Analog transmission, in which speech or data is converted directly into a varying electrical current, is suitable for local calls. For long-distance calls, the signal is digitized, or converted to a series of encoded pulses. A signal may be transmitted via coaxial and fiber-optic cables and microwave and longwave radio signals. A coaxial wire is an efficient transmission system. Fiber-optic cable transmits energy in the form of light pulses.
Optical fibers can handle tens of thousands of conversations simultaneously.
Such calls were very expensive. Microwave radio has the ability to inexpensively handle a large number of simultaneous conversations over the same link and can be relayed around the world by communications satellites. The first person to send a voice signal over wires was Scottish-born American inventor Alexander Graham Bell , a speech teacher in Boston, Massachusetts. Bell constructed a transmitter and a receiver, for which he received a patent in 1876. Other inventors also claimed to have invented the telephone at the same time .