Chapter 3 — Physics for computing | Engineering Critical infrastructure and sector security

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Chapter 3 — Physics for computing

Lesson map

Lesson 03 published. Map, example, EX006.

flowchart TD
  N0["Learning outcomes and connection"]
  N1["3.1 Electricity for the systems engineer"]
  N2["3.2 Signals, noise and links"]
  N3["3.3 Heat and lifetime"]
  N4["3.4 Physics and security"]
  N5["3.6 Practical activity"]
  N6["Chapter synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> N6
  N6 --> EX["Worked example"]
  EX --> Q["Exercise EX006"]

Logical drawing

flowchart LR
  N0["Learning outcomes and connection"]
  N1["3.1 Electricity for the systems engineer"]
  N2["3.2 Signals, noise and links"]
  N3["3.3 Heat and lifetime"]
  N4["3.4 Physics and security"]
  N5["3.6 Practical activity"]
  N6["Chapter synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> N6 --> Q["EX006"]

[Learning outcomes and connection]
    |
    v
[3.1 Electricity for the systems engineer]
    |
    v
[3.2 Signals, noise and links]
    |
    v
[3.3 Heat and lifetime]
    |
    v
[3.4 Physics and security]
    |
    v
[3.6 Practical activity]
    |
    v
[Chapter synthesis]
    |
    v
[Worked example] --> [Exercise EX006]

Learning outcomes and connection

After this chapter you can (1) use Ohm's law, power and energy to size backup supplies for control and communication equipment; (2) explain signal propagation, noise and attenuation well enough to read a link budget; (3) relate heat, temperature and component life to the reliability of field cabinets; and (4) identify physical side channels and electromagnetic effects that matter to security. It builds on Chapters 1–2 and prepares Chapter 8 (computer architecture) and Chapter 11 (networks).

3.1 Electricity for the systems engineer

Voltage $V$ (volts) drives current $I$ (amperes) through resistance $R$ (ohms): $V = IR$. Power is $P = VI$ watts; energy is power integrated over time, $E = \int P\,dt$, usually quoted in watt-hours. A field RTU drawing 0.5 A at 24 V uses 12 W; with an LTE modem at 6 W and a small Ethernet switch at 8 W the cabinet load is 26 W. A 24 V, 40 Ah battery stores nominally 960 Wh; usable energy is lower because batteries should not be fully discharged and capacity falls with age and cold. With a usable fraction of 0.6 and 85% conversion efficiency, the cabinet runs for $960 \times 0.6 \times 0.85 / 26 \approx 18.8$ hours. That number is a security requirement, not a maintenance detail: if a power outage is part of an attack, telemetry that dies after two hours blinds the operator exactly when needed. The Ukrainian grid attack of December 2015 included disabling uninterruptible power supplies at control centres, according to the E-ISAC and SANS defense use case and the CISA alert IR-ALERT-H-16-056-01.

3.2 Signals, noise and links

Signals travel through copper at a significant fraction of the speed of light, through fibre at roughly two-thirds of it, so a 100 km fibre adds about 0.5 ms one way — small compared with protocol and queueing delays (Chapter 36). Radio links lose power with distance and obstruction; engineers express gains and losses in decibels (dB), where +3 dB is about double the power and +10 dB ten times. A link budget adds transmitter power and antenna gains and subtracts path and cable losses; the remainder above the receiver's sensitivity is the fade margin. Shannon (1948) showed that a channel's capacity depends on bandwidth and signal-to-noise ratio, $C = B\log_2(1 + S/N)$. The security consequence: a jammer does not need to break any cryptography; it only needs to reduce $S/N$ below what the link needs. Availability of radio telemetry is a physical property before it is a cryptographic one.

3.3 Heat and lifetime

Electronic components fail faster when hot. A widely used engineering rule of thumb, derived from the Arrhenius relation, is that many failure mechanisms roughly double in rate for each 10 °C rise; treat it as a heuristic to be replaced by the manufacturer's data. A sealed roadside cabinet in summer sun can be far hotter than the air. Design implications: derate, ventilate or cool, and monitor cabinet temperature — a sudden rise can also indicate a door left open.

