While securing the data-network is crucial, it remains vulnerable despite the layers of protection installed, and the operators wouldn’t even know it — as demonstrated at the Stuxnet (2010), Irongate (2016) and Black Hat (2019) events, where PLCs were hacked and compromised without detection or operator awareness of such compromise.
This vulnerability can be solved by monitoring the most reliable source of information, the physical source – the raw (un-hackable) electrical signals at level 0 – sensors and actuators.