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Unformatted text preview: her higher layer user data structures) are input to the sending end of the layer, plus data routing control instructions (assignment of TC Packets or TC Files to particular MAPs). The input data are first broken into pieces (segments) which will fit into the data field of the lower layer TC Transfer Frames. Each segment must only contain a piece of ONE input data unit, i.e., a new segment must be started at each boundary between higher layer data units. Every segment is labelled with a Segment Header that conveys segment order. The segment is further labelled to identify with which multiplexing port it is associated, so that it may be properly routed and reconstructed at the receiving end: this is accomplished by assigning a MAP identifier to each segment, also conveyed in the Segment Header. The completed TC Segment is output to the layer below, the TC Transfer layer, for encapsulation within the data field of one TC Transfer Frame. Issue 6 Page 3-6 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE SEGMENTATION LAYER PEER PROCESS SENDING END RECEIVING END PACKETIZATION LAYER INPUT: TC PACKETS. TC FILES. OUTPUT: TC PACKETS. TC FILES. FUNCTION: SEGMENT SETS OF PACKETS TO FIT TRANSFER FRAMES. MULTIPLEX SEGMENTS BY MULTIPLEXER ACCESS POINT (MAPs). INTERFACE SERVICE PACKETS WITH TRANSFER FUNCTION: RECONSTRUCT SETS OF PACKETS FROM SEGMENTS. LAYER SEGMENTS OUTPUT: SEGMENTS AND DELIVERY CONDITIONS. NONE INPUT: SEGMENTS. TRANSFER LAYER Figure 3-4: TC Segmentation Layer At the receiving end of the TC Segmentation layer, TC Segments are received from the layer below, sorted by MAP, and the individual higher layer TC data units are reconstructed prior to passing them across the interface to the layer above. Since the Segmentation layer relies completely on the guaranteed delivery service of the layer below, it contains no layer-unique reporting procedures. Appreciating the functional utility of the multiplexing feature of the Segmentation layer requires an understanding of the characteristics of the underlying Transfer layer. The Transfer layer, which utilizes TC Frames as its data units, interfaces with the single physical data channel in the layer below it. The TC Frames themselves feature an independent multiplexing capability, since each frame may be assigned with its own "Virtual Channel" (VC) identifier. Individual TC Frames may (by giving them different VC identifiers) each carry different higher layer TC Issue 6 Page 3-7 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE data units: this therefore provides a Transfer layer multiplexing capability which is functionally similar to the Segmentation layer MAPs. However, since the Transfer layer contains the closed-loop retransmission procedures which control the delivery of TC Frames to the spacecraft, having a large number of VCs that are simultaneously "open" will increase the complexity of the reporting mechanism between the receiving and sending end of this layer. The MAP feature of the Segmentation layer allows the user data multiplexing to be performed ABOVE the...
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This document was uploaded on 03/06/2014.

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