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Unformatted text preview: INSTRUCTIONS. UNDEFINED INPUT: TC PACKETS.
TC FILES. SEGMENTATION LAYER Figure 3-3: TC Packetization Layer Packetization layer, containing control instructions which define the system conditions that
must exist before the TC application data are passed back across the interface to the TC System
At the receiving end of the TC Packetization layer, the named sets of TC application data are
extracted from the data fields of the TC Packets in the sequential order in which they were
given at the sending end, and passed to the layer above. Status reports may be formulated and
returned to the sending end of the layer (via Telemetry Packets) if information relating to the
correctness, completeness, and sequentiality of the received data is required by particular
mission processes: the CCSDS presently does not define these reports.
It should be noted that many of the telecommanding functions within and above the
Issue 6 Page 3-5 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE Packetization layer will probably be implemented within user application processes, such as the
instrument and subsystems themselves, rather than as external supporting services. 3 . 2 TC DATA ROUTING SERVICE
The TC Data Routing Service provides a pivotal function within the TC System by performing
the error-controlled communication of higher layer TC data through the ground-to-space data
link, relying on the lower layer TC Channel Service in order to perform its task. The
combination of the Data Routing and Channel Services provides users with a powerful
mechanism for the flow control and guaranteed transfer of TC data between the sending and
receiving ends of the TC System. The CCSDS therefore has focussed its attention on fully
specifying these services so that robust, mission-independent user support systems may be
rapidly developed. The Data Routing Service contains two layers: the TC Segmentation
layer and the TC Transfer layer. The detailed protocols and formats for the TC Data
Routing Service are specified in References  and . 3 . 2 . 1 TC SEGMENTATION LAYER
The TC Segmentation layer is optional, i.e., it may be bypassed if its services are not required.
The basic service of the Segmentation layer is to prepare variable-length TC data units from the
layer above (e.g., TC Packets, or other user-supplied data structures) for transfer through the
space data link, using the lower TC Transfer layer service. Since the data unit of the TC
Transfer layer (the TC Frame) has an upper length limit, the Segmentation layer must break the
higher layer TC data units into suitable sized pieces for insertion into the data field of the frame.
A second aspect of its service is the capability to multiplex together segments of data from
different TC data units for the purpose of flow control: to accomplish this, the Segmentation
layer provides "Multiplexer Access Points" (MAPs) to the layer above. Figure 3-4 depicts the
activities and interfaces of the TC Segmentation layer.
TC Packets (or ot...
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