PSTN refers to the Public Switch Telephone Network. It is the collection of the world’s interconnected public telephone networks.

The PSTN is a circuit-switch network or a dedicated circuit established for voice traffic, such as a telephone call. This set-up contrasts with packet-switching networks, wherein messages are broken down into packets and sent individually. The Internet is one example of a packet-switching network which follows the TCP/IP protocol.

PSTN, which used to be a network of fixed-line analog telephone systems, is now almost completely digital. Nonetheless, most of its subscribers are still linked by analog circuits. Fixed-line and mobile phones are now included in this new system set-up.

History and Architecture

PSTN traces its beginnings to the invention of the telephone and the early development of the telephone service. The first telephones were all in private use and had no networks as they were wired in pairs.

Employing the same technology used by telegraph systems, every telephone was later connected to a local telephone exchange all wired together with trunks. These networks were in turn wired together in a hierarchy that spans cities, countries, continents, and oceans.

A network was created using analog voice connections using manual switchboards. They were replaced by automated telephone exchanges, and later on by digital switch technologies. Nearly all switches now employ digital circuits among exchanges, whilst two-wire analog circuits are still used to connect most phones.

Before the advent of the Internet, the PSTN was vital for data transmission using circuit switching, just as it was used by voice communications. However, the PSTN is now becoming just another application of the Internet, with the voice traffic shifted to voice over internet protocol. PSTN would eventually shift from circuit switching to packet switching.

PSTN Digital Circuit

The PSTN uses the Digital Signal 0 (DS0), a 64 kbit/s channel originated by Bell Labs. This digital circuit carries a typical phone call from a calling party to a called party. Its audio digital sound is digitized using a pulse code modulation that operates at a rate of 64 kbit/s. The call is then transmitted via telephone exchanges from one end to another and switched with a signaling protocol used with the exchanges.

DS0’s are also known as timeslots. These timeslots are conveyed from the initial multiplexer to the exchange over the access network. A number of defined reference points are contained in these access networks. The V reference point is one particular reference point that is used between a primary multiplexer and exchange.