On- and off-hook are complementary states of a telephone set (or any device that interfaces with a public switched telephone network, PSTN) indicating whether the device’s circuit is closed (off-hook) or open (on-hook). The terms derive from the mechanical action of placing a handset on or lifting it from a cradle, which historically opened or closed the electrical loop required for telephone service.
Definition
- Off-hook: The condition in which the telephone circuit is closed, allowing current to flow between the telephone exchange and the device. This state is achieved by lifting the handset, pressing a line button, or otherwise signaling the desire to initiate or maintain a call. In modern electronic devices, off-hook may be indicated by a voltage drop on the line (typically from a nominal −48 V DC to a lower level) and the activation of a call‑progress tone or audio path.
- On-hook: The condition in which the telephone circuit is open, preventing current flow. This state is reached by placing the handset back in its cradle, pressing an “on‑hook” button, or otherwise terminating the call. The line returns to its idle voltage, and the exchange releases the circuit for other users.
Technical Operation
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Line Signaling
- The PSTN supplies a −48 V DC bias on the tip and ring conductors of a subscriber line.
- When a device goes off‑hook, the loop resistance falls (typically below 600 Ω), causing a measurable current (approximately 20 mA) that informs the central office switch that the line is seized.
- The central office provides a dial tone and, upon receipt of dialing information (pulse or DTMF), proceeds with call setup.
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Circuit Protection
- Modern exchanges include over‑current and fault detection to prevent damage from prolonged off‑hook conditions (e.g., “off‑hook alarm”).
- On‑hook detection is used to release call resources, terminate billing, and enable call waiting or other features.
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VoIP and Digital Systems
- In Voice over IP (VoIP) and digital PBX systems, “off‑hook” and “on‑hook” are simulated by signaling messages (e.g., SIP “INVITE” for off‑hook, “BYE” for on‑hook). The physical state of a handset may be abstracted, but the logical distinction remains essential for call control.
Historical Context
The terminology originates from the rotary‑dial era, where the handset rested on a metal cradle that mechanically completed or broke the circuit. Early telephone exchanges (e.g., the Strowger switch) relied on these electrical changes to route calls. The physical on‑hook/off‑hook distinction persisted through the transition to electronic switching and remains in contemporary telecommunication standards.
Related Standards
- ANSI/TIA‑603‑A: Defines the electrical characteristics of analog telephone interfaces, including on‑hook and off‑hook voltage and current specifications.
- ITU‑T Recommendation G.704: Addresses signaling in digital transmission networks, referencing circuit‑level status analogous to on‑hook/off‑hook.
- SIP (Session Initiation Protocol): Uses request methods that map to off‑hook (INVITE) and on‑hook (BYE, CANCEL) actions in IP‑based telephony.
Applications
- Call Initiation: Lifting the handset (off‑hook) signals the network that the subscriber wishes to place a call.
- Call Termination: Restoring the handset to its cradle (on‑hook) ends the call and releases network resources.
- Call Waiting and Hold: Devices may temporarily transition between on‑hook and off‑hook states under program control to manage multiple concurrent call sessions.
Safety and User Experience
Incorrect handling of on‑hook/off‑hook states can lead to “off‑hook” alarms (continuous off‑hook without termination) or “busy” signals if a line is already off‑hook. Devices typically provide audible feedback (e.g., a fast busy tone) to inform users of improper state changes.
This entry summarizes the established technical meaning of “on- and off-hook” within telecommunications, based on widely recognized industry standards and historical usage.