Chapter 1 — Computing systems
Lesson map
Lesson 01 published. Map, example, EX001.
flowchart TD
N0["Learning outcomes"]
N1["1.1 Many computers, few of them obvious"]
N2["1.2 Embedded versus general-purpose"]
N3["1.3 Availability first"]
N4["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4
N4 --> EX["Worked example"]
EX --> Q["Exercise EX001"]
Logical drawing
flowchart LR
N0["Learning outcomes"]
N1["1.1 Many computers, few of them obvious"]
N2["1.2 Embedded versus general-purpose"]
N3["1.3 Availability first"]
N4["Synthesis"]
N0 --> N1 --> N2 --> N3 --> N4 --> Q["EX001"]
[Learning outcomes]
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v
[1.1 Many computers, few of them obvious]
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v
[1.2 Embedded versus general-purpose]
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v
[1.3 Availability first]
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v
[Synthesis]
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v
[Worked example] --> [Exercise EX001]
Learning outcomes
You will (1) name the computing devices in a cold-storage warehouse and what each one does; (2) separate embedded controllers from general-purpose computers; (3) explain why uptime expectations differ; and (4) start the asset notebook used through the whole course.
1.1 Many computers, few of them obvious
The synthetic Northgate Cold Storage (NCS) warehouse keeps frozen food at −22 °C and chilled goods at 2–4 °C. Its obvious computers are the office PCs. Less obvious are two refrigeration controllers (PLC-R1 and PLC-R2) that run the ammonia compressors, an operator screen (HMI-C), a historian (HIST), forty wireless temperature loggers that report through a gateway (GW-LOG) to a cloud service, and IP cameras on the loading dock. A technologist's first job is to see all of them.
Priya's first notebook row, for GW-LOG, already shows why: if it stops, cooling continues but the temperature record has a gap.
1.2 Embedded versus general-purpose
A PLC runs one control program in a fixed loop and may stay powered for a decade. A Windows HMI runs many processes, receives updates and can be infected by ordinary malware. Loggers are microcontrollers with radio and a battery; they cannot run antivirus at all. Each class needs a different protection approach, which is why NIST SP 800-82r3 discusses OT device types separately.
Priya's notebook records the class in its type column, and the three classes lead to three plans. PLC-R1 and PLC-R2 are protected by controlling who can reach them, because they cannot run security software. HMI-C is hardened like a Windows host within the vendor's certified build. The forty loggers depend entirely on the vendor's firmware and on the gateway in front of them.
1.3 Availability first
If an office PC reboots, someone loses ten minutes. If PLC-R1 stops, compressor suction pressure drifts and the freezer warms by roughly a degree every 15–20 minutes; after a few hours stock worth hundreds of thousands may be condemned. That consequence shapes every later decision: when to patch, how to test, who must be present.
At NCS this is why a firmware fix for PLC-R1 waits for a maintenance window with the refrigeration contractor present, and why Priya's later firewall change in Chapter 27 has a two-minute rehearsed rollback: a missing flow that stops the historian recording for an hour matters more than the change itself.
Worked example (synthetic NCS)
Priya Nair, a technologist trainee, builds her asset notebook: device, type (embedded or general-purpose), function, location, who changes it, what happens if it stops. Her row for GW-LOG reads: "embedded gateway; forwards logger readings to the cloud; comms room; vendor updates it remotely; if it stops, temperature records have a gap and food-safety evidence is lost, but cooling continues."
Synthetic case — Northgate Cold Storage. NCS, Priya Nair and all device names are invented for teaching.
Errors and limits
Do not assume "air-gapped" without checking for cellular modems or vendor laptops. Warming rates depend on door openings and load; use the plant's own data before quoting one.
Exercises
- EX001 — Embedded or general-purpose? Classify HMI-C, PLC-R2 and a temperature logger, giving one reason each. Rubric: 3 = general-purpose, embedded, embedded with reasons; 2 = classes right, weak reasons; 1 = one right; 0 = none.
- EX002 — Consequence of a stopped gateway. State what NCS loses if GW-LOG fails for six hours and what it does not lose. Rubric: 3 = loses temperature records/alerts, cooling continues; 2 = one side; 1 = says freezers stop; 0 = none.
- EX003 — Notebook row for PLC-R1. Write the six-column asset-notebook row for PLC-R1. Rubric: 3 = all six columns plausible; 2 = four or five; 1 = name only; 0 = none.
Synthesis
Seeing every computer in a facility — and knowing what each one's failure costs — is the foundation of OT security work.