A dial-up modem converts digital [digital: data measured at discrete intervals, eg a digital watch typically moves from displaying one second to the next without displaying the values in-between ] signals from a computer to analogue [analogue: continuous data, eg the second hand on an analogue watch displays each second and the space in-between ] signals that are then sent down the telephone line. A modem on the other end converts the analogue signal back to a digital signal the computer can understand.
The maximum theoretical connection speed is 56Kbps [kilobits per second (Kbps): a measurement of the speed data is being transferred at ]. Dial-up is very slow by today’s standards and in real world use will typically result in a download [download: the transfer of a file or files from one computer connected to the Internet to another ] speed of roughly 5KBps [kilobytes per second (KBps): a measurement of the speed data is being transferred at ].
Phone calls cannot be made whilst connected to the Internet.