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

==Off-hook== thumb| Off hook telephone. 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. * A general description of one of two possible signaling states at an interface between telecommunications systems,<ref>{{cite web | url = https://archive.org/stream/bitsavers_westernEleEngineeringandOperationsintheBellSystem2_49741719/Engineering_and_Operations_in_the_Bell_System_2ed_1984 | title = Engineering and Operations in the Bell System, 2nd Ed | last = Rey | first = R. F. | date = 1984 | publisher = Bell Telephone Laboratories | access-date = 19 March 2017 | page = 267}}</ref> such as tone or no tone and ground connection versus battery connection. Note that if off-hook pertains to one state, on-hook pertains to the other. * The active state (i.e., a closed loop (short circuit between the wires) of a subscriber line or PBX user loop) * An operating state of a communications link in which data transmission is enabled either for (a) voice or data communications or (b) network signaling.<ref>Federal Standard 1037C</ref><ref>MIL-STD-188</ref>

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

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

==On-hook== thumb|1907 dial telephone, receiver on its 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 signaling 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 subscriber line or PBX user loop. * An operating state of a telecommunication circuit in which transmission is disabled and a high impedance, or "open circuit", is presented to the link by the end instrument(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.

== See also == * Disconnect supervision * Hook flash * Line signaling * Off-hook tone

== References == {{reflist}} {{FS1037C MS188}}

Category:Telephony signals