Plain Old Telephone Service (POTS)

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POTS stands for Plain Old Telephone Service and it is the basic analog telecommunications service provided by a local telco. Plain Old Telephone Service (POTS) was the only type of telephone service until the 1970s.

Plain Old Telephone Service - POTS networking
Plain Old Telephone Service

How It Works

Starting from your home or customer premises, two-pair copper twisted-pair wire runs to your local telco’s central office (CO). This copper wire connection forms what is known as the local loop.

The CO has switches that connect you to another local subscriber, to another CO, or to a long-distance provider, depending on whether your call is local or long distance. POTS is an inexpensive circuit-switched telecommunications service, but it supports data transfer speeds up to only 56 Kbps. It typically takes 15 to 30 seconds to establish connections for data transfer using modems.

POTS Characteristics

  • Bi-directional (full duplex) communications.
  • Using balanced signaling of voltage analogs of sound pressure waves on a two-wire copper loops
  • Restricted to a narrow frequency range of 300–3,300 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 ringback tone.
  • Dial pulse signaling of addresses.
  • BORSCHT functions.

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 1,700 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 in the United States. 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.