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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Yes, that’s the right direction in
my view.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>A couple of points to throw into the mix. For
the OBL stuff and the fact that we have to “pander” to such horrors
as 1553, there is a subtle reason for this. 1553, and other similar buses like ESA
OBDH impose some limits on the way you do things. In the old days, spacecraft
that adopted these buses were obliged to work within those limits and there
flight architecture and the way the software was designed was basically driven
by this. Subsequent missions tend to base themselves on what went before to a
greater or lesser degree and therefore generate specifications that are similar
to preceding spacecraft. When you design a spacecraft based on these specs, you
find that not surprisingly, they can be implemented quite nicely on 1553 like
buses. So we have a self reinforcing feedback system that is based on outdated
considerations that no one seems prepared to challenge seriously. This
fundamentally is why projects do not give proper consideration to the choice of
onboard bus, and why we keep having 1553 and its like thrust upon us. I think
the OBL concepts as they were originally conceived back in 2001 break this
unhealthy feedback loop and open the way to making a better informed selection
of the onboard bus, but we have not promulgated this concept sufficiently well.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>As for the application needs, onboard
systems are typically run on synchronous distributed deterministic schedules at
the moment, largely because that is the way they have been done before, and
that is what the current onboard buses seem to support best. There are a few
systems that are breaking away from this model (including things like OBOSS)
which are based on asynchronous operation and the exchange of short (10 to 20
octet) fully delimited messages. As Steve pointed out yesterday there are also
users who want to transfer enormous images and usually get dedicated links to
do this. However, while these latter applications might look like candidates
for a stream service, I would contend that this is actually a very risky and inefficient
way of transferring such data. In fact, I cannot imagine any data, even in
terrestrial applications, that is naturally suited to a streaming service!<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Let’s look very carefully at the
formats of the data that onboard users want to exchange, and the size and
frequency of exchange.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Chris. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span lang=EN-US
style='font-size:10.0pt;font-family:Tahoma;font-weight:bold'>From:</span></font></b><font
size=2 face=Tahoma><span lang=EN-US style='font-size:10.0pt;font-family:Tahoma'>
sois-tcons-bounces@mailman.ccsds.org
[mailto:sois-tcons-bounces@mailman.ccsds.org] <b><span style='font-weight:bold'>On
Behalf Of </span></b>dstanton@keltik.co.uk<br>
<b><span style='font-weight:bold'>Sent:</span></b> 01 April 2005 13:06<br>
<b><span style='font-weight:bold'>To:</span></b> sois-tcons@mailman.ccsds.org<br>
<b><span style='font-weight:bold'>Subject:</span></b> [Sois-tcons] TCONS
rationale</span></font><span lang=EN-US><o:p></o:p></span></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal style='margin-bottom:12.0pt'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'><br>
My initial thoughts were to look to the TCOA documentation to find the services
which they are demanding of the underlying transfer layer and which are not
provided by current transport services. However, there isn't much to be got
from their documents.<br>
<br>
We need to identify very succinctly the TCONS USPs (Unique Selling Points) in
bullet form and to identify the arguments for and against existing protocols
which may already have them. So, a first cut would be:<br>
<br>
Bit efficiency (arguments about IP header compression etc.)<br>
Priority (does Diffserv do the job?)<br>
Scheduling (Why?, Which applications?)<br>
<br>
The fundamental arguments against are likely to be (and I'm playing devils's
advocate here):<br>
<br>
Are we really likely to need to have to provide these services across
hetereogeneous busses? Are we compensating for bad spaceraft design and
(flameproof suit on) the failure of OBL to arrive at a standardised bus/LAN?<br>
<br>
Are these services actually required by real applications? <br>
<br>
Can these services be provided by other transport/network layers?<br>
<br>
Can these issues be solved by using conventional protocols across high capacity
busses? The space link guys have the laws of physics working against them. We
don't have such an excuse for having limited bandwidth. Do the power or
availability of rad hard devices arguments wash? If we have spacewire why are
we worried about bit efficiency and priority?<br>
<br>
Fundamentally we are missing a SOIS concept of operations and policies (one
such policy issue is why are we prepared to be iconoclastic at the network
layer and yet pander to entrenched positions at the bus/lan level, trying to
accommodate every flavour of bus that has ever flown or likely to be flown).
Looking at the documentation, ther's a lot of assertion without much in the way
of justification. This may be because I haven't seen some of the earlier work.<br>
<br>
I'm not trying to shoot everyone down here. It's just that if we aren't clear
in our own minds on these issues, it will be very difficult to defend
TCONS/SOIS at the CESG. <br>
<br>
Any thoughts?<br>
<br>
Dai<br>
<br>
<o:p></o:p></span></font></p>
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