NCS: Message Transfer Systems
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This lecture describes different choices for message transfer systems in a networked control system. The message transfer system is responsible for managing network communications between computers and software modules with the control system. We focus on systems that are build on top of the TCP/IP protocol stack. Design choices include how to encode information in packets, whether to broadcast or send packets point-to-point, and whether to retransmit packets on lost data transmission. Because of the closed loop nature of the networked embedded systems that we are programming, timing and latency are critical issues. We focus on the use of spread as a specific example of a low-level message transfer subsystem and describe how it can be used in a NCS context.
Lecture Materials
Reading
Missing. I'm still looking for a good reference on causality and message passing.
A Users Guide to Spread, J. Stanton. 2002. This is the documentation for the Spread Toolkit. The first and second chapters provide most of the information you need to understand the basic ideas, although the way in which Spread servers are configured, described in Chapter 3, is also useful.
The Spread Wide Area Group Communication System, Y. Amir and J. Stanton. Technical Report CNDS-98-4, The Center for Networking and Distributed Systems, The Johns Hopkins University, 1998. The paper provides a detailed technical description of how Spread works. It is mainly useful if you want to know more about what spread does. Requires some background in network protocols if you want to understand the details.
Additional Resources
- Spread Toolbox - The Spread Wide Area Group Communication System
- Neutral Message Language (NML) Programmers Guide - part of the NIST Realtime Control System library
- RoboFlag documentation - includes discussion of how RoboFlag network communications are implemented
