[Sis-csi] Cislunar Section 8

[Lee.Neitzel at EmersonProcess.com]

Having worked on the upper layers of the AOS, I find Adrian's comment
completely accurate. We developed the AOS with the idea in mind that
spacecraft, such as the space station, would need balanced protocols.
Therefore, AOS was architected so that the upper layers could be used
onboard in a peer manner and the entire stack used bidirectionally on the
space link, where the R/S applies.

 

  _____  

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 12:11 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
<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
<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
<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
<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.  



-- 
----------------------------------------------------------------------
  Keith Hogie                   e-mail: Keith.Hogie at gsfc.nasa.gov
<mailto:Keith.Hogie at gsfc.nasa.gov> 
  Computer Sciences Corp.       office: 301-794-2999  fax: 301-794-9480
  7700 Hubble Dr.
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