Plain old telephone service


PSTN stands for Public Switch Telephone Network, which is defined as the Global collection of interconnects. It was originally designed to support circuit-switched voice communication. The PSTN provides the traditional Plain Old Telephone Service (POTS) to residences and many other establishments.

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

figure [1]

Bandwidth:The basic PSTN network link supports 64 Kbps of bandwidth. The PSTN phone line which carries this bandwidth is basically a copper cable. The PSTN utilizes the SS7 (Signalling System#7) signaling protocol. There are the parts of the PSTN which is utilized for DSL, VoIP and other Internet-based network technologies. PSTN Standard:

The PSTN is created by the ITU-T which is basically a technical standard. It uses E.163/E.164 telephone numbers for addressing. Architecture :Traffic engineering was the earliest example of “PSTN” that delivers the quality of service (QoS) guarantees. The PSTN Architecture is shown below:

Figure [2]

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. Digital channel

The network was initially created by using analog voice connections through manual switchboards, automated telephone exchanges replaced most switchboards, and later digital switch technologies were used. Most switches now use digital circuits between exchanges, with analog two-wire circuits still used to connect to most telephones. A DS0 circuit is the basic granularity of circuit switching in a telephone exchange.

A DS0 is also known as a timeslot because DS0s are aggregated in time-division multiplexing (TDM) equipment to form higher capacity communication links. A Digital Signal 1 (DS1) circuit carries 24 DS0s on a North American or Japanese T-carrier (T1) line, or 32 DS0s (30 for calls plus two for framing and signaling) on an E-carrier (E1) line used in most other countries. In modern networks, the multiplexing function is moved as close to the end user as possible, usually into cabinets at the roadside in residential areas, or into large business premises.

The timeslots are conveyed from the initial multiplexer to the exchange over a set of equipment which is collctively known as the access network. The access network and inter-exchange transport of the PSTN use synchronous optical transmission, for example, SONET and Synchronous Digital Hierarchy (SDH) technologies.

There are a number of reference points defined within the access network, . Most of these are of interest mainly to ISDN but one – the V reference point – is of more general interest. This is the reference point between a primary multiplexer and an exchange. References: