The PSTN—a Historical Overview
The PSTN was created to interconnect telephones. Over time it grew to be the largest man-made technical artefact ever created. Now the old 2-wire analog PSTN is nearing its end days as many new buildings have only fiber connections into the network. We are looking into implications for users for a later article but we begin the series with a look at the interesting history of the PSTN – where we started from.
What Is the PSTN?

Until the latter half of the 20th century, the entire PSTN was analog. Each user had a connection to a pair of wires, which were routed via larger cables along with thousands of other pairs— ultimately arriving at an ’exchange’, or switch, in the telco ‘Central Office’ (CO). This was the ‘local’ part of the PSTN.
Signalling used a rotary ’dial’ to send pulses to the switch— hence ’dialing’.
The Main Network
The lines interconnecting Central Offices allocated an audio bandwidth of 4kHz for each call—sufficient for understandable voice communication. Rather than use one copper pair for each call, main network communication usually used frequency division multiplexing to combine multiple 4kHz channels into a larger communication link. These used microwave, satellite, and co-ax cable systems, including submarine cables.
The PSTN did not connect every Central Office to every other, except for in metro areas. Rather, they architected a hierarchy of COs, rather like a branch and tree system.
Office Phone Systems
Business offices had private branch exchanges (or PBXs) to connect PSTN lines to users. Direct In Dial (DID) technology gave users their own public number. Rotary dial evolved to touch tone dialing keypads.
Services
The voice telephone service that the PSTN carried is often referred to as POTS (Plain Old Telephone Service).
Apart form POTS, there are many other services that have grown up using the PSTN. These services converted the device interface into audio signals that could use the PSTN.
Prior to the broadband systems we know today, most data transmission used ‘modems’ to convert binary data into audio signals that could be sent over the dial-up PSTN. Many computer users back in the ‘70s and ‘80s will remember using dial-up modems to connect to a computer.
Facsimile transmission (aka fax) was used to scan documents and reproduce them at a remote location. The inter-connecting technology was the dial-up PSTN using built-in modems. The fact that fax machines contained an identifier code meant that a fax receipt showed sender, recipient and date/time—acting as verification of receipt. Even today, the fax receipt is valued by the medical industry and by many other industries as a verified transmission.
Many offices still have separate PSTN lines for fax machines and sometimes even for modems.
Background music (Muzak), including ‘elevator music’ and music-on-hold was delivered over the PSTN, although it usually used dedicated wide bandwidth feeds to COs to give more than 4kHz.
Legacy Problems
That the PSTN worked was never in doubt, but it was limited. Some users wanted more and saw the old-school telco monopoly as a barrier to the much needed support for data transmission. This started the next major step in the evolution of how POTS was provided and introduced an avalanche of new capabilities using digital technologies. More of that next time.
If you’d like to comment on this article or explore these ideas further, contact me at peter.
This article was published in the
November 2022
edition of The TMC Advisor
- ISSN 2369-663X Volume:9 Issue:6
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