|  | 4.3 | Basics of Signals and Noise in Communications Systems
| 4.3.3 | Representing one bit on a physical medium
Data networks have become increasingly dependent on digital (binary, two-state) systems. The basic building block of information is 1 binary digit, known as the bit or pulse. One bit, on an electrical medium, is the electrical signal corresponding to binary 0 or binary 1. This may be as simple as 0 volts for binary 0, and +5 volts for binary 1, or a more complex encoding. Signal reference ground is an important concept relating to all networking media that use voltages to carry messages. In order to function correctly, a signal reference ground must be close to a computer's digital circuits. Engineers have accomplished this by designing ground planes into circuit boards. The computer cabinets are used as the common point of connection for the circuit board ground planes to establish the signal reference ground. Signal reference ground establishes the 0 volts line in the signal graphics. With optical signals, binary 0 would be encoded as a low-light, or no-light, intensity (darkness). Binary 1 would be encoded as a higher-light intensity (brightness), or other more complex patterns. With wireless signals, binary 0 might be a short burst of waves; binary 1 might be a longer burst of waves, or another more complex pattern. You will examine six things that can happen to 1 bit: propagation attenuation reflection noise timing problem collisions