TFVN) is used to distinguish among different Transfer Frames. The values for the TC, TM, and AOS Transfer Frames are 1 (‘00’), 1 (‘00’), and 2 (‘01’), respectively. The numbers in the parentheses are binary encoded values that actually appear in the header of Transfer Frames. The TC and TM Transfer Frames, which share the same TFVN value, should be distinguished by management or configuration information. 4.3.3 SPACECRAFT IDENTIFIER The Spacecraft Identifier (SCID) is used to identify the spacecraft associated with the space link, but it must always be modified by the TFVN in order to identify the spacecraft uniquely. In other words, there is a set of SCIDs (each of which consists of 10 bits) for spacecraft that use the TC-SDLP and TM-SDLP (for which the value of TFVN is 1) and anot...
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This document was uploaded on 03/06/2014.

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