The earliest type of telephone line was referred to as open wire – unsheathed copper wires strung on telephone poles and secured by glass insulators. Because it was uninsured, this type of telephone line was highly susceptible to electromagnetic interference; the wires had be spaced about 12 inches apart to minimize the problem. Although open wire can still be found in a few places, it has almost entirely been replaced with cable and other types of communications media.
Cable is insulated wire, Insulated pairs of wires twisted around each other- called twisted- pair cable – can be packed into bundles of a thousand or more pairs. These wide-diameter cables are commonly used as telephone lines today and are often found in large buildings and under city streets. Even though this type of line is a major improvement over open wire, it still has many limitations. Twisted-pair cable is susceptible to various types of electrical interference, which limits the practical distance that data can be transmitted without being garbled. (To be received intact, digital signals must be “refreshed,” or strengthened, every 1 to 2 miles through the use of an amplifier and related circuits, which together are called repeaters. Although repeaters do increase the signal strength, which tends to weaken used for years for voice and data transmission, however, newer, more advanced media are replacing it.