Digital transmission is the sending information over a physical communications media in the form of digital signals. Analog signals must therefore be digitized first before being transmitted. However, as digital information cannot be sent directly in the form of 1s and 0s, it must be encoded in the form of a signal with two states, for example::
1. Two voltage levels with respect to earth
2. The difference in voltage between two wires
3. The presence/absence of current in a wire
4. The presence/absence of light.
Digital data transmission can occur in two basic modes, Parallel and Serial. Data within the width of the parallel bus matched to the word size of the computer system. Data between computer systems is usually transmitted in bit serial mode. Consequently, it is necessary to make a parallel-to-serial conversion at a computer interface when sending data from a computer system into a network and a serial-to-parallel conversion at a computer interface when receiving information from a network. The type of transmission mode used may also depend upon distance and required data rate.