[Sis-csi] IP Header Compression

[Jason A. Soloff]

Keith / Chris -
 
To give a lunar perspective, the LRO is being designed with a single
command rate (4kbps).  We have received an action at our peer-review to
include a second, lower rate (1kbps or 2kbps were suggested), but
nothing on the order of a few bps.  As for the human missions, at a
minimum, a spacecraft would have to support a voice loop (10-20kbps or
so) as the crew can always throw switches to "command" and that
bandwidth can be used to command from the ground in the event that the
crew could not respond.  Again, not on the order of the few bps you have
been describing for deep space robotics.
 
The other thing to consider is that the architecture should support
human missions beyond Cis-lunar space, and again, those will likely
require higher "minimum" rates than robotics.
 
- Jason
 
____________________________________________
"It's kind of fun to do the impossible." - Walt Disney
 
Jason A. Soloff
Chief Systems Engineer
Exploration Communication & Navigation Systems
Constellation Systems
 
NASA / Goddard Space Flight Center
Code 567 / B19 / S046
Greenbelt, MD 20771
 
Phone: (301)286-1368
Blackberry: (301)356-3708
Fax: (301)286-1750
E-Mail: Jason.A.Soloff at nasa.gov
 
 


________________________________

	From: sis-csi-bounces at mailman.ccsds.org
[mailto:sis-csi-bounces at mailman.ccsds.org] On Behalf Of Krupiarz,
Christopher
	Sent: Wednesday, September 07, 2005 11:12 AM
	To: Keith Hogie; sis-csi at mailman.ccsds.org
	Subject: RE: [Sis-csi] IP Header Compression
	
	
	Keith,
	 
	I do not have extensive R/F knowledge and, therefore, can't
really give you an answer regarding what we would see at the Moon.
However, part of the Cislunar charter is to extend, where possible, the
architecture to Mars where missions currently do have that low of a bit
rate.  The answer may be whether this falls into the "where possible"
category or not and your concern is very valid.  If there is someone
with greater R/F experience than I have on this list who could chime in
(in particular,thoughts on rates on the Moon), that would be very
helpful.
	 
	As for your second point, I'm not sure I follow.  Could you
elaborate a bit on how you would see commanding done in an emergency
situation without using an IP packet?  I probably got lost along the
way, but I made the assumption that in this architecture all packets
that a spacecraft received would be an IP packet.  The hardware decode
command (I think equivalent to our critical commands here) would just be
a bit string, but it would still be in an IP packet.
	 
	BTW, I'll have to double check (well, I guess this would be
triple check now ;), but I may have swapped emergency commanding with
emergency telemetry rates.  Emergency commanding is ~7.8 bps versus
emergency telemetry at 10 bps.
	 
	Chris

		-----Original Message-----
		From: sis-csi-bounces at mailman.ccsds.org
[mailto:sis-csi-bounces at mailman.ccsds.org] On Behalf Of Keith Hogie
		Sent: Tuesday, September 06, 2005 4:40 PM
		To: sis-csi at mailman.ccsds.org
		Subject: Re: [Sis-csi] IP Header Compression
		
		
		Chris,
		
		  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?
		
		  Also, for hardware decode commands like hardware
reset, I'm not sure if packet sizes or IP or CCSDS headers really
matter.  Isn't a hardware decode command just a string of bits
(hopefully long enough to be unique) that get grabbed by hardware
without any special packet knowledge.  That would mean that this bit
string can be carried inside any packet and the only length that matters
is the length of the hardware command bitstring.
		
		Keith Hogie
		
		Krupiarz, Christopher wrote:
		

			I was curious about thoughts as to if and where
we would address IP header compression in the Green Book.  On some our
missions (and I think this is typical at least in deep space), if we
have a reset of the system, the spacecraft may come up in an emergency
mode of receiving 10 bps.  Hence, every bit is quite valuable at this
point.  With a CCSDS header, we're looking at 6 bytes (plus a couple of
bytes if a secondary header is used).  If we move to IPv6, this becomes
40.  At 10 bps, that's an additional uplink time of 25-26 seconds before
a command can be received which is long enough to envision some
nightmare scenarios.  Clearly IP header compression would alleviate this
concern but I'm not sure where it fits or if it is needed in this doc (I
hope I didn't miss it somewhere).

			Chris 

			
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		  Keith Hogie                   e-mail:
Keith.Hogie at gsfc.nasa.gov
		  Computer Sciences Corp.       office: 301-794-2999
fax: 301-794-9480
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