A telecommunication network is required to transmit messages between any of its users (who are usually called customers or subscribers), and the messages may be conveyed by signals that are either digital or analogue. A digital signal can have only discrete values. The simplest digital signal is a binary signal having only two values. Binary-coded data from a digital computer is an obvious example. However, multilevel signals are also used. An analogue signal is a continuous function of time and at any instant it can have any value between limits set by the maximum power that can be transmitted. A speech signal is an obvious example. A television signal is a mixture of analogue and digital, since it transmits both picture contents and synchronizing pulses.