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I find the basic OSI model very helpful for network programming. I'm now getting started in SCADA, and having trouble mapping the various standards (protocols? networks?) to OSI.

Let's start with layers 1 and 2. In the network world, this is usually Ethernet. In SCADA, this seems to be usually either Ethernet or serial lines. Is that correct? Are there other common ones?

Now, it seems that Modbus is a very common protocol, and that Modbus can go over serial. So, that would put it at Layer 3, similar to IP? Or does it do something else?

Modbus can also go over TCP/IP, but I assume that's more of a tunneling than a higher layer.

What about other protocols, like DNP3? Where do they fit in?

In short: What are the important SCADA networks and protocols, and where do they fit in on the OSI model? What is there basic function?

SRobertJames
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  • The OSI layers are not to be found in the SCADA world, or anywhere else outside college or a textbook. OSI has been defunct for twenty years. Let it rest in peace. TCP/IP has its own reference model which in fact *pre-dates* OSI. You should be thinking in terms of that. – user207421 May 02 '17 at 21:47
  • Tend to agree with @EJP. It's not the first time I've heard this question and it always seems contrived and out of touch. – Kevin Herron May 02 '17 at 23:07
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    Like any model, it's a _model_, which means _approximation_. ("_All_ models are inaccurate. _Some_ models are useful.") In the networking world, OSI is a very useful one. If I say something operates at L3 versus L4, that conveys a concept that would otherwise take quite some time to explain. If I want more fidelity, we'll need to talk further (just like any term). – SRobertJames May 03 '17 at 01:27

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The two protocols you've mentioned, Modbus and DNP3, are both application layer (OSI layer 7) when run on TCP/IP.

When run over serial they include some data link layer definitions as well, but on TCP/IP it's all just encapsulated and part of the application layer.

Kevin Herron
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I found myself with the same questions while implementing IEC 60870 part 5. With the TCP alternatives for serial communication you go back from the transport layer to the data link layer. IEC 60870-5-104 does this with the so called APDU frames.

Pascal de Kloe
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