For LRWA's roadside cabinets, the temperature sensor should report to SCADA alongside the battery voltage, so a rising trend is visible before components age early.

3.4 Physics and security

Physical effects leak information and allow faults. Power consumption and electromagnetic emanations can reveal cryptographic keys (side channels); voltage or clock glitches can make a processor skip instructions; DRAM rows disturbed by repeated access can flip bits in neighbours, as Kim et al. (2014) demonstrated. In infrastructure, physical access to field cabinets is often easier than network access, so these attacks are not exotic. Tamper switches, sealed enclosures and keys held off-device are the physical controls.

At LRWA, the cabinet at each pumping station holds the RTU and, after TMRP, its key store.

3.5 Worked example: PS4 cabinet hold-up (synthetic)

LRWA's TMRP requirement R5 says field telemetry must survive a 24-hour grid outage. PS4's planned cabinet draws 26 W. Required usable energy at 85% efficiency is $26 \times 24 / 0.85 \approx 734$ Wh; at a 0.6 usable fraction, nominal capacity must be at least $734/0.6 \approx 1{,}224$ Wh, or about 51 Ah at 24 V. Engineers round up to a standard 65 Ah bank to allow for ageing (capacity often specified to end-of-life at 80%: $65 \times 0.8 = 52$ Ah, which still meets the requirement). A design that met R5 only when the battery was new would fail it in year three.

3.6 Practical activity

Without code: build a spreadsheet or a ten-line script that computes battery hold-up from load (W), nominal capacity (Wh), usable fraction, efficiency and end-of-life capacity factor. Reproduce §3.5, then add a 15 W cabinet heater that runs whenever the outdoor temperature is below 0 °C and recompute.

Use the 26 W cabinet load, 85% efficiency, 0.6 usable fraction and 80% end-of-life factor from the worked example, so your first result reproduces the 65 Ah choice. Then add the heater and see how much the requirement grows. Write the answer as a requirement check, not a single number: state whether the bank still meets R5's 24 hours at end of life in cold weather, and what you would change if it does not. Keep your units visible in every column.

3.7 Common errors and limits

Sizing for nameplate battery capacity rather than end-of-life, cold, usable capacity.
Forgetting that the communications path (cellular base stations, fibre repeaters) has its own power dependency outside your control.
Treating cryptographic protection as availability protection.
Rules of thumb (10 °C doubling) used as precise predictions.

Each of these errors would make R5 look met on paper while failing it in practice. Sizing for a new battery is the commonest, because the shortfall only appears in year three. Forgetting the carrier's power dependency matters at LRWA because the LTE network shares feeder F1 with PS3. Treating encryption as availability protection is the misunderstanding EX006 asks you to correct, and a rule of thumb used as a prediction gives a precise-looking number with no real basis behind it.

Exercises — chapter 3

EX005 — Hold-up with heater. Objective: size energy storage with a conditional load. Inputs: §3.5–3.6. Steps: compute the required nominal capacity at end of life if the 15 W heater runs for 10 of the 24 hours. Expected evidence: calculation with units. Rubric: 3 = correct energy, efficiency, usable and end-of-life factors applied; 2 = one factor missing; 1 = units wrong; 0 = none.

EX006 — Jamming versus encryption. Objective: separate confidentiality/integrity from availability. Inputs: a design review comment: "LTE telemetry is encrypted, so radio attacks are mitigated". Steps: write a 150-word reply that explains what encryption does and does not mitigate and proposes two physical or architectural controls. Expected evidence: reply. Rubric: 3 = jamming and S/N reasoning, controls that address availability (diverse path, local autonomy, detection of link loss); 2 = controls generic; 1 = misconception kept; 0 = none.

Chapter synthesis

Power, signal and heat are where cyber meets physical. Size backup energy for end-of-life conditions and the full outage the threat model includes; treat radio availability as a matter of signal-to-noise, not cryptography; design cabinets for heat; and remember that physical access opens side channels and fault attacks that no firewall sees.

Continue studying — chapter 03 of 50

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