View source for Plain old telephone service

'''Plain old telephone service''' ('''POTS''') is the voice-grade [[telephone]] service that remains the basic form of residential and small business service connection to the telephone network in most parts of the world. The name is a [[retronym]], and is a reflection of the telephone service still available after the advent of more advanced forms of telephony such as [[ISDN]], [[mobile phone]]s and [[VoIP]]. POTS has been available almost since the introduction of the public telephone system in the late 19th century, in a form mostly unchanged to the normal user despite the introduction of [[Touch-Tone]] dialing, electronic [[telephone exchange]]s and [[fiber-optic communication]] into the [[public switched telephone network]] (PSTN). 

The system was originally known as the '''Post Office Telephone Service''' (or '''System''') in many countries.  The term was dropped as telephone services were removed from the control of national [[post office]]s.

POTS services include:
* bi-directional, or [[Duplex (telecommunications)|full duplex]], voice path with limited frequency range of 300 to 3400 Hz: in other words, a signal to carry the sound of the human voice both ways at once;
* [[call-progress tone]]s, such as [[dial tone]] and [[ringing signal]];
* subscriber dialing;
* operator services, such as [[directory assistance]], [[long distance]], and [[conference call]]ing assistance;
* a standards compliant analog telephone interface including [[BORSCHT]] functions

In the United States, the [[Tip and ring|pair of wires]] from the central switch office to a subscriber's home is called a [[subscriber loop]]. It is typically powered by −48[[Volt|V]] direct current (DC) and backed up by a large bank of batteries (connected in series) in the central office, resulting in continuation of service during most commercial power outages. The subscriber loop typically carries a "load" of about 300 [[Ohm]]s, and does not pose a threat of electrocution to human beings (although shorting the loop can be felt as an unpleasant sensation).

Many [[calling feature]]s became available to POTS subscribers after computerization of telephone exchanges during the 1970s and 1980s. The services include:
* [[Voicemail]]
* [[Caller ID]]
* [[Call waiting]]
* [[Abbreviated dialing|Speed dial]]ing
* [[Conference call]] (three-way calling)
* [[Enhanced 911]]
* [[Centrex]]
* and other services.

The communications circuits of the [[PSTN]] continue to be modernized by advances in digital communications; however, other than improving sound quality, these changes have been mainly transparent to the POTS customer. In most cases, the function of the POTS [[local loop]] presented to the customer for connection to telephone equipment is practically unchanged and remains compatible with old [[Pulse dialing]] telephones, even ones dating back to the early 20th century.

Due to the wide availability of POTS, new forms of communications devices such as [[modem]]s and [[facsimile machine]]s are designed to use POTS to transmit digital information.

== Reliability ==
While POTS provides limited features, low [[Bandwidth (signal processing)|bandwidth]] and no [[mobile phone|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 [[High availability|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 "[[uptime|five nines]]" reliability standard. It is equivalent to having a dial-tone available for all but less than five minutes each year.

==See also==
*[[Public switched telephone network]]
*[[RJ11, RJ14, RJ25|RJ11]] - the type of telephone jack common in most of the world for single line POTS telephones
*[[DTMF]] - the tones used for pushbutton dialing
*[[25-pair color code]]
*[[Category 1 cable]]
*[[Twisted pair]]
*[[Foreign exchange office]] (FXO)
*[[Foreign exchange station]] (FXS)
*[[Network interface device]]

==External links==
*[http://www.ling.upenn.edu/~kurisuto/phone_wiring.html Doing your own telephone wiring]