When we speak of transmission media, we usually mean a mix of physical lines ranging from wire pairs to cable, and over the air transmission media, such as microwave and satellites.
The transmission media used in telecommunications networks vary both physically and in their carrying capacity. The "commonly used performance measurement of a telecommunications link is its bandwidth. This gives an indication of the range of frequencies which can be transmitted by these channels. The greater the bandwidth, the greater the rate at which data can be transmitted.
The channels or media can be grouped into two main divisions, i.e., line and' free space. In the line group, we have two metallic media, twisted pairs and coaxial cables and one optical medium - glass or optical fibres. In the free space category, we have radio propagation which includes broadcast (e.g., TV) and point to point (e.g., microwave signals). Let us discuss these in a greater detail.