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Unformatted text preview: e Virtual Channels, each using the AD Service, will be delivered in the
order initially transmitted.
For the Type-AD Transfer Frames, the automatic retransmission procedure makes use of
several variables, the most notable being the Receiver_Frame_Sequence_Number, V(R); the
Transmitter_Frame_Sequence_Number, V(S); the Next Expected Frame Sequence Number
contained in the CLCW, N(R); and the Frame Sequence Number in the Transfer Frame
Primary Header, N(S) (see figure 2-1). These variables are explained in more detail in
subsections 5.1 and 6.1.
END TR ANSFER
N(R) FARM-1 CLCW Figure 2-1: COP-1 Variables, Frame and Report Values
For transferring Type-AD Transfer Frames, if required, a flow control mechanism (‘Wait’
mechanism) is provided that permits the receiving end to signal that it does not have enough
resources to cope with incoming data.
For Type-BC Transfer Frames, COP-1 makes use of a very small number of variables, the
most notable being the ‘Lockout’ Flag in the CLCW for the ‘Unlock’ Control Command, and
the value ‘N(R)’ in the CLCW for the ‘Set V(R)’ Control Command.
2.2.2 EXPEDITED SERVICE (BD SERVICE) The Expedited Service (BD Service) is normally used either in exceptional operational
circumstances, typically during spacecraft recovery operations, or when a higher layer
protocol provides a retransmission capability.
Type-BD Transfer Frames are used for carrying FDUs of the BD Service. Type-BD Transfer
Frames are transmitted only once (i.e., no retransmission). There is no guarantee that all
Type-B FDUs are delivered.
In the FOP-1 at the sending end, Type-BD Transfer Frames are given immediate access in so
far as is permitted by delays in the lower layers and procedures by other traffic on the
communications link. At the receiving end, the FDUs carried by Type-BD Transfer Frames
will be passed to the next procedure immediately. CCSDS 232.1-B-2 Page 2-3 September 2010 RECOMMENDED STANDARD FOR COMMUNICATIONS OPERATION PROCEDURE-1 NOTES
1 Some implementations of FARM-1 may use the same buffer to deliver FDUs carried
by either Type-AD or Type-BD T...
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