Bandwidth is the difference between the upper and lower frequencies in a contiguous set of frequencies. It is typically measured in hertz, and may sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on context. Passband bandwidth is the difference between the upper and lower cutoff frequencies of, for example, an electronic filter, a communication channel, or a signal spectrum. In case of a low-pass filter or baseband signal, the bandwidth is equal to its upper cutoff frequency. The term baseband bandwidth always refers to the upper cutoff frequency, regardless of whether the filter is bandpass or low-pass.
Bandwidth in hertz is a central concept in many fields, including electronics, information theory, radio communications, signal processing, and spectroscopy. A key characteristic of bandwidth is that a band of a given width can carry the same amount of information, regardless of where that band is located in the frequency spectrum (assuming equivalent noise level). For example, a 5 kHz band can carry a telephone conversation whether that band is at baseband (as in your POTS telephone line) or modulated to some higher (passband) frequency.
In computer networking and other digital fields, the term bandwidth often refers to a data rate measured in bits per second, for example network throughput, sometimes denoted network bandwidth, data bandwidth or digital bandwidth. The reason is that according to Hartley's law, the digital data rate limit (or channel capacity) of a physical communication link is proportional to its bandwidth in hertz, sometimes denoted radio frequency (RF) bandwidth, signal bandwidth, frequency bandwidth, spectral bandwidth or analog bandwidth. For bandwidth as a computing term, less ambiguous terms are bit rate, throughput, maximum throughput, goodput or channel capacity.
Bandwidth is a key concept in many telephony applications. In radio communications, for example, bandwidth is the frequency range occupied by a modulated carrier wave, whereas in optics it is the width of an individual spectral line or the entire spectral range.
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