On- and off-hook

Intelephony,on-hookandoff-hookare 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 calledseizing the line.Off-hookoriginally referred to the condition that prevailed when telephones had a separate earpiece (receiver), which hung from itsswitchhookuntil 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 thetelephone line.

Off-hook

Off hook telephone.

The termoff-hookhas the following meanings:

• The condition that exists when atelephoneor otheruserinstrument is in use, i.e., duringdialingor communicating.
• A general description of one of two possiblesignalingstates at an interface between telecommunications systems,such as tone or no tone andgroundconnectionversus battery connection. Note that if off-hook pertains to one state,on-hookpertains to the other.
• The active state (i.e., a closed loop (short circuitbetween the wires) of asubscriber lineorPBXuser loop)
• An operating state of a communications link in whichdata transmissionis enabled either for (a) voice or data communications or (b)networksignaling.

On an ordinary two-wire telephone line, off-hook status is communicated to thetelephone exchangeby 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 apermanent looporpermanent signal.

The act ofgoing off-hookis also referred to asseizingthe line or channel.

On-hook

On hook telephone handset

The termon-hookhas 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 astoneor no tone, orgroundconnection 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 asubscriber lineorPBXuser loop.
• An operating state of atelecommunication circuitin which transmission is disabled and a highimpedance,or "opencircuit",is presented to the link by theend instrument(s). Note: during the on-hook condition, the link is responsive toringing signals.

The act ofgoing on-hookis also referred to asreleasing the lineorchannel,and may initiate the process ofclearing.

See also

• Disconnect supervision
• Hook flash
• Line signaling
• Off-hook tone

References

This article incorporatespublic domain materialfromFederal Standard 1037C.General Services Administration.Archived fromthe originalon 2022-01-22.(in support ofMIL-STD-188).