Pots

Plain old telephone service, or POTS, is a term which describes 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. more...

The name is a reflection of the telephone service still available after the advent of more advanced forms of telephony such as ISDN, mobile phones and VoIP. It 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 exchanges and fiber-optic communication into the public switched telephone network (PSTN).

The system was originally known as the Post Office Telephone Service or Post Office Telephone System in many countries. The term was dropped as telephone services were removed from the control of national post offices.

POTS services include:

• bi-directional, or 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 tones, such as dial tone and ringing signal;
• subscriber dialing;
• operator services, such as directory assistance, long distance, and conference calling assistance;
• a standards compliant analog telephone interface including BORSCHT functions

In the United States, the pair of wires from the central switch office to a subscriber's home is called a subscriber loop. It is typically powered by -48V 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 Ohms, and does not pose a threat of electrocution to human beings (although shorting the loop can be felt as an unpleasant sensation).

Read more at Wikipedia.org