telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...

, 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 the line''. ''Off-hook'' originally referred to the condition that prevailed when telephones had a separate earpiece (''receiver''), which hung from its

switchhook A telephone hook or switchhook is an electrical switch which indicates when the phone is hung up, often with a lever or magnetic button inside the cradle or base where a telephone handset resides. It takes its name from old wooden wall telephone ...

until the user initiated a telephone call by removing it. When off hook the weight of the receiver no longer depresses the spring-loaded switchhook, thereby connecting the instrument to the

telephone line A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or ot ...

Off-hook

The term off-hook has the following meanings: * The condition that exists when a

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 user instrument is in use, i.e., during dialing or communicating. * A general description of one of two possible

signaling In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

states at an interface between telecommunications systems, such as tone or no tone and ground connection versus battery connection. Note that if off-hook pertains to one state,

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 ...

pertains to the other. * The active state (i.e., a closed loop (

short circuit A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit ...

between the wires) of a

subscriber line 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 common ...

or PBX user loop) * An operating state of a communications link in which

data transmission Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal transmitted over a point-to-point o ...

is enabled either for (a) voice or data communications or (b)

network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

MIL-STD-188 MIL-STD-188 is a series of U.S. military standards relating to telecommunications. Purpose Faced with "past technical deficiencies in telecommunications systems and equipment and software…that were traced to basic inadequacies in the applicat ...

On an ordinary two-wire telephone line, off-hook status is communicated to the

telephone exchange A 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 syst ...

by a resistance short across the pair. When an off-hook condition persists without dialing, for example because the handset has fallen off or the cable has been flooded, it is treated as a ''permanent loop'' or

permanent signal Permanent signal (PS) in American telephony terminology, or permanent loop in British usage, is a condition in which a POTS line is off-hook without connection for an extended period of time.Telcordia Technologies, GR-505-CORE Issue 2, December ...

. The act of ''going off-hook'' is also referred to as ''seizing'' the line or channel.

On-hook

The term on-hook has the following meanings: * The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook. The weight of the receiver depresses the spring-loaded switchhook thereby disconnecting the idle instrument (except its bell) from the telephone line. * One of two possible

states, such as tone or no tone, or ground connection versus battery connection. Note: if on-hook pertains to one state, off-hook pertains to the other. * The idle state, i.e., an open loop of a

or PBX user loop. * An operating state of a

telecommunication circuit A telecommunication circuit is a path in a telecommunications network used to transmit information. Circuits have evolved over time from generally being built on physical connections between individual hardware cables, as in an analog phone swi ...

in which transmission is disabled and a high impedance, or "open circuit", is presented to the link by the

end instrument In the context of telecommunications, a terminal is a device which ends a telecommunications link and is the point at which a signal enters or leaves a network. Examples of terminal equipment include telephones, fax machines, computer termina ...

(s). Note: during the on-hook condition, the link is responsive to ringing signals. The act of ''going on-hook'' is also referred to as ''releasing the line'' or ''channel'', and may initiate the process of clearing.

References

{{reflist Telephony signals

Off-hook

On-hook

See also

References