1 411 transfer frames general the protocol data units

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Unformatted text preview: ansfer Frames used by the TC-SDLP, TM-SDLP, and AOS-SDLP are called TC Transfer Frames, TM Transfer Frames, and AOS Transfer Frames, respectively. Each Transfer Frame consists of a header which provides protocol control information and a data field within which SDUs are carried. 4.1.2 TC TRANSFER FRAMES The TC-SDLP uses variable-length Transfer Frames to facilitate reception of short messages with a short delay. The length of each TC Transfer Frame is contained in its header. The TC-SDLP uses another data unit called the Communications Link Control Word (CLCW). CLCWs are sent from the receiver to the sender of TC Transfer Frames and contain a report that describes the status of acceptance of TC Transfer Frames at the receiver. CLCWs are usually transferred with a service provided by the TM-SDLP or the AOS-SDLP. 4.1.3 TM AND AOS TRANSFER FRAMES The TM-SDLP and AOS-SDLP use fixed-length Transfer Frames to facilitate simple, reliable, and robust synchronization procedures over weak-signal, noisy links. Their length must be fixed on a particular Physical Channel during a Mission Phase and must be known to the receiver through a management activity before the actual reception occurs. The length of Transfer Frames must be determined according to the rules specified in reference [10]. 4.2 VIRTUAL CHANNELS The mechanism used by the Space Data Link Protocols for transferring data with different QoS (Quality of Service, mostly priority and latency in this case) requirements is the use of Virtual Channels. The Virtual Channel facility allows one Physical Channel to be divided into multiple separate logical data channels, each known as a Virtual Channel (VC) and identified by a Virtual Channel Identifier (VCID) (see figure 4-1). Each Virtual Channel carries a separate sequence of SDUs, which may have different QoS requirements from those carried on the other Virtual Channels. Each Transfer Frame transferred over a Physical Channel belongs to one of the Virtual Channels of the Physical Channel. CCSDS 130.2-G-1 Page 4-1 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS Virtual Channel 0 Virtual Channel 1 Physical Channel Virtual Channel 2 Figure 4-1: Virtual Channels Figure 4-2 shows an example that illustrates how Virtual Channels are used to transfer Packets with different QoS requirements. In this example, the Physical Channel has two Virtual Channels: VC 0 for high-priority traffic and VC 1 for low-priority traffic. In figure 4-2, a long, low-priority Packet (for memory upload or download, for example) is being transmitted on VC 1. Since this Packet is longer than what can be carried by the maximum-size Transfer Frame (if the TC-SDLP is used) or the fixed-length Transfer Frame (if the TM-SDLP or AOS-SDLP is used), it is carried by two consecutive Transfer Frames of VC 1. Then, when the first Transfer Frame carrying this low-priority Packet is being transmitted, a short, high-priority Packet (carrying an on-off command or an event report, for example) is generated. Since this high-priority Packet needs to be transmitted as soon as possible, a Transfer Frame of VC 0 is generated to carry this high-priority Packet and inserted...
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This document was uploaded on 03/06/2014.

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