d key systems Crossbar technology (Figure 34.2) used relays to make connec- 34.1.4 Telephones tions between wires in a physical matrix. Rather than responding to individual digits, all the all the required digits would be sent to the Telephones perform a number of fundamental purposes. On receipt control system which would then find a possible route through the of a signal from the telephone system indicating an incoming call switching matrix. While calculating the required number of matrix (ringing), the telephone is expected to deliver an audible and/or elements was still required, this method of operation was more visual signal to alert the called person. When the called person efficient since all possible paths through the matrix could be tried. answers the call (goes off-hook), the telephone must signal the The other major advantage of this method was that routeing infor- telephone system to remove the ringing signal and cut through the mation could be different to the dialled number information. This transmission path. The receiver in the telephone converts electrical made it much easier to achieve the goal of one telephone number energy representing the caller's voice into acoustical energy. The regardless of where the call originated from. transmitter in the telephone converts acoustical energy into electri- cal energy. A part of the telephone circuitry sends some of the electrical energy from the transmitter to the receiver so that the user can hear their own voice. This is called sidetone. When the called person disconnects the call (goes on-hook), the telephone must signal the telephone system to this effect. On an outgoing call, the telephone must request service from the telephone system (by going off-hook). The telephone system usually returns dial tone to the caller at this point. The telephone must now indicate to the telephone system the number of the called person. Early telephones disconnected and reconnected the circuit to the telephone system to send pulses (dial pulses) to indicate the numbe