What is the PSTN (Public switched telephone network)?

PSTN in a Nutshell

The PSTN at a local level consists of a private consumer’s telephone connecting via a local loop to local telephone exchanges, which in turn connect via optical access trunks to main exchanges called central offices (CO). The CO’s in turn connect via regional, national and international trunks to other public exchanges, which span the globe allowing a telephone to call another telephone on the other side of the world.

PSTN Local Loop

Originally, the PSTN was entirely analogue but today its core infrastructure is almost entirely digital with only the last mile or local loop that connects private households still being analogue. The local loop, which is the property of the Teleco, is predominately over long established copper wire twisted pair, which has a long return on investment time (ROI) and is very expensive to install and maintain.

The PSTN Architecture

When the PSTN first developed, it did so as a hierarchal model, whereby if a local call could not be satisfied by the local exchange it was passed by default up to a regional exchange. If the call could still not able be initiated by the regional exchange, it would be passed upwards to a national exchange, and so on. This was a beneficial model as it saved on the number of peering trunks, over landlines, required between regional and national exchanges. Today the PSTN is a much flatter model though still hierarchal in design. This is due to Sonet/SDH optical networks, which connect exchanges over vast optical rings that span the country.

ITU-T Standards and Practices

Telephone numbering and country codes falls under the control of the ITU-T and their E.163 and E.164 standards provide the single global address space for telephone numbers.

The PSTN does not only cover fixed line circuit based telephone networks, it also includes the new generations of mobile networks. The PSTN does not however include the internet based VoIP technologies, as these do not use PSTN infrastructure or standards. The voice quality and reliance of the PSTN is though the benchmark used too judge VoIP call quality.

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