Telephony

The Development of Telephony

Telephony slowly supplanted telegraphy as the key form of telecommunication between individuals after its invention in the mid-1870s. Most scholars credit Alexander Graham Bell with the invention and first public exhibition of the telephone. The telephone did not require the intermediation of telegraph operators trained in the codes used to transmit messages but instead allowed two individuals to directly communicate after being connected by an operator. The telephone carried audio messages as electromagnetic signals along transmission paths that allowed the messages to travel short distances. Early telephony relied on analog technology to carry calls over iron and then copper wires by converting audio sounds into electrical signals. These electrical signals were then converted into their original audio sounds at the receiving end.

Early analog telephony allowed calls to travel only over limited geographical distances because of the attenuation of electric currents as they traveled through the wires and the fact that only one call was routed through a particular wire at one time. Early telephones were directly linked to each other, also limiting their capabilities. New developments in telephony, however, soon increased the range of early telephones. The insertion of inductors allowed signals to travel longer, but still limited, distances while frequency division multiplexing allowed each wire to handle more than one simultaneous call through division along the frequency spectrum. Insulated cables were laid underwater, [Page 1629]including the first transatlantic telephone cable in 1956.

As the telephone's popularity grew, switching technology replaced earlier direct connections between telephones. A central office manned first by manual switchboard operators and later by automated switching exchanges established temporary links between telephones. Signal transmission paths could then be organized into networks that connected the various switching offices, calls could be switched to any telephone, callers could be alerted to an incoming call, and the circuit could then be cleared on its completion. Local area networks connect telephones in specific geographical areas, exchange area networks connect the central offices of local networks, and long-haul networks provide connections to long-distance exchanges. Trunk or junction lines connect telephone exchanges.

The next revolutionary development in telephony was the rise of digital transmission technologies and switches, which began replacing analog telephony in the 1960s. Digital telephony utilizes a binary code of ones and zeros to transmit audio and other signals along fiber optic networks with almost limitless bandwidth. The benefits of digital telephony include increased transmission capacity and quality as well as reduced costs. The mid-20th century also witnessed the introduction of satellite telephony and the first transatlantic telephone satellite, the Early Bird, as well as wireless telephony, although the latter did not achieve widespread global commercial use until the late 20th century.

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