[Sis-csi] IP Header Compression

[Keith Hogie]

Adrian J. Hooke wrote:

> At 01:40 PM 9/6/2005, Keith Hogie wrote:
>
>>   I agree we need to consider issues with small packets and low 
>> rates, but how low do we need to go.  In all of the missions I have 
>> seen (non deep space), the lowest data rates are 125 bps.  This is 
>> over an order of magnitude difference from your 10 bps. 
>>
>>   For the Cislunar environment, we need to figure out what some of 
>> our limits are.  Do we really want to burden the Cislunar design with 
>> issues that only relate to Deep Space?
>
>
> Aren't the Lunar missions supposed to be "training" for going to Mars? 
> For critical emergency commanding operations, shouldn't we be 
> developing a robust, unified, reliable, tested system that works 
> wherever you go?
>
 
  For emergency commanding I don't see any difference between Cislunar 
or Deep Space.  In both cases the solution is to send a string of bits 
that gets decoded by hardware and do not need any protocol.  The 
critical hardware commands are their own frame sync, authentication, and 
command all packed into a highly unique string of bits.  Most hardware 
decoders pick off the bits they are looking at directly from the 
receiver and don't involve any flight software.  This means that there 
is no complex packet processing and the hardware is just looking for  
particular sequence of bits.  The length of this sequence is not a 
function of any CCSDS or IP headers.  Getting the command to the 
spacecraft just requires radiating the proper string of bits.  The 
length of the hardware command is just a function of how many bits you 
think you need to make sure your command doesn't occur in normal data 
transfers. 

 My main concern is for all the other operational modes there is a very 
large disconnect between things that will work in a Cislunar environment 
and a long haul link to Mars.  If you consider the following round trip 
times (RTT):

0000.1 sec - Interaction between rovers, landers, (e.g. local environment)
0000.1 sec - Low-Earth orbit  ( a few hundred kilometers one-way)
0000.1 sec - Low-Lunar orbit ( a few hundred kilometers one-way)
0000.1 sec - Low-Mars orbit   ( a few hundred kilometers one-way)
0000.5 sec - Earth geosync orbit  (36,000 kilometers one-way)
0002.5 sec - Earth-to-Moon (384,000 kilometers one-way)
0010.0 sec - Earth to L1 or L2 (1,500,000 kilometers one-way)
------------------Limit of Cislunar domain
0366.0 sec - Earth to Mars (closest = 55.000.000 kilometers one-way, 6 
minute, RTT)
2673.0 sec - Earth to Mars (farthest = 401,000,000 kilometers one-way, 
45 minute RTT)

  When you look at distances like these there is a huge break between 
Cislunar ones and Mars.  In the Cislunar  area it is actually possible 
to do interactive things like interactive audio, video, and data 
access.  You can consider security protocols that negotiate security 
details.  At L1 and L2 things get a bit uncomfortable at 10 seconds RTT 
but that is still manageable.  At Lunar distances you can do most 
anything you do on Earth.  A 2.5 second delay is a bit long for some 
interactive operations but it is not really any longer than what happens 
when you surf the open Internet and hit a bit of congestion.  The main 
point is that out to L1 and L2 you can actually do interactive 
operations  This also applies to systems on Mars and orbiting around Mars. 

  However, when you move to the long haul link between Earth and Mars, 
the RTT jumps up to over 100 or 1,000 times that of the Earth and Moon.  
With a 6 to 45 minute RTT, you can't carry on an interactive voice or 
video conversation and lots of interactive data access just doesn't 
work.  On a Earth-to-Mars link you are forced to shift to an operations 
concept of two one-way links.  Operations must shift into email-like 
file store-and-forward or one-way streaming of data. 

   So I don't see any real problem with using the same hardware 
commanding solution in Cislunar or Earth-to-Mars scenarios.  Some file 
store-and-forward and one-way streaming operations will also work for 
both environments.  Of course any acknowledgments on the file-store-and 
forward will take lots longer. 

  My concern is that other there are lots of protocols and applications 
that will work fine in an interactive Cislunar environment but just 
don't work for Earth-to-Mars.  We don't want to limit our Cislunar 
solutions to only those that will also work for Earth-to-Mars.  I think 
we need to develop our Cislunar solutions and then see if any of them 
will also work in a Earth-to-Mars scenario. 

----------------------------------------------------------------------
  Keith Hogie                   e-mail: Keith.Hogie at gsfc.nasa.gov
  Computer Sciences Corp.       office: 301-794-2999  fax: 301-794-9480
  7700 Hubble Dr.
  Lanham-Seabrook, MD 20706  USA        301-286-3203 @ NASA/Goddard
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