Plain old telephone service (POTS) is 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:
- 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 to 3300 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 signal.
- Dial pulse signaling of addresses.
- BORSCHT functions.
The pair of wires from the central switch office 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.
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.
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 forms of communications devices such as modems and facsimile machines were initially designed to use traditional analog telephony to transmit digital information.
While POTS provides limited features, low bandwidth, and no mobile capabilities, it provides greater reliability than other telephony systems (mobile phone, VoIP, etc.). Many telephone service providers attempt to achieve dial-tone availability more than 99.999% of the time the telephone is taken off-hook. This is an often cited benchmark in marketing and systems-engineering comparisons, called the "five nines" reliability standard. It is equivalent to having a dial-tone available for all but about five minutes each year.