Unformatted text preview: ervice data units
supplied by the sending user are delivered to the receiving user. COP-1 provides
two services (Sequence-Controlled Service and Expedited Service) to support
these service types, as described in 2.2.1 and 2.2.2.
2.2.1 SEQUENCE-CONTROLLED SERVICE (AD SERVICE) The Sequence-Controlled Service (AD Service) is realized by an automatic ARQ procedure
of the ‘go-back-n’ type, with sequence-control mechanisms of both sending and receiving
ends and a standard report returned from the receiving end to the sending end.
For the AD Service, COP-1 ensures with a high probability of success that:
a) no FDU is lost;
b) no FDU is duplicated;
c) no FDU is delivered out of sequence.
Two types of Transfer Frames are used for providing the AD Service:
a) Type-AD Transfer Frames, for carrying FDUs;
b) Type-BC Transfer Frames, for carrying Control Commands for configuring COP-1
(‘Unlock’ and ‘Set V(R)’).
The AD Service is initiated by means of four distinct ‘Initiate AD Service’ Directives. Two
of these Directives result in the transmission of one of the two Control Commands (Type-BC
Transfer Frames). Each of the two Control Commands causes a well-defined action in
FARM-1, which is then reflected in the CLCW. A timer is used to cause retransmission of
the Control Command if the expected response is not received, with a limit on the number of
automatic retransmissions allowed before it is declared that there is a problem in initiating
the AD Service. The other two Directives allow the AD Service to be started without
waiting for an action by the receiving end (although one requires receiving a good CLCW).
Once COP-1 for a particular Virtual Channel has been initialized for AD Service, FDUs are
inserted into Transfer Frames and transmitted on that Virtual Channel in the order in which
they are presented to COP-1.
The AD Service guarantees in-order delivery of FDUs on a single Virtual Channel. Because
of the possibility of retransmission on only a single Virtual Channel, there is no guarantee CCSDS 232.1-B-2 Page 2-2 September 2010 RECOMMENDED STANDARD FOR COMMUNICATIONS OPERATION PROCEDURE-1 that FDUs on separat...
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- Spring '14
- Space Exploration, ........., European Space Agency, CCSDS