Loop start is a

telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that ...

supervisory protocol between a central office or

private branch exchange A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone syst ...

(PBX) and a subscriber

telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...

or other terminal for the purpose of starting and terminating a

telephone call A telephone call is a connection over a telephone network between the called party and the calling party. First telephone call The first telephone call was made on March 10, 1876, by Alexander Graham Bell. Bell demonstrated his ability to "ta ...

. It is the simplest of the telephone signaling systems, and uses the presence or absence of loop current to indicate the

off-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...

and

on-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...

loop states, respectively. It is used primarily for subscriber line signaling. An extension of the protocol that adds

disconnect supervision In telecommunication, supervision is the monitoring of a telecommunication circuit for telephony to convey to an operator, user, or a switching system, information about the operational state of the circuit. The typical operational states of trunks ...

is often called kewlstart.

Protocol operation

When the telephone is ''on-hook'', the potential of the ring conductor of the

local loop In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the commo ...

is held at a nominal level of -48V DC with respect to the tip conductor, provided by the

telephone exchange telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syste ...

or a

foreign exchange station Foreign exchange service (FX) is an access service in a telecommunications network in which a telephone in a given exchange area is connected, via a private line, as opposed to a switched line, to a telephone exchange or central office in another e ...

(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

resets the line to an idle state. For alerting an incoming call, the exchange rings the telephone by superimposing an

alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...

(AC) signal onto the line of between 40 V and 90 V. The frequency of the signal is 20 Hz. In early exchanges, the ringing power was produced by a hand-cranked generator in the operator console of the exchange. Loop start signaling cannot detect simultaneous seizure of the line on both ends, commonly called

glare Glare (derived from GLAss REinforced laminate ) is a fiber metal laminate (FML) composed of several very thin layers of metal (usually aluminum) interspersed with layers of S-2 glass-fiber ''pre-preg'', bonded together with a matrix such as epo ...

, when used on trunk lines. The protocol imposes no requirement to the central office end to inform the subscriber end of distant far-end disconnection conditions. Such conditions are properly recognized in ground start signaling, a protocol primarily used on trunks between switching systems.

Signaling extensions

Modern loop start trunks often provide additional methods supervision to avoid the far-end-disconnect problem. Answer supervision and

are extensions to alert a

foreign exchange office Foreign exchange service (FX) is an access service in a telecommunications network in which a telephone in a given exchange area is connected, via a private line, as opposed to a switched line, to a telephone exchange or central office in another e ...

(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 (''battery reversal'') for the duration of the call when it has been answered. For disconnect supervision the polarity may reverse back to its original state. Polarity switching results in reversals of the direction of loop current. When disconnect supervision is implemented without answer supervision, polarity reversal may still be used, but this is rarely implemented in public telephone networks. Typically, it is implemented by removal of battery voltage from the line for a short period of time, resulting in loss of loop current. Interruption of loop current is variously called open loop disconnect, or an

open switching interval In telecommunication, supervision is the monitoring of a telecommunication circuit for telephony to convey to an operator, user, or a switching system, information about the operational state of the circuit. The typical operational states of trunks ...

. Additional types of loop signaling are signals to the central office that the subscriber intends to initiate a second simultaneous call, a three-way conference call, or answer a second incoming call. This signal is called flashing or

hook flash On analog telephone lines with special services, a flash or register-recall signal is used to control functions on the public telephone exchange, PBX or VoIP ATA. The term "register-recall" in Europe refers to sending a discrete signal to alert t ...

, and is performed by interrupting the loop for a fraction of a second, typically at least 300 ms. The flash signal is longer than a rotary dial pulse, sometimes called a ''short flash'', and is shorter than the time required for the on-hook condition. Disconnect supervision is also known as calling party control (CPC), and forward disconnect.

Kewlstart

Kewlstart is a coined term created in the Asterisk PBX open-source software community for an extension of loop start signaling for FXS and

FXO Foreign exchange service (FX) is an access service in a telecommunications network in which a telephone in a given exchange area is connected, via a private line, as opposed to a switched line, to a telephone exchange or central office in another e ...

telephony interfaces which adds

. This type of on-hook/off-hook supervision was in use in step-by-step systems as well as electronic switching systems in North America for many decades. Disconnect supervision signals the called terminal device, typically an automated customer premises equipment, that the remote calling party has hung up. Disconnect supervision is implemented as an

(OSI), a period of several hundred milliseconds during which the loop current is interrupted. Some switching systems remove loop battery voltage for about 250 ms within 6 seconds after the far-end party disconnects.

References

{{reflist
Glossary of telecommunications terms
Telephony signals

Protocol operation

Signaling extensions

Kewlstart

See also

References