Loop start
Loop start is a telecommunications supervisory protocol between a central office or private branch exchange (PBX) and a subscriber telephone or other terminal for the purpose of starting and terminating a telephone call. It is the simplest of the telephone signaling systems, and uses the presence or absence of loop current to indicate the off-hook and on-hook loop states, respectively. It is used primarily for subscriber line signaling. An extension of the protocol that adds disconnect supervision is often called kewlstart. Protocol operation When the telephone is on-hook, the potential of the ring conductor of the local loop is held at a nominal level of -48V DC with respect to the tip conductor, provided by the telephone exchange or a foreign exchange station (FXS) interface. When the terminal initiates use of the line, it causes current to flow by closing the loop, and this signals the exchange 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 for a certain period of time, the subscriber equipment signals that it has finished using the line; the telephone exchange