Process Control Network Fundamentals
Domain 1 tests how well you distinguish a process control network (PCN) from enterprise IT and how you apply the Purdue model, control-loop priorities, and OT component roles.
Core rules
- OT prioritizes availability, safety, and integrity ahead of confidentiality; a reboot for patching can stop a production line.
- Purdue levels are 0 process, 1 basic control, 2 supervisory, 3 operations management, 3.5 DMZ, 4 enterprise, 5 external. Conduits should cross only where justified.
- A basic process control system (BPCS) manages the process; a safety instrumented system (SIS) independently brings the process to a safe state. Do not treat the SIS as just another PLC.
- Deterministic timing matters: scan cycles, I/O updates, and network latency can change physical outcomes.
Worked example
A remote wellhead has an RTU reporting to a SCADA master. The RTU and its sensors are Level 1; the SCADA master and HMI are Level 2; the historian sits at Level 3. A firewall between Level 2 and Level 3 permits only the historian's outbound replication and required SCADA updates. Reasoning: placing the RTU at Level 2 would blur control and supervision; allowing all Level 3 traffic into Level 2 would expose the control zone to enterprise threats. The conduit is narrow and directional.
Traps
Assuming every device speaks TCP/IP; putting the SIS on the same VLAN as the BPCS; treating a 24x7 uptime requirement as an excuse to skip segmentation; confusing the Purdue model with a physical floor plan.