 lines are rarely installed today but they are still in use in rural areas as subscriber lines or analog carrier systems with a small number of speech channels. 4.7.1.3 Coaxial Cable In a coaxial cable, stiff copper wire makes up the core, which is surrounded by insulating material. The insulator is encased by a cylindrical conductor. Introduction to Telecommunications Network Engineering The outer conductor is covered in protective plastic sheath. The construction of the coaxial cable gives a good combination of high bandwidth and excel- lent noise immunity. Coaxial cables are used in LANs (original 10-Mbps Ethernet), in antenna systems for broadcast radio and TV, and in high- capacity analog and digital transmission systems in telecommunications net- works and even in older generation submarine systems. 4.7.2 Optical Fiber Cables Optical fiber is the most modern of the transmission media. It offers a wide bandwidth, low attenuation, and extremely high immunity to external elec- trical interference. The fiber optic links are used as the major media for long-distance transmission in all developed countries and high-capacity coax- ial cable systems are gradually being replaced by fiber systems. An optical fiber has a central core (with a diameter around 8 or 60 um) of very pure glass surrounded by an outer layer of less dense glass. A light ray is refracted from the surface between these materials back to the core and it propagates in the core from end to end. The principle of optical cable trans- mission is presented in Figure 4.28. Compare the dimensions of optical fiber with the diameter of a human hair that is approximately 100 um. The principle of optical fiber transmission has been known for some decades. The breakthrough of optical fiber technology had been expected to occur ever since the first half of the 1970s. However, the development of fiber manufacturing technology and optical component technology was slower than expected, and the commercial breakthrough was delayed until the mid-1980s. Since that ti