[Sls-slp] FW: proximity-1 clarifications

Kazz, Greg J (313B) greg.j.kazz at jpl.nasa.gov
Thu Sep 24 23:49:18 UTC 2009

For your information.


From: Kazz, Greg J (313B)
Sent: Thursday, September 24, 2009 4:45 PM
To: 'Unnikrishnan E'
Cc: slp at mailman.ccsds.org
Subject: RE: proximity-1 clarifications

Dear Unnikrishnan E,

My answers are below in between ** **.

Best regards,

Greg Kazz

From: Unnikrishnan E [mailto:unnikrishnane at yahoo.com]
Sent: Thursday, September 17, 2009 11:10 PM
To: Kazz, Greg J (313B)
Cc: slp at mailman.ccsds.org
Subject: proximity-1 clarifications

Dear Sir,
We are implementing CCSDS proximity-1 protocol by referring to Recommended Standard CCSDSS 211.0-B-4 Blue Book July 2006 and Green book for additional information.  Request to clarify the following points for our implementation.
1.       SCID (Spacecraft ID)
Do the Proximity link and TC link on an Orbiter share the same SCID (space craft id) or two separate SCIDs will be allotted.
** The orbiter is only assigned one spacecraft ID for all space links. There is the concept of a Global SCID (GSCID) as well. It consists of the SCID plus the Transfer Frame Version Number. **

2.       ASM ( Attached Synchronization Marker)
When convolutional coding is not selected any error is allowed in the ASM (24 bits) for synchronization. ** implementation dependent as to how many bit errors are allowed in the ASM by the frame synchronization process – typically very few bit errors in the ASM are allowed **

When convolutional coding is selected is ASM also convolutionally encoded.  ** correct **

3.       Data Ambiguity Resolution
What is the suggested method for data ambiguity resolution if it is not performed by the Physical layer.
 Is phase ambiguity always resolved at the physical layer by eg., with differential modulation technique.
** CCSDS documents say to use ASM to resolve data ambiguity.  **
** I will forward your question about phase ambiguity resolution on to my colleague, Dennis Lee for an answer **
4.       Removal of IDLEs at receiving side
Is searching for the first occurrence of ASM in the received bits sufficient to remove the IDLEs.
** I don’t understand what you mean by “remove IDLE data” in this context, however IDLE data should be ignored until the first occurrence of the ASM is detected. **

5.       Synchronization Procedure
Is searching for the first occurrence of ASM in the received bits sufficient for synchronization. How any ‘bit slips’ or patterns of ASM in the data bits are taken care in the protocol.
** This ASM pattern was chosen as to minimize the probability of that pattern occurring in the data (low autocorrelation). **
6.       Variable and Fixed frame length.
Is there any difference in the synchronization procedure for variable length frame and fixed length frame. I came across a suggestion to use fixed frame length when BER is poorer than 10x-6. Please comment.
**Here again we are talking about implementation issues. Fixed length frame synchronization is easier to implement. However, We envisioned Prox-1 to transfer messages across the Proximity Link. Messages are typically of variable length and are most efficiently transported over an asynchronous data link. However some agencies have implemented Prox-1 having the transmitter generating fix length frames, but having the receiver handle both fixed and variable length frames.  From a performance point of view, the fixed length frame synchronization typically would achieve synchronization much faster than on a variable length frame. **

7.       Availability of Reference Implementations
Is any standard reference implementation (hardware or software) available for carrying out protocol testing.
** I will poll the CCSDS agencies to find out who has such an implementation and if it is available. For example, NASA/JPL has some of the Proximity-1 functions in a reference implementation, but I am currently uncertain about it’s availability. **

Thanking you,
Unnikrishnan E
Head, Digital Commn. Section,
Digital System Group,
ISAC, Bangalore,

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