In telecommunications, a loop start is a supervisory signal given by a telephone or PBX in response to the completion of the loop circuit, commonly referred to as 'off-hook'. When idle, or 'on-hook', the loop is at 48V DC (provided by the CO or FXS interface). When a telephone or device wishes to use the line, it causes current to flow by closing the loop (going off-hook), and this signals the FXS end to provide dial tone on the line and to listen for DTMF digits or a hookflash (see below). When the loop is opened and current stops flowing, the subscriber's equipment is saying that it has finished using the line; the CO line now makes the line idle again. When the FXS needs to ring the customer, it superimposes an AC signal onto the line at 20Hz. This signal is traditionally 88V, but most devices are tolerant of signals between 60V and 100V. The power to ring phones was historically supplied by a hand-cranked generator on the customer's and operator's telephones.
Loop start signaling is normally used by POTS lines and key systems.
An alternative to loop start is a ground start trunk.
Modern Loop Start Trunks also have methods of Answer supervision and Disconnect supervision to alert the FXO end of the line that the remote party has answered or hung up. Answer supervision usually takes the form of the CO reversing the polarity of the line for the duration of the call when it has been answered. This 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 CO 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 opening the loop for a fraction of a second (usually around 600ms) and closing it again. The flash is longer than a rotary dial pulse (sometimes called a 'short flash', see Dial pulsing), and is shorter than a hang-up (on-hook).