<tt><font size=2>> As part of this action, NASA has proposed to the
OLSG group that <br>
> future optical communications links use the CCSDS standard AOS at
<br>
> the frame level. That will ensure that all CCSDS protocols above
<br>
> AOS will be compatible with future optical communications links. <br>
> This is what is currently being done on NASA’s Laser Communications
<br>
> Relay Demonstration (LCRD) project in development. If this is
<br>
> accepted by the group, then the group can focus on standards needed
<br>
> below the frame level.</font></tt>
<br><tt><font size=2>> </font></tt>
<br><tt><font size=2>Dear Bernie,</font></tt>
<br><tt><font size=2> I must say that
I am a little puzzled by the NASA proposed approach. </font></tt>
<br>
<br><tt><font size=2>CCSDS has developed four protocols for the Data Link
Protocol Sublayer of the Data Link Layer:</font></tt>
<br><tt><font size=2>a)TM Space Data Link Protocol </font></tt>
<br><tt><font size=2>b)TC Space Data Link Protocol </font></tt>
<br><tt><font size=2>c)AOS Space Data Link Protocol </font></tt>
<br><tt><font size=2>d)Proximity-1 Space Link Protocol—Data Link Layer
</font></tt>
<br>
<br><tt><font size=2>The TM and AOS Space Data Link Protocols use fixed-length
Transfer Frames to facilitate robust synchronization procedures over a
noisy link, while the TC</font></tt>
<br><tt><font size=2>Space Data Link Protocol and the Proximity-1 Space
Link Protocol use variable-length Transfer Frames to facilitate reception
of short messages with a short delay. </font></tt>
<br>
<br><tt><font size=2>The services offered by the TM and AOS Space Data
Link Protocols (and therefore by their Transfer Frames formats) are largely
equivalent and share similar limitations. Therefore any limitation to constrain
optical links to single frame format should have a very sound rationale.</font></tt>
<br>
<br><tt><font size=2>Moreover, the Coding and Synchronization standards
applied to the downlink are independent from the frame format used at the
upper layer.</font></tt>
<br><tt><font size=2>The only constraints applied are the fixed-length
and the minimum/maximum) size of the frames provided.</font></tt>
<br><tt><font size=2>Therefore I am really puzzled by an approach that
would design the Data Link Coding & Sync Sublayer (for optical links)
based on a single frame structure.</font></tt>
<br><tt><font size=2>Note that this would not only be a limitation for
the current CCSDS asset, but may even jeopardize further evolution of the
Data Link Protocol Sublayer if its lower (sub)layer is too rigid.</font></tt>
<br>
<br><tt><font size=2>To summarize, I find perfectly admissible that the
NASA’s Laser Communications Relay Demonstration (LCRD) project selects
the AOS format, but I do not see enough element to justify this choice
as the only allowed one.</font></tt>
<br>
<br><tt><font size=2>Most likely this will be something to be investigated
in CCSDS.</font></tt>
<br>
<br><tt><font size=2>Best regards</font></tt>
<br>
<br><tt><font size=2>Gian Paolo</font></tt><PRE>This message and any attachments are intended for the use of the addressee or addressees only. The unauthorised disclosure, use, dissemination or copying (either in whole or in part) of its content is not permitted. If you received this message in error, please notify the sender and delete it from your system. Emails can be altered and their integrity cannot be guaranteed by the sender.
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