A fiber optic cable carries data as light through glass fibers rather than as electricity through copper. Light travels by total internal reflection between the core and the surrounding cladding, so the signal is immune to electromagnetic interference, unaffected by ground potential differences, and reaches kilometres where copper stops at 100 metres.
Cable selection starts with the fiber grade. Multimode fibers have a 50-micron core in which light travels in multiple modes: OM2 suits 1 Gigabit, OM3 reaches about 300 metres at 10 Gigabit and OM4 about 400 metres. Singlemode (OS2) fiber has a 9-micron core, light travels in a single mode and distances are measured in kilometres. Multimode is used for short data center links, singlemode between buildings and on carrier connections.
The second decision is cable construction. Indoor/outdoor LS0H cables serve both duct runs and building entries; the LS0H jacket emits little smoke and contains no halogens in a fire. Outdoor steel-armoured cables suit routes exposed to rodents, underground crossings and mechanical impact. SLT (single loose tube) stays compact at low fiber counts, while MLT (multi loose tube) lets fiber groups be managed separately at 24 fibers and above.
Fiber count should be chosen for the need five years out. Labour dominates the cost of a fiber route: trenching, pulling, splicing and termination make up most of the total. Pulling 12 or 24 fibers where 4 would do is therefore far cheaper than a second pull later. Unused fibers are stored as spares in the tray.
The most common installation mistake is violating the bend radius. Bending a fiber too far creates permanent loss that is invisible to the eye and appears only in measurement. Pulling force must also stay within the manufacturer's limit; the cable is pulled by its strength member, never by the fibers.