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.
An alternative to loop start signaling is ground start signaling.
Other signaling
Modern loop start trunks also have methods of answer supervision and disconnect supervision to alert a foreign exchange office (FXO) interface that the remote party has answered or hung up. Answer supervision usually takes the form of the central office reversing the polarity of the line for the duration of the call when it has been answered. This condition is called Reverse Battery or battery reversal. Disconnect supervision can take the form of the polarity reversing back, or removing voltage from the line for a short period of time (battery drop). Disconnect supervision explicitly provided over an interface using battery drop is known in the Asterisk PBX community as Kewlstart.
Another type of loop signaling is the ability for the customer to signal the central office that they would like to make a second simultaneous call, a three-way conference call, or answer a second incoming call. This signal is called flashing or hook flash, and is performed by interrupting the loop for a fraction of a second, usually around 600ms. The flash is longer than a rotary dial pulse, sometimes called a short flash, and is shorter than a hang-up (on-hook).