¥The tasks of communication is to encode
information as a signal level, transmit this signal, then decode
the signal at the receiving end.
¥An analog signal varies continuously with time, and has an
infinite number of possible signal levels.
¥A discrete signal changes only once during a certain time
interval. The signal value during this time interval is one sample, and the
interval length is called the sampling period.
Each sample has a infinite number of possible signal levels.
¥A digital signal is discrete, but each sample has a finite
number of possible signal levels.
The limited number of levels means that each sample transmits a single
information. It also means that
each sample can be represented as digital data, a string of
ones and zeroes.
¥A digital
signal is preferred in computer communications because computers already store
and process information digitally.