SCADA is Elephants and Turtles all the way down

We had just moved some of our SCADA engineering to the Invensys facility in West Durrington, Worthing, UK. Worthing is down by the beach not far west of Brighton on the English channel. Not far from Worthing, if you go up past Arundel, is Goodwood, the home of the famous racing circuit among other things.  I found the Cass Sculpture park there and wandering around on a weekend. I stumbled across Tony Cragg's public sculpture. I like this one in particular for it's topology, and for today's post I think it relevant in the discussion on the general form of SCADA systems. This post includes a Turchin style dialog on SCADA, what is it and what does it mean. It also includes a general discussion on metasystems and holons and how SCADA architectures themselves include aspect of these higher level concepts.  So if you are brave... read on...



A Turchin style Dialogue on SCADA [footnote1]

In this short dialog, Harry never actually meets Sally.  Harry calls Sally, in a kind of dial-a-friend way on skype because he just needs to know something a bit stronger about SCADA. As the dialog progresses, you will see that something which appears to be simple, can become complex fast, but stick with it....

HARRY	SALLY
I’d like to know a bit about SCADA, so maybe you could tell me.    ?	Sure, our company Invensys builds many systems which have SCADA. It can get a bit complex, are you sure you really want to know ?
Well yes, start anyway and if it gets too boring, I'll probably say enough and we can talk about something else. Come on, I have a few moments. So SCADA - what does it mean? What is it?	Well, SCADA itself is an acronym - SuperVisory Control and Data Acquisition, it's an acronym for the combination of those two functions.
So a SCADA can do two things, controls and gets data ?	Funny you should say that 'a SCADA' as though it is a thing. She said. It's more a system wide capability. OK but the SCADA part which connects to IO is called the Remote Terminal Unit.  Usually because it is in quite a remote place. It can also do useful local logic and data storage.
So it sounds like the remote terminal unit is just a remote part of a SCADA system ?	Yes it has SCADA capability, but it is only part of, a SCADA System.  You see the Remote Terminal Unit itself, doesn't know what to do, so it has to be told when to make a Supervisory Control. It also doesn't do anything much with the data it acquires, except to send it back to other parts of the SCADA system. Most commonly it communicates with a master terminal unit.
You mean a bit like an ATM ?	Well, yes, it is a bit that way, except that it is permanently attached to the remote plant rather than interacting with multiple users.  But yes it does do transactions of a kind back with the master terminal unit.
Oh ok ! – but then how does it talk back to the bank, er.. I mean the master terminal unit, the the ATM, the RTU SCADA thingy ?	Yes, you probably need to remember 3 key acronyms before we go any further. SCADA is a node and branch type architecture. The two kinds of nodes are… ·  MTU Master Terminal Unit. · RTU  Remote Terminal Unit. OK now the branch – the branch is what we will call a channel, which really is a communication link with a SCADA protocol stack on top of it all. Here I will draw you a drawing….
Ok I see the drawing – you have MTU at the centre with and arrow connecting and RTU. So you need all three components to make a SCADA system ?.. Why the arrow for the link ?	Yes you need all three components to make a SCADA system. The arrow is to show the general direction of data aggregation, and the fact that the SCADA protocol stack is asymmetric. The MTU aggregates the data and manages control for it’s subordinate RTUs. An MTU can aggregate all the data from many RTUs, possibly up to around 1200 or so. The MTU does this by communicating with the RTUs over channels. Each channel is a communications link running a SCADA protocol stack.  That’s where the protocol stack comes in.
Oh – so that’s what you mean about higher organisation.  The SCADA MTU kind of organises all the data from all the SCADA RTUs.	YES, you got it. Now the idea of master comes from the SCADA protocol stack. A SCADA protocol stack is normally divided into a Master and Slave pair.  The Master runs on the MTU and the Slave runs on the RTU. This is the asymmetric bit. This enables the MTU to manage many RTUs.
OK so the whole thing is like an interconnected system. Too simple really. Ok I think I have it now.	Hey Harry, relax, not so fast.. Yes like a Node and Branch System.  The MTU is a higher level node and connects by multiple branches to RTU nodes at the end of the branches.
Mmm … thanks for that.. but I can see now that SCADA is not just SCADA like it doesn’t describe things, it’s more like a hierarchy of things.	Yes, remember when they used to say the Earth was round, and therefore was just a something, and it was generally refuted by saying no… at the top of the world was an elephant, and the elephant stood on the back of a turtle and from then on it was turtles all the way down ? SCADA is like that.  Hang on … here is another drawing…
OK …… what have you done ??	OK.. now at the top of the SCADA World is the MTU.. pretend that is the elephant. At the bottom of the SCADA World is an RTU.. pretend there is actually a bottom turtle somewhere.… somewhere out in the never never. Look at that thing in the middle.  What you see there is an RTU[turtle]. Now what we have are SCADA protocol stacks on communication channels linking the RTUs. Each of the intermediate layers is an RTU which has MTU capability built in.
So I get it, you have to have an MTU elephant sitting on at least one RTU turtle, but you can stretch it out using these intermediate RTUs. Sort of like a turtle with an inner elephant ?	YES !! but look – you can have any number of inner elephants and a whole stack of turtles. This means you can organise SCADA in many different Node and Branch structures.
Well I guess that could get complex..	Well the architectures can get very complex, but the components are simple to imagine.  You remember ?
MPR = MTU,PROTOCOL STACK,RTU	Right.
OK but what about these SCADA protocol stacks.  It looks like they do some magic ?	Yes, well we use a special SCADA protocol. We normally use DNP3 or IEC 60870-5-101.  These are open protocols that can handle the SCADA messages.  They also handle communication failure which allows the RTU to go on doing it’s think quite independently of the MTU.
So when people ask me do I have a SCADA product, I really need to be able to say I have the three different capabilities, like RTU which goes out somewhere in the world. The MTU which sits somewhere higher up in the food chain, more centrally, and then the SCADA protocol stack, which has to exist in both.	Yes, it is also possible for Integrated Control and Safety Systems to have the MTU function built in.  This enables large Automation Systems to be able to collect data and do supervisory controls over any communications channel.
OK – that’s enough – thanks.. Sally.	OK Bye Harry...

