2.1 PSTN and ISDN

2.1 PSTN and ISDN

The Public Switched Telephone Network (PSTN) is the world wide network of public telephone systems. The PTSN is a circuit-switched system that operates using internationally agreed codes and unique telephone numbers.

The PSTN was initially developed as a fixed-line analogue telephone system and the one thing that every business had that connected them was the telephone. The use of public telephones in business and the growth of the use of computers led to the need for modems to allow the use of the analogue telephone system.

The PTSN has evolved into an almost completely digital system that now incorporates mobile telephone and computer technology.

Note: Sometimes the PTSN is referred to as the Plain Old Telephone System (POTS).


Integrated Services Digital Network (ISDN) was developed to use the existing circuit-switched telephone network system for digital transmission of voice and data. The use of ISDN allowed for higher speed and better quality than a traditional 56 kbps dialup connection.

ISDN is a set of protocols used to provide voice and data services over a circuit switched network.

There are two main types of ISDN: BRI and PRI

Basic Rate ISDN (BRI)

BRI consists of 2 bearer (B) channels for transferring data which operate at 64 Kbps each. The other channel is known as the delta (D) channel and operates at 16 Kbps. The D channel is used for link management purposes and signalling. This is known as 2B+1D

Primary Rate ISDN (PRI)

PRI depends on the country but a typical PRI is 23 bearer (B) channels at 64 Kbps for data and 1 D channel at 64 Kbps. This is known as 23B+1D.

Each B channel is a clear data path and with the use of multiple B channels ISDN can provide more bandwidth than other services, e.g. certain leased services. BRI gives a total usable bandwidth of 128 kbps whereas PRI gives a bandwidth of 1.44 Mbps

ISDN is a dial-on-demand service that is used when required.