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<DIV dir=ltr align=left><SPAN class=590480214-30092005><FONT face=Arial
color=#0000ff size=2>While we're pushing to get as much data as possible over
IP, there will probably still be for a long time some data that wants to sit
directly on top of the space link. These could be legacy applications that
run directly over space data packets and don't want to move (and know that they
will not need to go multi-hop), or 'special' commands that just can't deal with
the overhead of IP. The canonical example of the latter is the 'tumbling
spacecraft' problem, where the spacecraft is spinning and it won't hold still
long enough to point its antenna at Earth to receive the 'stop that'
command. This actually occurred, and the saving grace was being able to
transmit (over and over and over) a very short 'safe the spacecraft' command
that was hardware decoded. While you're right that one might look for a
particular bit stream that happens to be an IP packet in the hardware decoder,
there might be advantage to being able to do it in fewer bits. Then again,
there's security, which would seem to be sort of mutually exclusive with 'short
command that causes the spacecraft to do something sort of drastic (like safe
mode).'</FONT></SPAN></DIV>
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color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=590480214-30092005><FONT face=Arial
color=#0000ff size=2>All that said, the way I can see bit rate playing into this
is that we're really interested in how many bits are needed for emergency
command, and how many bits we can inject into the spacecraft before it goes off
and does something else (spins, reboots again, ...). We may need to have
two answers, one for people who absolutely require security (may as well use IP
and IPSEC here) and one for people who don't (allows very short commands but
requires a direct data link connection to the thing doing the
resetting)?</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=590480214-30092005><FONT face=Arial
color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN
class=590480214-30092005> <FONT
face=Arial color=#0000ff size=2>--keith</FONT></SPAN></DIV><BR>
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<FONT face=Tahoma size=2><B>From:</B> sis-csi-bounces@mailman.ccsds.org
[mailto:sis-csi-bounces@mailman.ccsds.org] <B>On Behalf Of </B>Krupiarz,
Christopher<BR><B>Sent:</B> Wednesday, September 07, 2005 11:12
AM<BR><B>To:</B> Keith Hogie; sis-csi@mailman.ccsds.org<BR><B>Subject:</B> RE:
[Sis-csi] IP Header Compression<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV><SPAN class=658080714-07092005><FONT face=Arial color=#0000ff
size=2>Keith,</FONT></SPAN></DIV>
<DIV><SPAN class=658080714-07092005><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=658080714-07092005><FONT face=Arial color=#0000ff size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=658080714-07092005><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=658080714-07092005><FONT face=Arial color=#0000ff size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=658080714-07092005><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=658080714-07092005><FONT face=Arial color=#0000ff size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=658080714-07092005><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=658080714-07092005><FONT face=Arial color=#0000ff
size=2>Chris</FONT></SPAN></DIV>
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<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left><FONT
face=Tahoma size=2>-----Original Message-----<BR><B>From:</B>
sis-csi-bounces@mailman.ccsds.org [mailto:sis-csi-bounces@mailman.ccsds.org]
<B>On Behalf Of </B>Keith Hogie<BR><B>Sent:</B> Tuesday, September 06, 2005
4:40 PM<BR><B>To:</B> sis-csi@mailman.ccsds.org<BR><B>Subject:</B> Re:
[Sis-csi] IP Header Compression<BR><BR></FONT></DIV>Chris,<BR><BR> 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. <BR><BR> 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?<BR><BR> 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.<BR><BR>Keith Hogie<BR><BR>Krupiarz,
Christopher wrote:<BR>
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<P><FONT face=Arial size=2>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).</FONT></P>
<P><FONT face=Arial size=2>Chris</FONT> </P><PRE wrap=""><HR width="90%" SIZE=4>
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</PRE></BLOCKQUOTE><PRE class=moz-signature cols="72">----------------------------------------------------------------------
Keith Hogie e-mail: <A class=moz-txt-link-abbreviated href="mailto:Keith.Hogie@gsfc.nasa.gov">Keith.Hogie@gsfc.nasa.gov</A>
Computer Sciences Corp. office: 301-794-2999 fax: 301-794-9480
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