PSTN stands for Public Switched Telephone Network, also knows as the Plain Old Telephone System (POTS). It is basically the inter-connected telephone system, over which telephones calls are made via copper wires.

PSTN is based on the principle of circuit switching. Therefore when a call is made, particular dedicated circuit activates which eventually deactivates when call ends. Telephone calls transmits as analogue signals across copper wires.

Fixed telephone subscription refers to a natural person or legal entity connected to the public switched telephone network (PSTN). Number of fixed telephone subscriptions is measured as a number of active fixed telephone lines (PSTN lines).

ISDN stands for Integrated Services Digital Network. it is designed for a completely digital telephone/telecommunications network. ISDN is designed to carry voice, data, images, video, etc. It is also designed to provide a single interface (interms of both hardware and communication protocols) for hooking up phones, fax machines, computers, video phones, video-on-demand systems, etc.

One key difference between ISDN and PSTN is the duplex mode. Analog transmissions operate at half-duplex; they can only send data or receive data, not both at the same time. Digital transmissions operate at full duplex they can send and receive data the same time.