 is to be dialed, the dialing plate with finger holes is rotated clockwise to the end and released. While homing, the switch is break- ing the line current periodically and the number of these periods indicates the dialed digit. For example, digit 1 has one period, 2 has two periods, and 0 has 10 periods or cycles. Mechanics make the homing speed approxi- mately constant and each period is about 100 ms long with a 60-ms break (Figure 2.4). This method for the transmission of digits has also been used for signaling between exchanges and then it is known as loop disconnect signaling. The value of the loop current differs slightly from country to country and it is also dependent on line length and supply voltage, for example. Typi- cally it is from 20 to 50 mA, high enough to control old generation electro- mechanical switches that used pulses to control directly the rotating switches of the switching matrix of an exchange. 2.3.3 Tone Dialing Currently telephones include electronic circuits that make possible the implementation of better means for signaling. Digital exchanges do not require high-power pulses to drive the selectors as old electromechanical switches did. However, subscriber lines are still, and will be, supplied by a -48- or -60-V battery SO that telephones continue to operate independent of the electric power supply. Electronic telephones use 50- to 500-uA current The Telecommunications Network: An Overview all the time to supply power to their electronic circuitry, which is needed for number repetition, abbreviated dialing, and other additional features of modern telephone sets. Modern telephones usually have 12 push buttons (keys A to D of Figure 2.5 are not included in an ordinary subscriber set) for dialing, each generating a tone with two frequencies. One of the frequencies is from the upper frequency band and the other from the lower band. All frequencies are inside the voice frequency band (300-3,400 Hz) and can thus be transmitted through the network from end to end, when the speech connectio