ectronics, and the supplied voltage and current are directly used for speech transmission in addition to signaling functions that include the detection of on/off-hook condition and dialing. Modern electronic tele- phones would not necessarily need this if they could take their power from a power socket at home. However, getting the power supply from the exchange is still an important feature because it ensures that the telephone network operates even in emergency situations when the power network may be down. 2.3.1 Setup and Release of a Call Each telephone has a switch that indicates an on- or off-hook condition. When the hook is raised, the switch is closed and an approximately 50 mA of current starts flowing. This is detected by a relay giving information to the control unit in the exchange (Figure 2.2). The control unit is an efficient and reliable computer (or a set of computers) in the telephone exchange. It acti- vates signaling circuits, which then receive dialed digits from subscriber A. (We call a subscriber who initiates a call subscriber A and a subscriber who receives a call subscriber B.) The control unit in the telephone exchange con- trols the switching matrix that connects the speech circuit through to the called subscriber B. Connection is made according to the numbers dialed by subscriber A. When the call is being routed to subscriber B, the telephone exchange supplies to the subscriber loop a ringing voltage and the bell of subscriber B's telephone starts ringing. The ringing voltage is often about 70V ac with a 25-Hz frequency, which is high enough to activate the bell on any telephone. The ringing voltage is switched off immediately when an off-hook condition The Telecommunications Network: An Overview is detected on the loop of subscriber B, and then an end-to-end speech circuit is connected and the conversation may start. Figure 2.3 shows the signaling phases on a subscriber loop. When the exchange detects the off-hook condition of a subscriber loop, it informs us with a dial tone that we 