If you got this far in the blog post you are doing well.  Congratulations.  Now for the bit about the sculpture in the park.


As can be seen by the dialogue above, SCADA consists of two key nodal components and a link or branch component called a protocol stack.  The protocol stack itself is where all the connectivity magic happens, but each higher level MTU has to aggregate and make more and more higher level sense of the data it receives from the RTUs which it has to aggregate.


The other key aspect is that the combination of Channel/RTU can be constructed recursively, whereby it each intermediate RTU can also act as an MTU host and can enable engineers to create multi-layered systems which reach out to wide varieties of locations to do supervisory control at those locations. This kind of system is particularly prevalent in oil and gas systems, with higher levels of bandwidth applied further up the hierarchy as the total amount of aggregated data increases.


The combination of the MTU, RTU, with a Protocol Stack makes SCADA like a metasystem depending on your point of view. One key aspect of modern SCADA protocols is their ability to transport object data, where that object can be either simple data types or higher level objects.


Turchin's original metasystem transition was based on the subject at the lower level of the transition being operated on by functions at a higher level and so on.  Thus an organism, in this case an automation system can do higher levels of control and automation at  higher metasystem transition levels.  At the bottom layer, the RTU may be just pulling in data from one or two IO.  At the next layer up, it could be an well separator or plant.  At the next layer up even higher levels of data combination and control can be done.


I sincerely hope you may have got down to this point, but feel free to think about the topic.  SCADA is or isn't SCADA the way you first imagined it, just like the earth is a sphere, not an elephant standing on a turtle on turtles all the way down, or at least depending on your point of view.




Chris Smith
Feb 2012.


copyright.


Footnotes and Reference...
A Dialog on Metasystem Transition : Valentin Turchin - City of New York University 1999.