Modern, automated POTS is characterized by several aspects: • Bi-directional (
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 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
ringing tone •
Pulse dialing and
dual-tone multi-frequency signaling (DTMF) •
BORSCHT functions: battery feed (B), over-voltage protection (O),
ringing (R),
signaling (S), coding (C), hybrid (H), and test (T) • Loop start, ground start and
E&M signalling 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 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 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 types of communications devices, such as
modems and
fax machines, were initially designed to use traditional analog telephony to transmit digital information. ==Historical services==