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Unformatted text preview: (optionally uncorrectable) error be estimated to have
occurred within any TC Codeblock within a given sequence, the remainder of the sequence is
discarded and no further data are passed to the layer above until the Coding layer is reset by the
detection of another Start sequence. There are no reporting mechanisms between the receiving
and sending ends of the Coding layer. 3 . 3 . 2 TC PHYSICAL LAYER
The service of the TC Physical layer is to provide a physical connection, via radio signals,
between the transmitting station and the receiving spacecraft. Figure 3-8 depicts the activities
and interfaces of the TC Physical layer.
Inputs to the sending end of the Physical layer are CLTUs, plus control information from the
layer above defining the requested transmission services. The Physical layer controls the
activation and deactivation of the physical channel by invoking various "Physical Layer
Operations Procedures" (PLOPs).
A PLOP consists of sequential application of different "Carrier Modulation Modes" (CMMs).
The CMMs include an unmodulated carrier, a carrier modulated with an Acquisition sequence,
a carrier modulated with TC symbols corresponding to one CLTU, and a carrier modulated
with an Idle sequence. Using an appropriate PLOP, the CLTUs are radiated to the spacecraft
as physical waveforms. Issue 6 Page 3-13 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE PHYSICAL LAYER
END CODING LAYER INPUT: CLTUs OUTPUT: "DIRTY" SYMBOL
PROVIDE FUNCTION: CREATE PLOPs.
MODULATE WAVEFORMS. SERVICE PHYSICAL FUNCTION: SYMBOL DETECT.
DEMODULATE. SYNCHRONIZE. CONNECTION
TO S/C PHYSICAL
PHYSICAL WAVEFORMS. NONE INPUT: MODULATED
PHYSICAL WAVEFORMS. PHYSICAL MEDIUM Figure 3-8: TC Physical Layer At the receiving end of the TC Physical layer, the modulated radio frequency waveforms are
received, detected, demodulated, and symbol-synchronized: the Acquisition sequence provides
a preamble for synchronization purposes. The synchronized "dirty" symbol stream is delivered
to the layer above, along with control information describing the status of the rf processes. 3 . 4 INTER-LAYER DATA EXCHANGE
The present set of CCSDS Recommendations for telecommand deals primarily with the
specification of the layered architecture and the data structures and protocols which operate
ACROSS each layer. The mechanisms for transferring TC data and their associated control
Issue 6 Page 3-14 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE instructions BETWEEN the layers are currently left unspecified. However, the CCSDS is
currently developing a concept for a general set of "standard data interchange structures"
(Reference ) which could facilitate such inter-layer communication, particularly at the
sending end of the TC System. Instances of standard data interchange structures known as
"Standard Formatted Data Units" (SFDUs) will probably be developed to perform the
interconnection of the sending-end layers: at present these are left as potential items of fut...
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- Spring '14