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Unformatted text preview: provided by the underlying
Packetization layer in order to initiate the data transport process. (3) The TC Packetization layer formats TC application data into end-to-end
transportable data units called TC Packets. A TC Packet is a basic user data unit
that is transported "up" to the spacecraft by the TC System: it is virtually identical
to a Telemetry Packet, which is the basic user measurement data unit that is
transferred "down" to the user through the CCSDS Telemetry System. The
Packetization layer utilizes the underlying services of the TC Data Routing Service
to accomplish its functions. The TC DATA ROUTING SERVICE contains the intermediate two layers: the TC
Segmentation layer and the TC Transfer layer. The Data Routing Service supports the errorcontrolled transmission and retransmission of standard TC Packets or TC Segments through
the data link to the spacecraft, or other user data structures from non-standard higher layers.
(1) Issue 6 The TC Segmentation layer provides flow control services. Recognizing that
user data units from the layer above may be too long for efficient handling by lower
layer processes, the Segmentation layer is available to break them into smaller
communications-oriented pieces (TC Segments) for transfer through the space data
channel. It also provides "Multiplexer Access Points" (MAPs) which allow
different user data units to be multiplexed together for data flow control purposes.
TC Segments are of proper size for placement into the data unit of the next lower
layer, the TC Transfer Frame. If the user data units are already short enough to be
Page 2-5 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE compatible with insertion into the TC Transfer Frame, and the MAP feature is not
required, the entire Segmentation layer may be bypassed.
(2) The TC Transfer layer uses its own data structure, the TC Transfer Frame, to
reliably transport TC Packets, TC Segments or other higher layer user data
structures through the telecommand channel to the receiving spacecraft. As the
heart of the CCSDS Telecommand System, the Transfer layer offers a range of
delivery service options designed to satisfy various mission needs. It contains the
retransmission procedures required to reliably deliver TC Transfer Frames to the
spacecraft, plus the facility to multiplex these frames together into "Virtual
Channels" (VCs). The Transfer layer uses the underlying TC Channel Service to
accomplish its role. The TC CHANNEL SERVICE contains the bottom two layers: the TC Coding layer and
the TC Physical layer. The Channel Service supports the radiation of telecommand information
through the physical uplink path to the spacecraft.
(1) The TC Coding layer satisfies the TC System requirement for the error-free
delivery of commands to the spacecraft, by encoding the TC user data from the
layer above to protect them against noise-induced errors during transmission
through the underlying ground-to-spacecraft radio frequency (rf) channel. The
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This document was uploaded on 03/06/2014.
- Spring '14