Plain old telephone system

Plain old telephone service (POTS) is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. POTS was the standard service offering from telephone companies from 1876 until 1988 when the now-obsolete Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) was introduced, followed by cellular telephone systems, and Voice over IP (VoIP). POTS remains the basic form of residential and small business service connection to the telephone network in many parts of the world. The term reflects the technology that has been available since the introduction of the public telephone system in the late 19th century, in a form mostly unchanged despite the introduction of Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN).

POTS is characterized by several aspects:

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 1700 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 ft. or more.)

Many calling features became available to telephone subscribers after computerization of telephone exchanges during the 1980s. The services include voicemail, Caller ID, call waiting, speed dialing, conference calls (three-way calling), enhanced 911, and Centrex services.

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