The pair of wires from the central office switch to a subscriber's home is called a subscriber loop. It carries a direct current 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 1,700ohms, which translates into a maximum loop length of 18,000 feet or 5 km using standard 24-gauge wire. 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, 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.
Reliability
Although POTS provides limited features, low bandwidth, and no mobile capabilities, it provides greater reliability than other telephony systems. 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. However, POTS depends upon a hardwire connection from each household to the phone company. Many new housing developments are being offered which do not have such a connection so these homes depend upon a VOIP non-hardwire linkage to the phone company. Thus they are dependent upon home internet service which can fail for several reasons. Today, these have been replaced with optical glass fibres. Light is sent into one end of a glass or plastic cable at an angle large enough so that it will not be able to pass out of the cable into the surrounding air. The message is sent by light pulses and is more efficient and reliable.
