A transmission medium can be defined as the environment in which a signal is transmitted, be it material (electrical or optical wires, air) or non-material (vacuum). A vacuum is a transmission medium through which only electromagnetic waves are propagated. The material transmission media are structured into two main groups:

• Conductive media, in which the information is transmitted in the form of electrical impulses. Example of these media are twisted-pair cable and coaxial cable.
• Dielectric media, in which the information is transmitted in the form of radioelectrical or optical signals. Examples of these media are the atmosphere and optical fibre.

The propagation of signals over one of these media is what we call transmission. The success of the transmission of information in telecommunications networks depends on two main factors: the quality of the signal transmitted and the quality of the transmission medium used. In addition, there are natural forces that can resist transmission and modify the original charactertistics of the signals, which end up being degraded by the time they reach their destination. The most significant impairments are: attenuation, noise and distortion. These are looked at in respect to a communications channel, which is defined as a means of unidirectional transmission of signals between two points. When the communication is two-way it is more usual to talk of a circuit (with two channels: one for each direction of communication).