Loop start
Encyclopedia
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.
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).
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
or PBX in response to the completion of the loop circuit
Local loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...
, commonly referred to as 'off-hook
Off-hook
In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...
'. When idle, or 'on-hook', the loop is at 48V DC (provided by the CO
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
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
Dial tone
A dial tone is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed...
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
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
line now makes the line idle again. When the FXS needs to ring the customer, it superimposes an AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
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
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
lines and key systems.
An alternative to loop start is a ground start trunk.
Other signaling
Modern Loop Start Trunks also have methods of Answer supervisionAnswer supervision
Answer supervision is a term in telephony, describing a situation whereby the called party indicates to the central office that the call is being answered. -CAS E&M Signaling Basics:...
and Disconnect supervision
Disconnect supervision
Disconnect supervision is a term in telephony describing signaling between the telephone exchange and a connected party used to indicate that the connected call is being disconnected....
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
Disconnect supervision
Disconnect supervision is a term in telephony describing signaling between the telephone exchange and a connected party used to indicate that the connected call is being disconnected....
explicitly provided over an interface using battery drop is known in the Asterisk PBX
Asterisk PBX
Asterisk is a software implementation of a telephone private branch exchange ; it was created in 1999 by Mark Spencer of Digium. Like any PBX, it allows attached telephones to make calls to one another, and to connect to other telephone services including the public switched telephone network and...
community as Kewlstart
Kewlstart
Kewlstart is a coined term for an extension of Loop start signaling which adds Disconnect supervision through the use of an Open Switching Interval . In addition to the operation afforded by loop start, the CO signals the terminal end that the distant party has hung up...
.
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
Hook Flash
Hook flash or flash is a button on a telephone that simulates quickly hanging up then picking up again . This action can signal the telephone exchange to do something. A common use of hook flash is to switch to another incoming call with the call waiting service...
, 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).