Loop start

Loop start is a telecommunications supervisory signal provided by a telephone or private branch exchange (PBX) equipment in response to the establishment of a closed local loop, commonly referred to as the off-hook state. When idle, or on-hook, the loop potential is held a nominal 48V DC, provided by the telephone exchange or a foreign exchange station (FXS) interface. When a terminal initiates use of the line, it causes current to flow by closing the loop, and this signals the FXS end to provide dial tone on the line and to expect dial signals, in form of DTMF digits or dial pulses, or a hook flash. When the loop is opened and current stops flowing, the subscriber equipment signals that it has finished using the line; the telephone exchange resets the line to an idle state. When the FXS rings a telephone, it superimposes an alternating current (AC) signal onto the line. The most common ringing frequency is 20Hz. This amplitude is typically approximately 90V, but regulations prescribe that a minimum of 40V must be recognized. The ringing power was historically produced by a hand-cranked generator in the operator console of the exchange, or attached to a customer's telephone.