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Unformatted text preview: lecommand Page 6 3. Position of the systematic retransmission
FOP-1 is a procedure in the Transfer Sublayer of the Data Link Layer. The Transfer Sublayer receives
a data unit from the next layer or sublayer above, typically the Segmentation Sublayer. Within the
Transfer Sublayer, the data unit is called a Frame Data Unit (FDU) and it is placed in the Transfer
Frame Data Field of a TC Transfer Frame.
What is the best position within the layers for the actions that implement the proposed systematic
retransmissions? The options for the position fall into three obvious groups:
Option 1 - Before FOP-1
Option 2 - Inside FOP-1
Option 3 - After FOP-1 3.1 Option 1: Before FOP-1
Could the systematic retransmission be implemented before FOP-1?
Section 2.4 above discusses how to cause multiple transmissions by setting different values for FOP-1
variables, such as the timer. This is complex to automate and is not the best way to implement the
systematic retransmissions of frames.
The proposed systematic retransmission of frames depends on existing behaviour in FARM-1 at the
receiving end to reject duplicate frames, and this depends on the frame sequence numbers. The frame
sequence numbers are set by FOP-1, so this excludes any mechanism for submitting the same FDU
multiple times to FOP-1.
Note: It would not be completely impossible to manipulate the FOP-1 sequence number from outside.
For example, a method to repeat the submission of a sequence of FDUs could include terminating
FOP-1, setting its internal sequence number and then initialising it again. However, this would be
extremely complex and would need more management than the methods described in section 2.4
above. To produce a hammer pattern of retransmissions would need even more intervention. TCretrans7 2008Sep09 I.B. + M.A. DE LANDE LONG
SOFTWARE + CONSULTANCY
Options for repeated retransmissions for telecommand Page 7 3.2 Option 2: Inside FOP-1
Clearly, the systematic retransmission can be implemented inside FOP-1. How can it be done
effectively and efficiently, but with a minimum of modifications to the existing FOP-1...
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- Spring '14