This is called synchronous communication. The transmission capacity of an electronic line such as a communications network. When expressed in Hertz, the frequency may be a greater number than the actual bits per second, because the bandwidth is the difference between the lowest and highest frequencies transmitted. Some telephone companies are replacing traditional telephone lines with fiber optic cables. In the future, almost all communications will employ fiber optics.
Fiber optics is more expensive and more fragile.
Digital transmission occurs with discrete intervals. For example, when you turn the volume up on an analog radio the sound goes higher in a continuous movement. The same action with a digital radion causes the sound to rise in steps. Sometimes it is hard to set the volume at your comfort level - it is sometimes still too low or a bit too high. The term has often been erroneously used to specify bits per second.