Telephone

The intelligibility, naturalness, and audibility of transmitted speech are the main requirements for high quality telephone transmission. In technical terms, these requirements mean that: all the harmonic components of the human voice in the frequency range from 300 to 3,400 Hz pass through the communication channel; the loss of a signal during passing the channel does not exceed 30 dB; and the level of noise arising from all types of interference be at least 35 dB lower than the level of the main signals.

Alexander Graham Bell on the telephone at the formal opening of telephone service between New York and Chicago in 1892. U.S. National Aeronautics and Space Administration.

The quality of the telephone service is greatly dependent upon the structure of the networks by which subscribers are connected. The number of subscribers making telephone calls at one time is always substantially less than the total amount of subscribers. That is why the number of channels in the commercial telephone system is considerably less than the number of subscribers served by a central office (usually by a factor of 7-10 in a local office and 200-250 in toll exchanges). Because of such a design, a connection may be blocked when the telephone traffic is high. The quality of the automatic telephone service can be defined as the percentage of blocked (refused) calls during the hours of the heaviest telephone traffic. To reduce the incidence of channel overload, good planning based on statistical analysis is used.

Another way to improve network quality is to increase the amount of channels that can be carried by a single underlying medium. For example, in 1940, a coaxial cable could carry 600 voice channels. By the early 1950s, this amount increased to 1,860 voice channels and reached the number 13,200 in the 1980s. The microwave radio system experienced similar bandwidth growth, with 2,400 channels in the 1950s increased to 42,000 channels in the 1980s. The potential of the fiber optic technology promises even faster progress in this direction.