Sending and receiving data via cables (e.g., telephone lines or fibre optics ) or wireless relay systems.
Because ordinary telephone circuits pass signals that fall within the frequency range of voice communication (about 3003,500 hertz), the high frequencies associated with data transmission suffer a loss of amplitude and transmission speed. Data signals must therefore be translated into a format compatible with the signals used in telephone lines. Digital computers use a modem to transform outgoing digital electronic data; a similar system at the receiving end translates the incoming signal back to the original electronic data. Specialized data-transmission links carry signals at frequencies higher than those used by the public telephone network. See also broadband technology ; cable modem ; DSL ; ISDN ; fax ; radio ; teletype ; T1 ; wireless communications .