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Unformatted text preview: l Service to
perform its functions. The Data Routing Service contains two distinct layers of data handling
(1) A SEGMENTATION layer, which prepares the user data units for transfer by
forming them into suitable-sized routable pieces and permitting these pieces to be
multiplexed together for transmission through the command channel. (2) A TRANSFER layer, which encapsulates the routable pieces of user data into
transfer data structures and controls their transmission through the channel,
including any retransmissions needed to ensure delivery of the routable pieces in
sequence without gaps or duplication. The detailed specification of the retransmission mechanism, called the Command Operation
Procedure, is given in Part 2.1 of the Telecommand Recommendation (Reference ).
The Data Routing Service provides an interface between the high-layer “user” elements of
the TC System and the low-layer communications channel through which data flow.
Because the underlying Channel Service inherently includes a noisy signal path, there is a
finite probability that it will introduce an error. Since any error in the user’s data will render
them invalid, it is desirable to break large user data units into relatively small pieces (which
will thus each have a lower probability of being invalidated by transmission error than if the
entire user data unit were sent contiguously). This improves system throughput efficiency
since only small pieces have to be retransmitted when errors are detected. However, there
may also be situations in which the user data units are very small. For efficient transfer, the
Data Routing Service provides the capability to group these small units into larger pieces.
Analysis of the performance of the Channel Service and Data Routing Service under various
conditions is contained in Reference . CCSDS 202.0-B-2 Page 2-1 CCSDS HISTORICAL DOCUMENT November 1992 CCSDS HISTORICAL DOCUMENT
CCSDS RECOMMENDATION FOR TELECOMMAND: DATA ROUTING SERVICE APPLICATION PROCESS LAYER TELECOMMAND,
PART 3: DATA
MANAGEMENT SERVICE COMMAND
SYSTEM MANAGEMENT LAYER
TC APPLICATION DATA
PACKETIZATION LAYER TC USER DATA UNIT SEGMENTATION LAYER TELECOMMAND,
PART 2: DATA
ROUTING SERVICE TC FRAME DATA UNIT
TRANSFER LAYER TC TRANSFER FRAME CODING LAYER
TRANSMISSION UNIT TELECOMMAND,
PART 1: CHANNEL
SERVICE PHYSICAL LAYER PHYSICAL
WAVEFORM Figure 2-1: Telecomm...
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- Spring '14