POTS is the name for the original analog telephone connection and stands for Plain Old Telephone Service, Post Office Telephone Service or Post Office Telephone System.


Depending on the interpretation, this can have the following (conflicting) meanings:

The analog endpoint of the PSTN, so the classic analog telephone line at the user's home. The abbreviation Public Switched Telephone Network is then suitable. If one means this definition, it is better to use analog telephony to avoid misunderstandings (although this term is also a bit vague).
The entire telephone network when it still worked analog. So it is actually meant PSTN in the analog era. If one means this definition, it is better to use telephone network or PSTN to avoid misunderstandings. The acronym for Publicly Operated Telephone System is suitable, as is the Post Office Telephone System (in Great Britain, for example, the telephone network was managed by the postal service for much of the twentieth century). However, the term POTS was only introduced retroactively in the 1970s when the new ISDN technology emerged, and it was necessary to have a specific name for the analog technology used until then. POTS also stands for "Plain Old Telephony System".

In the rest of this article, we will start from the first definition, as it is most consistent with the definition of other terms.

Note that the term PSTN is often also used as “analog counterpart to ISDN”. However, this definition is incorrect. PSTN is the name for the entire telephone network (analog or digital), while ISDN only refers to the end points of this. Consequently, it makes sense to use POTS in the first definition above so that it refers to the analog endpoints. Thus, in this terminology, the entire telephone network is PSTN (analog or (nowadays fully) digital), and this has user endpoints as either POTS (analog) or ISDN (digital).


All signals pass over only two conductors (except ground):

Basic signals:

standby signal
ringing signal (an AC voltage of several tens of volts)
tones to the user: dial tone, alarm tone, busy tone, overload tone, information tone, switch-on tone
dialing signals (originally pulse dialing, nowadays almost exclusively tone dialing)

Signals added later:

earth: the 'white button' that briefly connects the b-wire to earth, giving a signal to a PABX. This requires a third wire, the ground wire.
flash: official Register Recall (special key that briefly interrupts the line, usually used with a PABX or house telephone exchange) or for call waiting.
rate pulses: to operate a cost counter, traditionally a symmetrical alternating voltage with respect to earth
caller ID information (not all countries use the same standard for this)
2nd call signal


At rest, the line has a voltage of about 48 V. Many home telephone exchanges give a much lower voltage, and this is usually sufficient. The alternating current for the bell (formerly called alarm clock) is superimposed on this.

When the handset is lifted, the voltage drops to a much lower value. A current of about 30 mA will flow.

The impedance on the telephone line (a twisted pair) is 800 ohms.

A POTS line consists of a twisted pair of wires. A regular telephone cable often contains four wires plus an earth and can therefore be used for two connections.

The following names and vein colors are common.


Polarity to Earth

The phone uses negative voltages to ground. The two wires of the phone are called tip (point) and ring. The tip and ring designations come from the connectors operators used to use: the TRS (tip-ring-sleeve) connector. (Ring has nothing to do with ringing a phone here.)

Tip is the Earth and Ring is the active signal. The reason for the negative voltage is not arbitrary. When wires were first laid underground, it was discovered that copper is eaten away by electrolytic processes when it comes into contact with moist earth. Since the first insulation consisted of gutta-percha that only works well for a decade, negative voltage was chosen. After all, the electrolytic process eats away the positive pole. Because the copper wire was negative, it was not affected if the insulation started to go bad.

Polarity between a and b

Most modern telephone exchanges sometimes reverse polarity, as follows:

at rest: positive (a positive with respect to b)
lifted the handset, dial number: positive
finished dialing: negative †
    the correct number of digits has been chosen (if the number length is known)
    an incorrect number was dialed or the connection could not be established (with busy signal)
    the caller stops dialing, ringing during dialing
conversation: negative
incoming call (bell rings): negative (plus superimposed AC voltage)
call ended, the other party hanged up: positive with busy tone
discharge state: positive with lower amperage (the handset is next to the hook without a telephone connection)

† If a domestic telephone number is selected, the polarity will usually reverse immediately after the last digit. If you choose a foreign telephone number, the local exchange does not know how long the number is and the polarity will reverse if there is some time to ring.

Application of polarity

Most telephones are not sensitive to polarity. It is therefore possible to exchange the two wires without any problem and reversing the polarity has no effect. However, subscriber equipment can respond to polarity. For a home telephone exchange, the polarity has the advantage that it can be continuously determined that the outside line is busy:

When a call comes in, call power is repeatedly placed on the line. However, the polarity reverses before the first ringing signal.
If the caller hangs up again before the incoming call has been answered, this is immediately noticeable because the polarity reverses. If the polarity does not reverse, the hanging can only be determined because there is no ringing for some time (a few seconds).
If the other party hangs up at the end of a call, the busy tone is (usually) heard, but it is difficult to detect by machine. Reversing the polarity is clear.

A simple application of the polarity is the baby monitor, only consisting of a diode in series with the device, the horn of which is next to the device. When a call is made, the diode immediately ensures that the call is answered, so that one can hear what is happening in the room.

The polarity is not always reversed with switchboards and cable connections.


In the 1990s, in addition to telephony, POTS was also used to connect to the Internet. The dial-up speed with a modem is then a maximum of 56 kbps.