A time-domain reflectometer (TDR) sends a voltage step down the cable's copper power/telemetry conductor and watches the source terminal. The conductor is a transmission line with characteristic impedance Z0; wherever it meets an impedance Zf that differs from Z0 (a break, a short, corrosion, a crush), part of the wave reflects back:
ρ = (Zf − Z0) / (Zf + Z0) [voltage reflection coefficient]
V(t) = V0 for t < t_rt
V(t) = V0·(1 + ρ·e^(−2αd)) for t ≥ t_rt (reflected step arrives)
t_rt = 2d / (v_factor·c) d = fault distance, c = 299 792 458 m/s
d_computed = v_factor·c·t_rt / 2 (inverts the same formula to confirm the reading)
An open circuit (Zf → ∞) gives ρ → +1: the trace jumps up to roughly double amplitude, same polarity as the incident step. A short circuit (Zf → 0) gives ρ → −1: the trace collapses to zero, inverted polarity. A partial fault sits in between and reflects only part of the power (P_reflected = ρ², P_transmitted = 1 − ρ²) — the rest keeps propagating past the fault.
Below the waveform, a small routing model treats the tested cable as one of three physical paths between landing stations. Its available capacity is derated by (1 − ρ²) — full when healthy, zero when fully open or shorted. Traffic (a fixed 100 Gbps demand) is greedily assigned to the lowest-latency path with spare capacity first, spilling into slower backup routes when the primary can't carry it, exactly like real submarine-cable failover.
- Cable map — landing stations, the fault marker at its true proportional position, and a looping schematic pulse (position to scale, timing compressed for visibility).
- TDR trace — voltage vs. real elapsed time in milliseconds, to scale from the current distance/velocity-factor/impedance settings.
- Network panel — the three routes, each path's derated capacity, and the resulting blended latency and unserved-traffic percentage.