science

On- and off-hook

On- and off-hook is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand On- and off-hook rather than just read about it. In short: 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.

Key takeaways

On- and off-hook belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect On- and off-hook to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of On- and off-hook from memory before moving on to harder problems.

Reference excerpt

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

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, 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. 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

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

This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).

Illustrations

Worked examples

Example 1 — a first encounter with On- and off-hook

Start with the simplest possible case. Write down what On- and off-hook claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to On- and off-hook before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about On- and off-hook ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of On- and off-hook

In research: On- and off-hook appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses On- and off-hook in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: On- and off-hook is common in secondary-school and first-year university syllabi. It links to neighbouring topics Telecommunications stubs, Telephony signals, so understanding it makes those chapters shorter.
In everyday life: Look for On- and off-hook outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study On- and off-hook in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what On- and off-hook means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain On- and off-hook out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is On- and off-hook in simple terms?

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.

Why does On- and off-hook matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study On- and off-hook?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on On- and off-hook.