Digital Systems Bandwidth

In a digital communication system, bandwidth has a dual meaning. In the technical sense, it is slang for baud, the rate at which symbols may be transmitted through the system. It is also used in the colloquial sense to describe channel capacity, the rate at which bits may be transmitted through the system (see Shannon Limit). Hence, a 66 MHz digital data bus with 32 separate data lines may properly be said to have a bandwidth of 66 MHz and a capacity of 2.1 Gbit/s — but it would not be surprising to hear such a bus described as having a “bandwidth of 2.1 Gbit/s.” Similar confusion exists for analog modems, where each symbol carries multiple bits of information so that a modem may transmit 56 kbit/s of information over a phone line with a bandwidth of only 12 kHz. A related metric which is used to measure the aggregated bandwidth of a whole network is bisection bandwidth.

In discrete time systems and digital signal processing, bandwidth is related to sampling rate according to the Nyquist-Shannon sampling theorem.

Bandwidth is also used in the sense of commodity, referring to something limited or something costing money. Thus, communication costs bandwidth, and improper use of someone else's bandwidth may be called bandwidth theft.

When Additive white Gaussian noise is present in a digital communication channel, the Shannon—Hartley theorem gives the relationship between the channel's bandwidth, the channel's capacity, and the Signal-to-noise ratio (SNR) ratio of the system.