Coax

In short: Coaxial cable — a cable type with a central inner conductor surrounded by insulation, a shielding braid and an outer sheath, classically used for cable television and older networks.

In more detail: The concentric design (hence “coaxial”) shields the signal well against electromagnetic interference. In networking it has largely been replaced by twisted-pair cables and fibre, but is still widespread in cable internet (DOCSIS).

In Depth

Physical structure

From the inside out, a coaxial cable consists of four layers: the central copper inner conductor (carries the actual signal), an insulating layer (dielectric), a braided or foil shield (diverts electromagnetic interference before it reaches the inner conductor and also serves as the return conductor for the signal) and an outer plastic sheath as mechanical protection. This concentric (“coaxial”, from the Latin for “common axis”) arrangement is the reason for its good resistance to interference — unlike twisted-pair cables (see Cat cables), the shielding sits directly and completely around the conductor, instead of relying on the principle of twisted wire pairs whose interference cancels each other out.

Impedance and cable types

An important, often overlooked parameter is the characteristic impedance: network and RF antenna cables mostly use 50-ohm cable (e.g. RG-58), while classic cable television and cable internet work with 75-ohm cable (e.g. RG-6, which for modern applications has largely replaced the thinner, less well shielded predecessor standard RG-59). If cables with mismatched impedance are combined, signal reflections occur at the junction, which noticeably worsen the signal quality — a common but often overlooked mistake in DIY installations.

Connectors

Among the most common connector types are BNC connectors (Bayonet Neill-Concelman — a quick bayonet lock with a quarter turn, common in older network installations and professional video technology) and F connectors (screw lock, the standard for cable television and satellite systems at home). A defective or badly fitted connector is in practice one of the most common causes of poor reception or unreliable cable internet connections — often more likely than a defect in the cable or modem itself.

Historical role in networking

In classic networking, coaxial cable was used for early Ethernet variants such as 10BASE2 (“thin Ethernet”, thinner cable) and 10BASE5 (“thick Ethernet”, thicker, more robust cable), in which all devices hung in series on a single long cable (bus topology) — back then, a single cable break or a loose termination at one of the cable ends brought the entire segment down, which was one of the main reasons why star-shaped twisted-pair cabling with a central switch prevailed in the 1990s: in a star topology, a cable defect only affects the one device concerned, no longer the whole network.

Use today: DOCSIS

Today coaxial cable lives on mainly in cable television and the cable internet built on it, standardised via the DOCSIS specification (Data Over Cable Service Interface Specification). Newer DOCSIS versions (3.1 and 4.0) now achieve gigabit speeds over the same physical coaxial cable that was originally only designed for analogue cable television — through more efficient modulation methods, not through better cables. Coaxial cable typically bridges the “last mile” between a fibre distribution node in the street cabinet and the household (hybrid fibre-coax, HFC), while the rest of the route already runs on fibre.

See also: Fibre, Cat categories and specifications, Ethernet