[Sls-slp] Questions about Proximity -1 space data link protocol

[Kazz, Greg J (312B)]

Dear Xiongwen He,

As a member of NASA, I can answer any ambiguities you have with the CCSDS Proximity-1 Space Data Link standard. However, I cannot answer any questions concerning the implementation details of this CCSDS recommedation. However, I have included the entire Space Link Protocol (SCP) working group mailing list, in case any of the members would like to answer or comment on your implementation related questions.

Please see my responses between **…** below.

Best regards,
Greg Kazz
Chairman CCSDS SLS-SLP WG

From: he xiongwen <hexw501 at hotmail.com<mailto:hexw501 at hotmail.com>>
Date: Saturday, September 10, 2016 at 2:52 AM
To: "Kazz, Greg J (313B)" <greg.j.kazz at jpl.nasa.gov<mailto:greg.j.kazz at jpl.nasa.gov>>
Subject: Questions about Proximity -1 space data link protocol


Dear Mr. Greg Kazz,


         I am from China Academy of Space Technology(CAST), and we are trying to adopt Proximity -1 space data link protocol to CAST standard.During the process, we find that some issues are not very clear, and I am looking for help from you.


     (1)   In the Proximity -1 data link layer recommendation,  there is a description as follows

“6.2.3.9 RECEIVING_SCID_BUFFER
RECEIVING_SCID_BUFFER shall be used in the frame validation process to compare a
received spacecraft ID value with that held within this buffer. This buffer may be loaded by
a directive from the local vehicle controller, or it may be loaded with the spacecraft ID
contained in the first valid received frame.”

"6.7.2 FRAME VALIDATION
c) If the Spacecraft ID field in the transfer frame header does not contain the value equal
to the RECEIVING_SCID_BUFFER for all frames received (i.e.,
Remote_Spacecraft_ID, MIB parameter) when the RECEIVING_SCID_BUFFER is
non-zero and the Source-or-Destination Identifier value equals ‘0’, i.e., source, and
Test_Source is true, a session violation has occurred and the vehicle controller is
notified (see annex D).
When the RECEIVING_SCID_BUFFER is zero and
Test_Source is true, the value of the SCID field in the header of the first received
frame whose Source-or-Destination ID is source is loaded into
RECEIVING_SCID_BUFFER and thereafter used for validation for the remainder of
the session. (See also related 6.2.4.2.)"


The question is:
a) is RECEIVING_SCID_BUFFER always equal to Remote_Spacecraft_ID, MIB parameter?
** Not necessarily. The test mentioned above still needs to be made. Depends upon the topology of the network involved. **
In a situation "When the RECEIVING_SCID_BUFFER is zero and
Test_Source is true, but the value of the SCID field in the header of the first received
frame whose Source-or-Destination ID is source  is not equal to Remote_Spacecraft_ID"
will the SCID also be loaded to the RECEIVING_SCID_BUFFER? If not, what actions will be taken? ** Yes. RECEIVING_SCID_BUFFER is the recommended data store for this information **
b） Is the value Remote_Spacecraft_ID loaded to RECEIVING SCID BUFFER by using directive “SET RECEIVING SCID BUFFER” . ** Per paragraph 6.3.3.1.7 SET RECEIVING SCID BUFFER
shall be used by the vehicle controller to load the RECEIVING_SCID_BUFFER. When the value of the SCID is known a priori, then the value in the Remote_Spacecraft_ID is used. When that value is not known a priori, then it can be determined by examining the SCID found in the first valid transfer frame received.
.
On what condition will this directive be used? ** see answer immediately above**

(2) in Figure 5-1: Proximity Time Tag Recording
Is  Proximity Time Tag Recording process responsible for transfer the collection of tags to time Correlation Process? ** No. Please see note 5.3: NOTE
When time correlation data sets canbe transferred, the time correlation process can be performed. The actual implementation details of this process are outside the scope of this specification. **
Do you have any examples of  time Correlation Process, what do this process usually do ? ** I cannot provide you with implementation examples**
Is Start Time Tag Collection Process command sent by ground or by on board process?** This is an implementation choice . No comment **When will the Initiator begin to collect the time tags? ** When the time tag activity is active I.e., once the initiator has built and sent the SET CONTROL PARAMETERS directive per paragraph 5.2.6 **
How do the initiator know the SET CONTROL PARAMETERS directive has been received by RESPONDER? ** Doesn’t absolutely know if it was or was not received. But it doesn’t matter to the initiator. It will only supply what it can supply. If it receives transfer frames from the responder than it will also time tag those frames as well as the ones it is transmitting. It is a best efforts activity as specified in chapter 5. **

(3) in Figure 5-2: Transferring Time to a Remote Asset
Between"Extract the enterprise time from
the TIME DISTRIBUTION directive
providing it to the responder’s
master clock" and the transfer of "TIME DISTRIBUTION directive", is there any delay? I see the two arrows are connect directly in the figure. ** The figure is only notional and per CCSDS figures in blue books are not normative. Clearly there will be a time delay between extracting the time and providing it in directive form **
is the action "Extract the enterprise time from the TIME DISTRIBUTION directive providing it to the responder’s
master clock" implemented by proximity-1 protocol or upper layer application? ** I can only say that this function is not in the scope of Proximity-1 Space Data Link Protocol **
How does TIME DISTRIBUTION directive initiated ? By ground command or on board command? ** This is an implementation choice. No comment. **

I would appreciated if you could reply and help me clarify the questions. **Best regards, Greg Kazz **

Best Regards,
Xiongwen He

 China Academy of Space Technology(CAST)
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