Question 1

What is the PSTN (Public Switched Telephone Network)?

Accepted Answer

The PSTN, or public switched telephone network, is the combined telephone networks of the world's carriers, connected so that a number on one can be dialed from another. No single company owns it. Landline and mobile carriers both belong to it, and a service that gives you an ordinary phone number reaches it through a carrier. It began as a network of copper lines and circuit switches, and parts of it now run on IP.

Question 2

What is the difference between PSTN and PBX?

Accepted Answer

The PSTN is the public telephone network that lets a phone number on one carrier reach a number on another. A PBX is the phone system one organization runs for itself. It gives people extensions and decides how calls are routed, answered and transferred. A business normally uses both, because the PBX handles calls inside the organization and a PSTN connection carries calls to and from outside phone numbers.

Question 3

Which legacy devices are still at risk when PSTN/POTS lines are retired or moved to VoIP?

Accepted Answer

The devices at risk are the analog ones that use a copper phone line as their call path. That covers fire and burglar alarm dialers, elevator emergency phones, telecare and medical alert base stations, fax machines, dial-up card terminals, gate and entry intercoms, and the modems that report on building equipment. Some dial by themselves and some wait for a person to press a button. A device of the same kind that reports over a cellular or IP connection does not depend on the retiring analog line for its calls. Its own connection is a separate thing to check. For the analog ones there are three things to settle: whether the line is going away, whether its replacement carries the device's signals, and what powers that replacement in an outage.

Question 4

How does the PSTN work?

Accepted Answer

The PSTN connects a call in two layers that run side by side. Signaling messages pass from switch to switch to find the dialed number and ring it. A voice path then opens between the two phones and stays up until someone hangs up. On the older parts of the network, that path is a reserved 64 kbit/s circuit. On the newer parts it is a stream of IP packets, and gateways convert between the two where a call passes from one kind of network to the other.

Question 5

Is PSTN secure, and can PSTN calls be encrypted?

Accepted Answer

The PSTN gives a call no end-to-end encryption. Its security rests on how each carrier along the path protects its own network, and a caller can't see that. Parts of a call can be encrypted. The IP leg between your phone or app and your provider can use TLS for signaling and SRTP for audio, and an IP link between two carriers can be protected the same way. Each of those protections runs between two devices that share the keys, and it ends at the device that decrypts the call to pass it on. End-to-end encryption needs compatible equipment or software at both ends, and an ordinary call to a phone number doesn't arrange that.

Question 6

What is a PSTN gateway, and what do FXS and FXO ports do?

Accepted Answer

A PSTN gateway converts phone calls between telephone circuits and IP packets, so equipment built for one kind of network can work with the other. FXS and FXO are the two ends of an analog phone line. An FXS port plays the phone company's part and supplies dial tone and ringing, so you plug a telephone or fax machine into it. An FXO port plays the telephone's part, so you plug an existing analog line into it.

Question 7

What is the difference between PSTN and VoIP?

Accepted Answer

The PSTN is a network, and VoIP is a way of carrying a call. The public switched telephone network is the carriers' networks, connected so that a number on one can be dialed from another. Voice over IP sends a conversation as data packets over an IP network. The two overlap. A VoIP service can connect to the PSTN, and carriers use IP inside the PSTN itself. The practical comparison is narrower: a traditional phone line against a phone service delivered over a data connection.

Question 8

What is the difference between PSTN and POTS?

Accepted Answer

POTS is a kind of phone line, and the PSTN is the public network that line connects you to. POTS stands for plain old telephone service: one analog voice line that a telephone company delivers over a pair of copper wires. The PSTN, the public switched telephone network, is the networks of the telephone carriers, connected so that a number on one can be dialed from another. A POTS line is one way onto the PSTN. A mobile phone, a SIP trunk and a cloud phone service are others.