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Unformatted text preview: ove to be carried from the ground
to the spacecraft. If the layer above the Segmentation layer is the Packetization layer, the TC
User Data Unit will be a TC Packet. If the layers above the Segmentation layer do not
conform to the CCSDS telecommand architecture, the TC User Data Unit may be any other
higher-layer user data structure. Certain services of the Segmentation layer are provided only
for TC Packets. CCSDS 202.0-B-2 Page A-5 CCSDS HISTORICAL DOCUMENT November 1992 CCSDS HISTORICAL DOCUMENT
CCSDS RECOMMENDATION FOR TELECOMMAND: DATA ROUTING SERVICE TRANSFER LAYER:
The bottom layer of the Data Routing Service, which performs the transfer of user data
structures to the receiving spacecraft.
TYPE-A (ACCEPTANCE MODE) FRAMES:
TC Transfer Frames which have a flag set indicating that they are to be tested against the
Frame Acceptance Check.
TYPE-B (BYPASS MODE) FRAMES:
TC Transfer Frames which have a flag set indicating that they are NOT to be tested against
the Frame Acceptance Check, but should be delivered as soon as they pass the Frame
A Type-B Control Command which resets a FARM LOCKOUT condition.
VIRTUAL CHANNEL (VC):
Virtual Channels are provided by the Transfer layer, which interfaces with the single physical
channel in the layers below and presents the service of apparently separating this single
channel into multiple “virtual” paths to the Segmentation layer.
A Virtual Channel (VC) is simply a unique, multi-bit ID code assigned to a particular
sequence of TC Transfer Frames to enable all the frames which are members of that sequence
to be identified. When different Virtual Channel IDs are assigned to different TC Transfer
Frame sequences, the sequences can be multiplexed onto the physical channel at the sending
end and then sorted at the receiving end back into their proper parent sequences. Necessarily,
when Virtual Channels are not used, the physical channel in concept is the same as one
Virtual Channel. Virtual Channels provide an alternative method for multiplexing user data
structures if the multiplexing feature of the Segmentation layer is not used.
An indication from the FARM, cont...
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