Coaxial Cabling is a form of network cabling used primarily in older Ethernet networks and in electrically noisy industrial environments. The name «coax» comes from its two-conductor construction in which the conductors run concentrically with each other along the axis of the cable.
Coaxial cabling has been largely replaced by twisted-pair cabling for local area network (LAN) installations within buildings, and by fiber-optic cabling for high-speed network backbones.
How Coaxial Cable Works
Coaxial cabling generally consists of a solid copper core for carrying the signal, covered with successive layers of inner insulation, aluminum foil, a copper braided mesh, and outer protective insulation. A solid conductor provides better conductivity than a stranded one, but is less flexible and more difficult to install. The insulation is usually PVC (polyvinyl chloride) or a non-stick coating; the aluminum foil and copper mesh provide shielding for the inner copper core. The mesh also provides the point of grounding for the cable to complete the circuit.
Coaxial cabling comes in various types and grades. The most common are
- Thicknet cabling, which is an older form of cabling used for legacy 10Base5 Ethernet backbone installations. This cabling is generally yellow and is referred to as RG-8 or N-series cabling. Strictly speaking, only cabling labeled as IEEE 802.3 cabling is true thicknet cabling.
- Thinnet coaxial cabling, which is used in 10Base2 networks for small Ethernet installations. This grade of coaxial cabling is generally designated as RG-58A/U cabling, which has a stranded conductor and a 53-ohm impedance. This kind of cabling uses BNC connectors for connecting to other networking components, and must have terminators at free ends to prevent signal bounce.
- ARCNET cabling, which uses thin coaxial cabling called RG-62 cabling with an impedance of 93 ohms.
- RG-59 cabling, which is used for cable television (CATV) connections.
In addition, a number of special types of coaxial cabling are sometimes used for certain networking purposes. An example is twinax cabling, which consists of two conductors first enclosed in their own insulation and then enclosed in a single copper mesh and insulating jacket. Twinax is used in legacy IBM networks for connecting AS/400 systems to 5250 terminals. Other more exotic varieties include triax, quadrax, and ribbon types of coaxial cables.
Coaxial cabling for electromagnetic interference
Coaxial cabling is often used in heavy industrial environments where motors and generators produce a lot of electromagnetic interference (EMI), and where more expensive fiber-optic cabling is unnecessary because of the slow data rates needed. Coaxial cabling is also used frequently in IBM mainframe and minicomputer environments. A device called a splitter can be used to fork one coaxial cable into two – for example when connecting two 3270 terminals to one IBM mainframe system. A splitter is used at either end of the connection so that the signals for both terminals can be sent over a single coaxial cable. Coax multiplexers can be used to connect eight or more terminals to a single controller.