If the sle forward space packet protocol service

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Unformatted text preview: orward Space Packet Protocol Service (reference [20]) is used on the ground to forward Space Packets (reference [13]) to be transmitted to a spacecraft using the TC-SDLP, the algorithm specified in reference [20] must be used for multiplexing Virtual Channels and MAP Channels. Otherwise, CCSDS does not recommend any standard algorithm for multiplexing MAP or Virtual Channels, and it is the responsibility of the project to ensure that the multiplexing algorithm used for that project actually satisfies the requirements. 3.2.4 TRANSFER TIMING 3.2.4.1 General The timing at which SDUs supplied by the sending users are transferred over the space link and delivered to the receiving users differs depending on the protocol and the service. 3.2.4.2 Transfer Timing of TC-SDLP The TC Space Data Link Protocol does not specify any rules for the timing of the transfer of SDUs supplied by the sending user. The user may submit SDUs at any time, but the timing of actual data transfer is determined by the service provider in accordance with missionspecific rules (such as multiplexing algorithms or scheduled release times) and may depend on the data traffic at the time of transfer. 3.2.4.3 3.2.4.3.1 Transfer Timing of TM-SDLP and AOS-SDLP General The TM and AOS Space Data Link Protocols have three service types (asynchronous, synchronous, and periodic) that determine the timing at which SDUs supplied by the sending user are transferred over the space link. 3.2.4.3.2 Asynchronous Service In asynchronous service, there are no timing relationships between the submission of SDUs by the service user and their transfer by the service provider over the space link. The sending user may request data transfer at any time, but the timing of actual data transfer is determined by the service provider in accordance with mission-specific rules (such as multiplexing algorithms) and may depend on the data traffic at the time of transfer. CCSDS 130.2-G-1 Page 3-4 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS Receiving User Sending User Queue Space Data Link Protocol (Service Provider) Figure 3-2: Asynchronous Service Model In this service type (figure 3-2), each SDU from a sending user is placed in a queue, the contents of which are sent to a receiving user in the order in which they were presented. Although transmission errors may prevent delivery of some SDUs, the service provider attempts to transfer all SDUs provided by the sending user exactly once. The key feature of this service type is that all of the SDUs from the sending user are transferred, and transferred only once. 3.2.4.3.3 Synchronous Service In synchronous service, the transfer of SDUs is synchronized with the release by the service provider of Transfer Frames associated with the service. The transfer timing may be periodic or aperiodic. In this service (figure 3-3), each SDU from a sending user is placed in a buffer that can hold only one SDU; the content of the buffer is sent to a receiving user at the time when a Sending User Buffer Receiving User Space Data Link Protocol (Service Provider) Figure 3-3: Synchronous Service Model CCSDS 130.2-G-1 Page 3-5 December 2007 CCSDS REPOR...
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