PSTN (Public Switched Telephone Network) Explained

July 10, 2017

What is the PSTN?

The PSTN or Public Switched Telephone Network is the complete formation of the world’s circuit-switched telephone networks which are operated by national, regional, or local telephone operators and provide the infrastructure and services for public telecommunication.

Although originally only intended to provide continuous, real-time voice communications for an average duration of three minutes or less, with a bandwidth of just 64 Kbps and delivered via twisted copper wire; PSTN quickly evolved both in its physical form and technical capabilities.

Now, nearly all PSTN components have moved in to the digital realm, however much of the access method for landline telephone remains this analogue, copper wire connection. This is mainly because the cost for telecom operators to replace the ‘last mile’ of copper wire with fibre optic cable or higher bandwidth media is expensive. Particularly when the development in wireless alternatives have made significant strides over the last few years.

Reliance on the PSTN

The number of people using landlines has been in significant decline over the last ten years, however a common misconception is that this means that PSTN therefore becomes less integral to modern infrastructure. In reality, the opposite is true. That’s because, aside from the method used to physically connect to PSTN exchanges, nearly everything else regarding call routing and processing remains the same.

The modern PSTN has adapted to the requirements of users. Now, alongside the original copper wire, it includes fibre optic cables, cellular networks, microwave transmission links, communications satellites and undersea cables. All of which have a larger amount of bandwidth available and therefore support much more than just voice communications.

How exactly does the PSTN work?

The ‘journey’ of a phone call begins with the initial dial tone and ends when the phone is hung up.

To start this process an individual user connects to the local exchange. Typically, if it is an individual (domestic) user with one line, the call goes straight to the local exchange, whereas if it is a group (such as a business) with multiple lines, the call will be managed by a PBX (private branch exchange) first and then passed to the local exchange.

At this point the calls journey is dependent on its destination. It may need to be pushed to internet service providers, cellular providers, international carriers, interexchange carriers etc. It is at this stage that the call can pass through many different layers of technology before reaching another user.

Breaking down the exact parts of equipment a call travels through can become confusing as the PSTN is comprised of a complex web of interconnected, nodes and transmissions links. However, although different infrastructures exist at the local, regional and national levels, the pieces of equipment within each section function in the same way.

You can break down the node types in to four variations:

1 – CPE (customer premises equipment)

Customer premises equipment refers to devices such as telephones, routers, switches, residential gateways, set-top boxes, fixed mobile convergence products, home networking adapters and Internet access gateways that enable consumers to access communications service providers’ services and distribute them around their house via a local area network (LAN).

A CPE can be an active equipment, as the ones mentioned above or a passive equipment such as analogue-telephone-adapters or xDSL-splitters.

2 – Transmission/Transport

The transmission (or ‘transport’ as it is sometimes known) node consists of the equipment and media that carry information between nodes of a network. This can include things like amplifiers, repeaters, multiplexers, digital cross-connect systems, and digital loop carriers.

3 – Access

The access node is responsible for connecting a user to the core network. Access equipment is typically installed within the telephone exchange or a roadside serving area interface cabinet.

You can read more about the details of the access network here.

4 – Core/Switching

In telecommunications, the core network is the central element of a network that provides services to customers who are connected by the access network. There are a number of services that the core network provides, but one of the main functions is to route telephone calls across the PSTN (public switched telephone network). This includes deciding when to setup, hold, charge, and release connections, and getting that information to the correct outlets that maintain and bill for each section of the network.

Typically, the core networks provide aggregation, authentication, call control/switching, charging, service invocation and gateways.

You can read more about the details of the core network here.

The induvial parts of the PSTN are constantly evolving and updating to fulfil the needs of an ever-increasing user base, however the fundamental structure of the networks have endured and become fundamental to global connectivity.

As the importance of global telecommunications increases and more bandwidth is required, this importance will only increase.


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