Chapter 3 — Physics for computing | Engineering Network and communications security

Open, free learning for anyone who wants to protect people and technology. No account is required to read this text or its worked answers.

Lesson map

Lesson 03 published. Map, example, EX007.

flowchart TD
  N0["Learning outcomes"]
  N1["3.1 Propagation delay"]
  N2["3.2 Wireless coverage"]
  N3["3.3 Electrical and physical layer"]
  N4["3.4 Power and heat"]
  N5["Measuring delay in practice"]
  N6["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> N6
  N6 --> EX["Worked example"]
  EX --> Q["Exercise EX007"]

Logical drawing

flowchart LR
  N0["Learning outcomes"]
  N1["3.1 Propagation delay"]
  N2["3.2 Wireless coverage"]
  N3["3.3 Electrical and physical layer"]
  N4["3.4 Power and heat"]
  N5["Measuring delay in practice"]
  N6["Synthesis"]
  N0 --> N1 --> N2 --> N3 --> N4 --> N5 --> N6 --> Q["EX007"]

[Learning outcomes]
    |
    v
[3.1 Propagation delay]
    |
    v
[3.2 Wireless coverage]
    |
    v
[3.3 Electrical and physical layer]
    |
    v
[3.4 Power and heat]
    |
    v
[Measuring delay in practice]
    |
    v
[Synthesis]
    |
    v
[Worked example] --> [Exercise EX007]

Learning outcomes

You will (1) relate signal propagation to latency; (2) understand wireless coverage; (3) consider physical layer security.

3.1 Propagation delay

Signals in fibre travel at roughly two thirds of the speed of light, about 200 kilometres per millisecond. Between two Ashcombe offices 40 kilometres apart, one-way propagation is about 0.2 milliseconds; most observed delay comes instead from queues and processing. This tells Kofi where latency budgets go and why adding a firewall hop matters more than distance within the district.

Distance still sets a hard minimum: every kilometre of fibre adds about five microseconds each way, whatever equipment sits at either end. That floor cannot be engineered away, so a link between distant offices always adds delay, and protocols or controls that need many round trips feel slower across it. Measured latency well above the propagation floor points to queuing or processing instead.

3.2 Wireless coverage

Wi-Fi signals weaken with distance and walls. Access points placed for good indoor coverage often also cover car parks, so guest and staff networks can be heard outside buildings. NIST SP 800-153 recommends considering coverage when securing wireless networks. Kofi notes that physical leakage is a reason to require strong authentication and encryption rather than relying on the building as a boundary.

Radio signal strength falls quickly with distance and walls, so coverage maps never stop neatly at building edges. A guest network that is usable in a car park is reachable by anyone parked there. Engineers therefore plan access point placement and transmit power with coverage boundaries in mind, and treat wireless as reachable from outside regardless.

3.3 Electrical and physical layer

Network cabinets in offices hold switches whose ports anyone with physical access could use. Physical security of cabinets and disabling unused ports are basic controls. Kofi records cabinet locations and port counts during site visits.

The physical layer is where many security assumptions quietly rest. An unused wall port that is still patched to a switch gives anyone in the room network access, and a cable run through a shared ceiling can be tapped or disconnected. Disabling unused ports, labelling patch panels and locking wiring closets are physical controls that support every logical control above them. Engineers should record these in the same inventory as switches and rules.

3.4 Power and heat

Firewalls and switches need stable power and cooling. A security appliance that overheats in a cupboard becomes a reliability problem. Kofi checks that each site's cabinet has ventilation and an uninterruptible power supply.

Network equipment fails when power is lost or when cooling fails, so availability depends on facilities as much as configuration. Uninterruptible power supplies give minutes, not hours, and a closet without ventilation can overheat a switch on a summer weekend. Engineers estimate load in watts, check supply runtime, and decide which devices must stay up during an outage, such as core switches and the firewall, before choosing what may shut down.

Measuring delay in practice

Kofi checks his estimates against measurements supplied by the council's network team. Round-trip times between offices average about three milliseconds, so propagation accounts for well under a fifth of the delay. Most of the rest comes from switches, routers and the existing edge devices. This matters for the design: adding a firewall hop at each site will add processing delay of the same order as current equipment, so the latency budget must be set from measurement rather than from distance. He records the measured baseline so later chapters can compare against it.

Worked example

For a 40 km fibre path, propagation is 40 / 200 = 0.2 ms one way, small compared with a measured 3 ms round-trip time dominated by equipment.

Synthetic case. Distances are synthetic.

Errors and limits

Copper and wireless have different propagation and loss characteristics; do not apply fibre figures universally.

Exercises

EX007 — Propagation estimate. Estimate one-way propagation over 120 km of fibre.
Show marking criteria
Rubric: 3 = 0.6 ms with method;; 2 = value;; 1 = vague;; 0 = none.
EX008 — Car park coverage. Why does Wi-Fi reaching a car park matter for security?
Show marking criteria
Rubric: 3 = outsiders can connect or listen; need auth and encryption;; 2 = exposure;; 1 = vague;; 0 = none.
EX009 — Unused ports. Give one physical layer control for office cabinets.
Show marking criteria
Rubric: 3 = locked cabinets or disabled unused ports;; 2 = lock;; 1 = vague;; 0 = none.

Synthesis

Physics sets latency floors and explains why wireless and physical access shape security design.

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