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2 g 1 page 3 5 december 2007 ccsds report concerning

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Unformatted text preview: T CONCERNING THE SPACE DATA LINK PROTOCOLS Transfer Frame associated with the service is transmitted. The transmission timing of Transfer Frames is determined by the service provider according to mission-specific rules (which are usually known to the user). The key feature of this service type, which is essentially time-division multiplexing, is that the timing of data transfer is driven by the transfer mechanism, not by individual service requests from the user. Thus a particular SDU from a sending user might be sent once, several times (if the ‘new’ value is not placed in the buffer soon enough), or not at all (if one value is replaced by a second before the service provider can send it). Periodic Service Periodic service is a special case of synchronous service in which SDUs are transferred at a constant rate. Periodic transfer from service interface to service interface is provided with a specified maximum delay and a specified maximum jitter at the service interface. A synchronous service is periodic if the channel associated with the service (which is either a Virtual Channel or a Master Channel) transfers SDUs at a constant rate. (Virtual Channels and Master Channels are logical data channels established over a space link and will be explained in 4.2 and 4.3, respectively). For periodic services, all SDUs are sent only once if the sending user supplies SDUs at the same rate as the rate at which the service provider transfers them. 3.2.5 CHANNELS For each instance of a service, SDUs are transferred over a channel (either a Physical Channel, a Master Channel, a Virtual Channel, or a MAP Channel (TC-SDLP only)). In addition to the Virtual Channels and MAP Channels explained in 3.2.3, the Space Data Link Protocols use Physical Channels and Master Channels. Physical Channels are explained in 2.2. A Master Channel is a set of Virtual Channels associated with one spacecraft. In most cases, a Physical Channel has only one Master Channel, but it may consist of multiple Master Channels. For explanation on how these channels are identified and multiplexed, see 4.3. 3.3 3.3.1 SERVICES PROVIDED BY THE SPACE DATA LINK PROTOCOLS GENERAL Table 3-1 shows the services provided by the Space Data Link Protocols categorized by the types of SDUs transferred by the services. A schematic summary of these services is found in annex B. CCSDS 130.2-G-1 Page 3-6 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS Table 3-1: Services Provided by the Space Data Link Protocols Type of Service Data Units TC Space Data Link Protocol TM Space Data Link Protocol AOS Space Data Link Protocol Packets with authorized PVN MAP Packet Service, VC Packet Service Packet Service Packet Service Fixed-length private data (None) VC Access Service VC Access Service Variable-length private data MAP Access Service, VC Access Service (None) (None) Short fixed-length data (None) VC FSH Service, MC FSH Service, VC OCF Service, MC OCF Service Insert Service, VC OCF Service Bit stream (None) (None) Bitstream Service Transfer Frames VC Frame Service, MC Frame Service VC Frame Service, MC Frame Service VC Frame Service, MC Frame Service The Encapsulation Service can be used for Packets which do not have authorized PVNs, as described in 3.3.3. 3.3.2 SERVICES TO TRANSFER PACKETS General The Space Data Link P...
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