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.
Read more about Loop Start: Other Signaling
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