the latter passes through a transmission medium.

Noise is always present and forms an instrinsic part of all communications. It is usually divided into four categories:

1. Thermal noise: caused by the agitation of electrons in a conductor due to changes in temperature.
2. Intermodulation noise: caused when two signals of frequencies f1 and f2 transmitted in the same medium produce a spurious signal at frequencies that are a linear combination of the previous ones (for instance the frequency fn = f1 + f2).
3. Points of noise: short-term disturbances caused by an electrical spark, a drop in voltage, atmospheric interference, etc. These do not tend to be a problem for analog signals but are the prime cause of errors in digital transmission. For instance, the interference that causes a peak of energy of 0.005 seconds will not significantly degrade a telephone conversation, but it will have affected 48 bits that were being transmitted at a rate of 9,600 bit/s.
4. Coupling or crosstalk: an effect produced by electromagnetic inductions between different conductive transmission media in close proximity. This is a common source of noise in telephony, and can be seen when two different conversations interfere with each other.