Digital transmission: a few states

Binary transmission: exactly two states (1 and 0)

Binary is a special case of digital

Sender and Receiver associate one or more bits with each state

Simplest case: High state = 1, Low state = 0

If four states, might have the following:

Highest = 11

Second highest = 10

Next highest = 01

Lowest = 00

Time is divided into fixed-length clock cycles

Modems: a few thousand clock cycles per second
LANs: millions of clock cycles per second


The line is kept in one of only a few possible states (conditions) during each time period - this is why the signal must be kept constant

At the end of each time period, the line may change abruptly to another of these few states

In analog transmission, the state of line can vary continuously and smoothly among an infinite number of states

States can be signal strengths, voltages, or other measurable conditions

Human voice is analog; telephone mouthpiece generates analogous electrical signal