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.
22.214.171.124 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.
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
126.96.36.199 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
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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- Spring '14
- Space Exploration, European Space Agency, Protocol data unit