Bandwidth is two different concepts in analog systems and digital systems. In analog systems, bandwidth refers to the pass bandwidth of the system. As for the pass band, I want to have a band pass filter. Basically, the output power drops to the interval between the two frequency points at the original half (-3dB). The unit is Hz. In digital systems, bandwidth is usually referring to data transmission rate, and units are bit / s or baud / s. For binary conditions, the bandwidth is exactly equal to the rate.
