[Sis-csi] Green book thoughts

[David Carek]

 From Lee.Neitzel at EmersonProcess.com, 4/19/2006 10:20 AM Eastern:
...
> which ultimately leads to cost management. We, in the process control 
> industries, do not make our money on protocols. They are a cost to us. 
> Therefore, they are carefully crafted to be simple and bundled to keep 
> costs low.  As you know, there are significant fixed and variable costs 
> associated with design, implementation, testing, and deployment of each 
> layer protocol.
...

Very nicely put - same goes for space flight software.  I developed the 
flight and ground software command and control interface for several 
shuttle experiments.  We used TCP to the onboard data system (a custom 
protocol was used for the air to ground segment - see 
http://scp.grc.nasa.gov/siw/presentations/Session_A/A_04_Carek.pdf). 
For our needs UDP would have been the way I would have preferred. 
Having a connection oriented TCP service added to our development time 
and caused us to spend time testing additional failure trees and figure 
out the vendor's TCP stack. We also had to add additional record 
boundary markers (since TCP is stream oriented from the applications 
perspective) on both flight and ground systems.

When dealing with space flight software you want to keep everything as 
simple as possible. The simpler it is written the easier it is to verify 
- and the more reliable it will be.

Also I'd recommend using your own 32 bit CRC if data integrity is 
important.  Never assume you data won't get mucked up somehow - even if 
the link providers tell you it will be reliable.

For space applications you want a link layer that is flexible enough to 
support multiple data types and protocols (e.g. TCP/IP, UDP/IP, CCSDS 
space packet, bit stream, etc.) - then let the functional requirements 
of the system drive the selection.


-- 
   David A. Carek, P.E.
   NASA Glenn Research Center