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Plain old telephone service (POTS) is the voice-grade telephone service that is based on analog signal transmission, that was common before the advent of advanced forms of telephony such as Integrated Services Digital Network (ISDN), cellular telephone systems, and voice over Internet Protocol (VoIP). It 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).
The technology of plain old telephone service is characterized by several aspects:
In the United States and Australia, the pair of wires from the central switch office to a subscriber's home is called a subscriber loop. It is powered by direct current (DC), typically at a nominal voltage of 48V, supplied by a large bank of batteries in the central office facilities, resulting in continuation of service during most commercial power outages. The subscriber loop typically represents an electrical load of about 300 ohms, and does not pose a threat of electrocution to humans, although shorting the loop may be felt as an unpleasant sensation.
Many calling features became available to telephone subscribers after computerization of telephone exchanges during the 1970s and 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.