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Changes to Hail PDU as a Result of a Single Lander Communicating With Multiple Orbiters. Matthew Cosby Space Link Protocols Working Group Berlin 2011 16 th – 20 th May 2011. Contents slide. 1 Issue and Scenario 2 Proposed Changes 3 Proposed Changes to the Standard. - PowerPoint PPT Presentation
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Changes to Hail PDU as a Result of a Single Lander Communicating With Multiple Orbiters
Matthew CosbySpace Link Protocols Working GroupBerlin 2011
16th – 20th May 2011
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Contents slide
1 Issue and Scenario
2 Proposed Changes
3 Proposed Changes to the Standard
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1Issue and Scenario
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1 Issue and Scenario
Issue:
• Forward and Return sequence numbers are not reset at hail
− The Standard relies on the sequence numbers being the same at the start of a pass
− This synchronisation is performed outside the protocol (either zeroed at power up or set by the vehicle controller)
• There is only a Set V(R) directive, not a Set V(S)
Scenario :
• A Lander on the surface with two or more Orbiters acting as a communications relay
• The Lander is not power cycled during the passes of the different Orbiters
• The Lander does not have its sequence numbers (N(R) and N(S)) reset by the vehicle controller between the 2 passes
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1 Pass of the First OrbiterScenario :
• For the first pass the Lander and Orbiter have been powered up before the start of the link session. Both the Lander’s and Orbiter’s sequence numbers are set to zero
• In this scenario the link session is established and continues with the transfer of data
• Once the link session has ended, then the Lander’s sequence numbers are now the last values used during this pass
N(R) = 0, N(S) = 0
Start Communication Finish
N(R) = n, N(S) = n
n = between 0-255
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1 Pass of the Second OrbiterScenario :
• For the second pass the new Orbiter will have been powered up before the start of the link session with its sequenced numbers set to zero
• The Lander has the sequence numbers set to the values at the end of the last pass
• The Lander’s and Orbiter’s sequence numbers will not be in sequence and as a result no Sequenced Communications will be able to be performed
• There is only a 1 in 256 chance that the sequence numbers will be the same
N(R) = n, N(S) = n
Start No Communication Finish
N(R) = n, N(S) = n n = between
0-255
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2Proposed Changes
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2 Proposed Changes
Required Changes:
• Include a directive in the Hail PDU that synchronises the sequence numbers
There are 3 Implementation options :
1. Either, include the “Set V(R)” directive in the Hail PDU. This directive meaning needs to change to allow the resetting of both N(R) & N(S) on the remote side.
2. Or, include the “Set V(R)” directive in the Hail PDU and a new “Set V(S)” directive. This new directive can be a modification of one of the spare bits of the current Set V(R) directive.
3. Or, include the “Set V(R)” directive in the Hail PDU and the remote system initiates a Set V(R) directive as the first response to the Hail.
After conducting a document review, Option 3 requires less of a change to the standard
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3Proposed Document Changes
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3 Proposed Document Changes
Required Document Changes (Option 3):
• Table 6-3: S33
• Table 6-4: S13
• Table 6-7: E3
• Table 6-10: E30
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