Editing Plain old telephone service (section)

==Characteristics==
Modern, automated POTS is characterized by several aspects:<ref>{{cite book|last1=Coll|first1=Eric|title=Telecom 101|date=2008|publisher=Teracom Training Institute|isbn=978-1894887014|url=http://www.telecom101.com}}</ref>
*Bi-directional ([[Duplex (telecommunications)|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&nbsp;Hz, called the [[voiceband]], which is much less than the human hearing range of 20–20,000&nbsp;Hz
*[[Call-progress tone]]s, 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 (telephony)|ringing]] (R), [[Signaling (telecommunications)|signaling]] (S), coding (C), hybrid (H), and test (T)
*Loop start, ground start and [[E and M signaling|E&M signalling]]

The [[Tip and ring| pair of wires]] from the central office switch to a subscriber's home is called a [[local loop|subscriber loop]]. It carries a direct current (DC) [[volt]]age 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{{nbsp}}[[ohm]]s, which translates into a maximum loop length of {{convert|18,000|ft|km|disp=or|abbr=out|0}} using standard 24-[[American wire gauge| 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 {{convert|50,000|ft|km|disp=sqbr}} or more.)

Many [[calling feature]]s 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]], [[Abbreviated dialing|speed dialing]], [[conference call]]s (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 [[modem]]s and [[Fax|fax machines]], were initially designed to use traditional analog telephony to transmit digital information.