The map feature of the segmentation layer allows the

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Unformatted text preview: Transfer layer, i.e., on one Virtual Channel, thus potentially simplifying the Transfer layer reporting and reducing its associated telemetered traffic. Conversely, it is important to recognize that this potential simplification of the Transfer layer is bought at the price of the increased complexity and communications overhead associated with the Segmentation layer. Missions are therefore free to decide whether or not to implement a Segmentation layer: if the higher layer TC data units (e.g., TC Packets) are all short enough to fit within a maximumlength TC Transfer Frame (and the reporting complexity is acceptable), the Virtual Channel feature of the TC Transfer Frame may be used for multiplexing, and the Segmentation layer may be completely omitted. It will be noted when reading Reference [3] that the CCSDS Recommendations theoretically allow up to 64 Virtual Channels in the Transfer layer, each with up to 64 MAPs attached to it within the Segmentation layer. This theoretically huge multiplexing capability is an artifact of the decision to provide alternative multiplexing mechanisms, and a real implementation will usually use a much more restricted repertoire of MAP and VC capabilities. 3 . 2 . 2 TC TRANSFER LAYER The basic service of the TC Transfer layer is the GUARANTEED error-free communication of higher layer TC data units to the receiving end of the layer above, correct and without omission or duplication, and in the same sequential order in which they were received from the layer above at the sending end. THIS GUARANTEED SERVICE IS CENTRAL TO THE OPERATING PHILOSOPHY OF THE TC SYSTEM: the Transfer layer is therefore the "core" of the standard CCSDS telecommanding concept. In order to provide this service, the TC Transfer layer draws upon the supporting lower layer TC Channel Service. Figure 3-5 depicts the activities and interfaces of the TC Transfer layer. The sending end of the TC Transfer layer encapsulates each higher layer TC data unit (e.g., TC Packet, TC Segment or a non-standard user data structure) within the data field of a TC Transfer Frame: one (and ONLY one) TC data unit is inserted into the data field of each frame. The header of the TC Frame contains key data link control information such as spacecraft and Virtual Channel identification, frame sequence number, and frame length. The operating configuration of the TC Transfer layer is specified by control instructions received from higher layers. The sending end of the Transfer layer formulates special TC Transfer Frames called "Control Commands" which it transmits to a "Frame Acceptance and Issue 6 Page 3-8 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE TRANSFER LAYER PEER PROCESS SENDING END RECEIVING END SEGMENTATION LAYER INPUT: SEGMENTS. DELIVERY CONDITIONS. OUTPUT: SEGMENTS. FUNCTION: ENCAPSULATE SEGMENTS AND DELIVERY CONDITIONS INTO TRANSFER FRAMES. MULTIPLEX FRAMES BY VIRTUAL CHANNEL. MONITOR FRAME ACCEPTANCE. INITIATE TRANSMISSION/ RE...
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This document was uploaded on 03/06/2014.

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