[Sis-csi] Cislunar Section 8

[Scott,Keith L.]

For Chris's example I think AOS both ways is fine.  I suspect his intent
was mainly to show heterogeneity of data link layers, which is good.  We
could show AOS on the downlink and TC on the uplink, but given the
current directions within ESMD towards more symmetric communications,
AOS both ways may be more representative of how things will look in the
future.  Considering that there are proposals to use link encryption for
security, I suspect that R/S decoding in space would not be considered
onerous.
 
When we get to the next level of detail,we will need to worry about the
data link layer mechanisms, as they have a bearing on the ability to
effect emergency operations (I'm thinking very low-level hardware
commands).  At that point we will need to come up with a _small_ set of
Recommended approaches for data flows.  I see this is populating the
general pipe diagrams we build in the Green book with specific
protocols.
 
        --keith

________________________________

	From: sis-csi-bounces at mailman.ccsds.org
[mailto:sis-csi-bounces at mailman.ccsds.org] On Behalf Of Keith Hogie
	Sent: Wednesday, August 24, 2005 1:11 AM
	To: sis-csi at mailman.ccsds.org
	Subject: Re: [Sis-csi] Cislunar Section 8
	
	
	Adrian J. Hooke wrote: 

		At 12:17 PM 8/23/2005, Keith Hogie wrote:
		

			3 - The protocol stack diagram is nice but I'm
not sure about the AOS boxes.  Normally AOS is used for data coming down
from space.


		Not so - the original architecture that was developed
for ISS was completely symmetric - the AOS frame was intended to be used
as either a unidirectional or a bi-directional space link protocol. See
http://public.ccsds.org/publications/archive/701x0b3.pdf page 2-3.
		


	I checked that document on Tues. and saw that is said that AOS
could be used either way.  But the comment was that missions have always
used it on the downlink only.  If there are any missions using AOS in a
two-way mode it would be good to know about them.  Are there any ground
systems or satellite systems that can use AOS on the uplink?  
	 
	

			It also has Reed-Solomon coding on it.  The
diagram uses AOS in both directions.  Are we proposing the use of AOS
and Reed-Solomon coding both ways.  This would require R/S encoders at
ground stations and decoders installed on spacecraft?


		The CCSDS space link protocols are cleanly layered:
http://public.ccsds.org/publications/archive/130x0g1.pdf
		
		In particular, the current AOS Space Link Protocol as
defined in:
		http://public.ccsds.org/publications/archive/732x0b1.pdf
is decoupled from its underlying coding layers.
		
		The coding layer that underlies AOS or conventional TM
space links is defined in:
http://public.ccsds.org/publications/archive/131x0b1.pdf and note that
it is NOT confined to R-S coding. It may be also expected to evolve as
new codes (such as LDPC) mature.
		
		

	All of those documents describe how the sync mark, R-S
codeblocks, and data link framing are all directly related to each
other.  They all show that the same sync information is used for both
the FEC coding and data framing.  This is not the FEC and data link
decoupling that has been used in commercial satellite modems where the
link coding is separate from the data link framing.  
	
	-- 
	
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	  Keith Hogie                   e-mail:
Keith.Hogie at gsfc.nasa.gov
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