The key feature of this service which is essentially

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Unformatted text preview: fer Frames is determined by the service provider according to mission-specific rules (usually known to the user). The key feature of this service, 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 service data unit from a 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). CCSDS 132.0-B-1 Page 2-5 September 2003 CCSDS RECOMMENDATION FOR TM SPACE DATA LINK PROTOCOL Sending Use r Re ce ivin g Use r B u ffe r Pro vi d e r Buffer c ont ent is t ransferred onc e per Trans f er Frame Trans f er f rom buf f er at s ending end to receiv ing end Figure 2-4: Synchronous Service Model Periodic Service Periodic service is a special case of synchronous service in which service data units 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. There are three cases in which a synchronous service is periodic: a) if the service is associated with a Virtual Channel (or a Master Channel), and that Virtual (or Master) Channel produces Transfer Frames at a constant rate, then the service is periodic; b) if the service is associated with a Master Channel and there is only one Master Channel in the Physical Channel, then the service is periodic. For periodic services, all service data units are sent only once if the user supplies service data units at the same rate as the rate at which the service provider transfers them. 2.2.3 SUMMARY OF SERVICES General Eight services are provided by the TM Space Data Link Protocol. Five of them (Packet, Virtual Channel Access, Virtual Channel Frame Secondary Header, Virtual Channel Operational Control Field, and Virtual Channel Frame) are provided for a Virtual Channel. Three of them (Master Channel Frame Secondary Heade...
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This document was uploaded on 03/06/2014.

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