Bandwidth is a measure of the information carrying capacity of a communications channel, whether it be of the wired or wireless type.
In analogue systems bandwidth is measured as a frequency in Hertz (Hz), and it is calculated as the difference between the highest and lowest frequencies of a band. The term bandwidth can apply to various types of equipment, e.g. audio, radio, video, etc., and in each case the bandwidth is calculated by subtracting the lower frequency limit of the band from its upper frequency limit.
The range of frequencies that a particular signal will occupy can also be referred to as its bandwidth (as all signals inherently occupy a certain amount of bandwidth), or alternatively it may mean the range of frequencies that a piece of equipment will respond to. Furthermore, because the rate at which digital data flows through a system will depend on the bandwidth, this data rate can also be shown to be a measure of the system's bandwidth.
If the bandwidth of a system is too narrow for a particular signal to pass through it, then there will be some distortion to that signal, or loss of its information content.
Because bandwidth is closely related to the signal frequencies being transmitted, it gives an indication of the capacity of the system to cope with different signals. This means that a wide bandwidth will allow higher data rates, and conversely a narrow bandwidth will restrict the data rate. A voice signal requires only about a 3 kHz bandwidth, whereas a TV video signal needs about 6 MHz of bandwidth, because it contains much more information. Signals outside the band of interest are usually suppressed, in order to prevent interference to or from other equipment.
Bandwidth has a general meaning of how much information can be carried in a given time period (usually a second). For example, a broadband link (i.e. one with a wide bandwidth) may be able to carry enough information to present moving video images. If one link or piece of equipment in a communication network has a much smaller bandwidth than the rest, then this will determine the data rate for the whole system.