[Css-csts] FW: Ambiguous specification of CLTU transfer buffer behavior

[Wolfgang.Hell at esa.int]

John,

Firstly you are correct that the F-CLTU specification does not prescribe any
specific implementation of the interface between the CLTU buffer and the
modulator. In practice, in particular if also higher command rates shall be
reported, I would assume that most implementations will feature a double
buffer as to make sure that radiation of consecutive CLTUs can be contiguous
without idle sequence in between except where that is imposed by the
delay-time specified by the service user.

The real-world implementations that I have seen so far all provided ample
buffering so that parameter cltu-buffer-available is of hardly any interest
from an operational perspective as it won't be filled up anyway in any
realistic scenario.

Besides that I feel that the specification is not ambiguous in the sense that
the provider is supposed to report the available buffer at the point in time
when the CLTU-TRANSFER-DATA response is generated. I concede that because of
the unspecified input buffer to the modulator two different implementations
with the same buffer capacity under the same operational condition may report
different values for the available buffer capacity. I do not regard that to
be a problem.

The above is my personal view on this issue and other members of the WG may
have a different view. If so, I would appreciate to get such input as soon as
possible so that I can take that into account before sending the updated CLTU
spec to the CCSDS secretariat.

Best regards,

Wolfgang




                                                                             
             "John Pietras"                                                  
             <john.pietras at gst.                                              
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                                        [Css-csts] FW: Ambiguous             
             30/05/2008 18:41           specification of CLTU transfer       
                                        buffer      behavior                 
                                                                             
                                                                             
                                                                             
                                                                             
                                                                             
                                                                             




Members of the CSTSWG:

The following email exchange describes an ambiguity encountered in the
defined behavior of the CLTU buffer in the FCLTU specification. If you have
an interest in this issue, please comment on whether the behavior of the CLTU
buffer should be more tightly defined in the FCLTU specification.

Best regards,
John

From: John Pietras
Sent: Friday, May 30, 2008 12:33 PM
To: 'Boxell, Jeff'; css-csts at mailman.ccsds.org
Cc: Michael Stoloff; Kazz, Greg J (X-JPL); Peter Shames; Erik Barkley; Fred
Brosi; Petkevich, Michael; Kluksdahl, Norman C. (JSC-DD22); Hill, Frank9
Subject: Ambiguous specification of CLTU transfer buffer behavior

Jeff,
It’s possible that the ambiguity that you have encountered was intentionally
left “loose” to accommodate some flexibility in vendor implementations and
user requirements, but I am not certain of that. I’ll forward the pertinent
part of your note to the membership of the Cross Support Transfer Service
Working Group, which is responsible for maintaining the SLE specifications.

Best regards,
John


From: Boxell, Jeff [mailto:jeff.boxell at lmco.com]
Sent: Friday, May 23, 2008 11:55 AM
..
Subject: RE: SLE provider

… I would like to get your thoughts on a possible upgrade to the SLE Spec
based the difficulties we experienced having 1 SLE User implementation talk
to two different vendor SLE provider implementations (one vendor user for
real-time operations, another used strictly for simulations).  Each vendor
managed the reporting of the CLTU buffer level differently.  One vendor
extracted a CLTU to be radiated and immediately started radiating the CLTU;
the other vendor extracted a CLTU and staged it for radiation so that 2 CLTUs
were typically outside the CLTU buffer (one being radiated, the other waiting
to be radiated).   To keep our buffer level management and reporting in sync,
we asked the SIM vendor to tailor their implementation to meet the other’s
implementation and they willingly did this.  Together we looked at the Specs
and saw that this level of detail was not explicitly specified.  Can you give
me some feedback on what might be done to prevent others from encountering
this problem (e.g. should I submit an update to the Spec)?
…

Jeff Boxell
Lockheed Martin Mission Services
Houston, Tx
281-853-2240_______________________________________________
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