PSTN is the connection of many wired communication endpoints. The communication is circuit-switched in nature. Originally, PSTN endpoints were fixed-line analog telephone systems, also referred to as Plain Old Telephone Systems (POTS). However, most have completely converted to digital systems such as ISDN, and some of them are digital towards the core side but have wired analog function on the last mile, from the exchange center to the user location.
Note
A Circuit-Switched system has a dedicated path for communication. It offers a high quality of service and constant bit delay, as all the data traverses the same path. On the other hand, packet-switched systems move data in packets where each packet is independently transmitted through a different path that is dynamically decided. At the destination, the original message is reassembled from the received packet in the proper sequence.
The following diagram shows a typical PSTN setup for home subscribers and enterprise network...