[Sls-ocm] Help with OLSG Action Concerning Applicable CCSDS Protocols / Tranfer Frame vs. AOS Frame

[Gian.Paolo.Calzolari at esa.int]

> As part of this action, NASA has proposed to the OLSG group that 
> future optical communications links use the CCSDS standard AOS at 
> the frame level.  That will ensure that all CCSDS protocols above 
> AOS will be compatible with future optical communications links. 
> This is what is currently being done on NASA?s Laser Communications 
> Relay Demonstration (LCRD) project in development.  If this is 
> accepted by the group, then the group can focus on standards needed 
> below the frame level.
> 
Dear Bernie,
        I must say that I am a little puzzled by the NASA proposed 
approach. 

CCSDS has developed four protocols for the Data Link Protocol Sublayer of 
the Data Link Layer:
a)TM Space Data Link Protocol 
b)TC Space Data Link Protocol 
c)AOS Space Data Link Protocol 
d)Proximity-1 Space Link Protocol?Data Link Layer 

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
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. 

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.

Moreover, the Coding and Synchronization standards applied to the downlink 
are independent from the frame format used at the upper layer.
The only constraints applied are the fixed-length and the minimum/maximum) 
size of the frames provided.
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.
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.

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.

Most likely this will be something to be investigated in CCSDS.

Best regards

Gian Paolo
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