The models shown below are intended only to

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Unformatted text preview: eriodic) that determine how service data units supplied by the user are transferred in protocol data units over a space link. The models shown below are intended only to illustrate the characteristics of services. They are not intended to guide or restrict design of on-board or ground systems. 2.2.2.2 Asynchronous Service In asynchronous service, there are no timing relationships between the transfer of service data units supplied by the service user and the transmission of Transfer Frames generated by the service provider. The user may request data transfer at any time it desires, but there may be restrictions imposed by the service provider on the data generation rate. In this service (figure 2-3), each service data unit 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 CCSDS 132.0-B-1 Page 2-4 September 2003 CCSDS RECOMMENDATION FOR TM SPACE DATA LINK PROTOCOL transmission errors may prevent delivery of some data units, the service provider attempts to transfer all data units provided by the user exactly once. The timing of data transfer is determined by the provider in accordance with mission-specific rules, and may depend on the traffic at the time of transfer. The key feature of this service is that all of the service data units from the sending user are transferred, and transferred only once. Sending User Receiv ing User Prov ide r Trans f er f rom s ending end t o receiv ing end Figure 2-3: Asynchronous Service Model 2.2.2.3 Synchronous Service In synchronous service, the transfer of service data units is synchronized with the release of either (1) Transfer Frames of a Virtual Channel, (2) Transfer Frames of a Master Channel, or (3) all Transfer Frames of a Physical Channel. The transfer timing may be periodic or aperiodic. In this service (figure 2-4), each service data unit from a sending user is placed in a buffer that can hold only one service data unit; the content of the buffer is sent to a receiving user at the time when a Transfer Frame is transmitted. The transmission timing of Trans...
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