science

Plain old telephone service

Plain old telephone service 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 Plain old telephone service rather than just read about it. In short: Plain old telephone service (POTS), or rarely publicly offered telephone service, is a retronym for voice-grade telephone service that employs analog signal transmission over copper loops. The term encapsulates a technology that has been available since the introduction of the public telephone system in the late 19th century, remaining largely unchanged despite the introduction of innovations such as Touch-Tone dial…

Key takeaways

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

Reference excerpt

Plain old telephone service (POTS), or rarely publicly offered telephone service, is a retronym for voice-grade telephone service that employs analog signal transmission over copper loops. The term encapsulates a technology that has been available since the introduction of the public telephone system in the late 19th century, remaining largely unchanged despite the introduction of innovations such as Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN). POTS was the standard service offering from telephone companies in the United States until c. 1988, when the Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) was introduced, followed by the development of cellular telephone systems and voice over internet protocol (VoIP). POTS has largely been phased out in many developed regions, though it remains in limited use in rural areas and legacy applications, including systems such as fire and security alarms, medical alert systems, fax machines and credit card machines. In the United States and other countries, telecommunications providers have increasingly retired copper-based networks in favor of fiber-optic and IP-based infrastructure.

Characteristics

Modern, automated POTS is characterized by several aspects:

Bi-directional (full duplex) communications. Using balanced signaling of voltage analogs of sound pressure waves on a two-wire copper loop Restricted to a narrow frequency range of 300–3,300 Hz, called the voiceband, which is much less than the human hearing range of 20–20,000 Hz Call-progress tones, such as dial tone and ringing tone Pulse dialing and dual-tone multi-frequency signaling (DTMF) BORSCHT functions: battery feed (B), over-voltage protection (O), ringing (R), signaling (S), coding (C), hybrid (H), and test (T) Loop start, ground start and E&M signalling The pair of wires from the central office switch to a subscriber's home is called a subscriber loop. It carries a direct current (DC) voltage at a nominal voltage of −48V when the receiver is on-hook, supplied by a power conversion system in the central office. This power conversion system is backed up with a bank of batteries, resulting in continuation of service during interruption of power to the customer supplied by their electrical utility. The maximum resistance of the loop is 1,700 ohms, which translates into a maximum loop length of 18,000 feet or 5 km using standard 24- gauge wire. (Longer loops are often constructed with larger, lower-resistance 19-gauge wire and/or specialized central office equipment called a loop extender. They may be 50,000 feet [15 km] or more.) Many calling features became available to telephone subscribers after computerization of telephone exchanges during the 1980s in the United States. The services include voicemail, caller ID, call waiting, speed dialing, conference calls (three-way calling), enhanced 911, and Centrex services. The communication circuits of the public switched telephone network continue to be modernized by advances in digital communications; however, other than improving sound quality, these changes have been mainly transparent to customers. In most cases, the function of the local loop presented to the customer for connection to telephone equipment is practically unchanged and remains compatible with pulse dialing telephones. Due to the wide availability of traditional telephone services, new types of communications devices, such as modems and fax machines, were initially designed to use traditional analog telephony to transmit digital information.

Historical services

In countries where telephone systems were originally operated by the postal service administrations, the systems were known as post office telephone service as early as 1912 In Australia, New Zealand, and the United Kingdom the telephone system was a government service, under Post Office control, until privatisation in the 1970s and 1980s.

Worked examples

Example 1 — a first encounter with Plain old telephone service

Start with the simplest possible case. Write down what Plain old telephone service 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 Plain old telephone service 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 Plain old telephone service 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 Plain old telephone service

In research: Plain old telephone service 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 Plain old telephone service 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: Plain old telephone service is common in secondary-school and first-year university syllabi. It links to neighbouring topics Telephone services, so understanding it makes those chapters shorter.
In everyday life: Look for Plain old telephone service 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 Plain old telephone service in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Plain old telephone service 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 Plain old telephone service out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Plain old telephone service in simple terms?

Why does Plain old telephone service 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 Plain old telephone service?

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 Plain old telephone service.