Fibre-optic cable

Fibre-optic cable carries signals as light through a thin core of glass or plastic rather than as electrical current through copper. Single-mode fibre uses a narrow core for long distances; multimode fibre uses a wider core for shorter runs. Compared with twisted-pair and coaxial cable, fibre supports higher bandwidth over longer distances.

Its security properties come from the physics. Light in the fibre is not affected by electromagnetic interference or crosstalk, so fibre suits electrically noisy environments such as plant floors. It also does not radiate the electromagnetic emanations that copper does, so it cannot be eavesdropped on from a short distance the way some copper can. That does not make it untappable. An attacker with physical access can bend or splice the cable to draw off light, and such taps can be hard to spot without monitoring for signal loss. Protecting the cable route and encrypting the traffic therefore still matter. The trade-offs are cost and fragility.

Exam relevance: when a scenario among the transmission media options needs resistance to interference, long runs, or protection against emanation-based eavesdropping, fibre is likely to be the expected choice. A candidate should not treat fibre as impossible to tap: physical protection and encryption remain the controls.