45 451 transfer of packets general how packets are

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Unformatted text preview: mal overview of the procedures for transferring Packets with Transfer Frames. The exact procedures are specified in the Recommended Standards (references [1], [2] and [3]). 4.5.2 TRANSFER OF PACKETS BY THE TC SPACE DATA LINK PROTOCOL If MAPs (see 4.3.6) are used, Packets whose lengths exceed the maximum length that can be carried by a Transfer Frame are sent by dividing them (i.e., by segmentation) into portions that are compatible with insertion into Transfer Frames. In this case, each Transfer Frame contains either a portion of a large Packet, a single complete Packet, or multiple complete Packets. The portions of a divided Packet must be transferred in consecutive Transfer Frames of the MAP Channel without being interlaced with other Packets or portions of other Packets in the same MAP Channel. If MAPs are not used, segmentation is not performed and each Transfer Frame contains either a single complete Packet or multiple complete Packets. When extracting multiple Packets from a single Transfer Frame, the length field of the Packets is used. Therefore, the location and meaning of the length field of the Packets transferred over the space link must be known to the service provider. (The Packets transferred by the TC-SDLP are limited to those with PVNs authorized by CCSDS.) CCSDS 130.2-G-1 Page 4-7 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PROTOCOLS 4.5.3 TRANSFER OF PACKETS BY THE TM AND AOS SPACE DATA LINK PROTOCOLS Packets are inserted into a Transfer Frame concatenated together until it is full. If a Packet exceeds the Transfer Frame length, it will be split, filling the Transfer Frame completely, and starting a new Transfer Frame on the same Virtual Channel with the remainder. Construction of the next Transfer Frame will continue with the concatenation of Packets until it overflows. The First Header Pointer field of the Transfer Frame will be set to indicate the location of the first octet of the first Packet occurring within the Transfer Frame. The service provider may generate an Idle Packet in the absence of sufficient Packets supplied from the sending users to fill a Transfer Frame at release time. The format of the Idle Packet is defined by reference [13]. If it is necessary, the service provider may generate a Transfer Frame without any Packets using the methods specified in the Recommended Standards (references [2] and [3]). At the receiving end, the service provider extracts Packets from Transfer Frames. The First Header Pointer of Transfer Frames will be used in conjunction with the length field of each Packet contained within the Transfer Frames to provide the delimiting information needed to extract Packets. Therefore, the location and meaning of the length field of the Packets transferred over the space link must be known to the service provider. (The Packets transferred by the TM-SDLP and AOS-SDLP are limited to those with PVNs authorized by CCSDS.) CCSDS 130.2-G-1 Page 4-8 December 2007 CCSDS REPORT CONCERNING THE SPACE DATA LINK PR...
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This document was uploaded on 03/06/2014.

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