Another communication transmission medium is the coaxial cable. It has better shielding than twisted pairs, so it can span longer distances at higher speeds. Two kinds of coaxial cable are widely used. One kind, 50-ohm cable is commonly used for digital transmission and is the subject of this section. The other kind, 75-ohm cable, is commonly used for analog transmission and will be described in the next section. This distinction is based on historical, rather than technical factor, (e.g., early dipole antennas had an impedance of 300 ohms, and it was easy to build 4:1 impedance matching transformers).
A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity. The bandwidth possible depends on the cable length. For 1 km cables, a data rate 1 or 2 Gbps is feasible. Longer cables can also be used, to be widely used within the telephone systems but have not largely been replaced by fiber optics on long-haul routes. In the United States alone, 1000 km of fiber is installed every day (counting a 100 km bundle with 10 strands of fiber as 1000 km). Coaxial cables are still widely used for cable television and some local area networks.
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