<P>Agreed. At present we have, on the one hand, synchronous, inflexible busses which support rigidly planned, inflexible operations and they bot to some extent feed off each other. In many ways, the operators are very comfortable with knowing what information is on the bus to an increment of a clock cycle. However, these busses do not easliy support message-based applications.</P>
<P>I believe SOIS need to take a bold line. SOIS need to make the case for succesful operations using a flexible asynchronous communications model. CCSDS took this line in the space link. It took 10 years for publication to general acceptance but now packet TM/TC is CCSDS's only true success. It was achieved by laying aside current practice in the large agencies and looking to future requirements. I beleive SOIS needs to play just such a long game and not try to buy instant acceptance by adopting too much of the current approach. Outside of the large agencies who have risk and cost pressures to reuse design and equipment over the next few mission cycles, there are missions who would welcome flexible standardised interfaces particularly at the bus/LAN level if only to settle the interfaces between the academic institutions providing payloads and the designers of the bus. 10 years ago, on STRV, we'd have been glad of an international standard for packet transfer to impose on everyone. Without a big stick (like, for instance, a CCSDS recommendation) we had to negotiate interfaces individually.</P>
<P>We need to take the example of packet TM/TC, play to the missions who don't have any institutional intertia and possibly wait for many of the current generation of project managers to expire.<BR>
<BR>
<BR>
<BR>
<B>On Fri, 1 Apr 2005 14:16 , Chris Plummer <c.plummer@skynet.be> sent:<BR>
<BR>
</P></B>
<BLOCKQUOTE style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #f5f5f5 2px solid; MARGIN-RIGHT: 0px"><DEFANGED_META content="text/html; charset=us-ascii" http-equiv="Content-Type"><DEFANGED_META name="Generator" content="Microsoft Word 11 (filtered medium)"><!--
v\:* {behavior:url(#default#VML);}
o\:* {behavior:url(#default#VML);}
w\:* {behavior:url(#default#VML);}
.shape {behavior:url(#default#VML);}
--><!--
<!--
/* Font Definitions */
@font-face
{font-family:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman";}
a:link, span.MsoHyperlink
{color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{color:purple;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-reply;
font-family:Arial;
color:navy;}
@page Section1
{size:595.3pt 841.9pt;
margin:72.0pt 90.0pt 72.0pt 90.0pt;}
div.Section1
{page:Section1;}
-->--> <DEFANGED_BODY lang=EN-GB link="blue" vlink="purple">
<DIV class=Section1>
<P class=MsoNormal><FONT face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial">Yes, that’s the right direction in my view.<o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial">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 face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial">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 face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial">LetR17;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 face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial">Chris. <o:p></o:p></SPAN></FONT></P>
<P class=MsoNormal><FONT face=Arial color=navy size=2><SPAN style="FONT-SIZE: 10pt; COLOR: navy; FONT-FAMILY: Arial"><o:p> </o:p></SPAN></FONT></P>
<DIV>
<DIV class=MsoNormal style="TEXT-ALIGN: center" align=center><FONT face="Times New Roman" size=3><SPAN lang=EN-US style="FONT-SIZE: 12pt">
<HR tabIndex=-1 align=center width="100%" SIZE=2>
</SPAN></FONT></DIV>
<P class=MsoNormal><B><FONT face=Tahoma size=2><SPAN lang=EN-US style="FONT-WEIGHT: bold; FONT-SIZE: 10pt; FONT-FAMILY: Tahoma">From:</SPAN></FONT></B><FONT face=Tahoma size=2><SPAN lang=EN-US style="FONT-SIZE: 10pt; 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 face="Times New Roman" size=3><SPAN style="FONT-SIZE: 12pt"><o:p> </o:p></SPAN></FONT></P>
<P class=MsoNormal style="MARGIN-BOTTOM: 12pt"><FONT face="Times New Roman" size=3><SPAN style="FONT-SIZE: 12pt"><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></DIV></BLOCKQUOTE><BR>