End of the PSTN

By Peter Aggus

The Public Switched Telephone network was created to interconnect telephones using copper wire connections. Copper is on its way out, being replaced by digital delivery to the customer, usually over fibre optic cable. New technology is great for telephones, but other services work best on the PSTN and we rarely think about them. How did we get to this? What do we need to do?

PSTN and POTS

Until the latter half of the 20th century, the entire PSTN was analog, and it was the largest man-made technical artefact ever created. A single user was connected to the CO (central office) with a pair of copper wires. The service provided by this technology is referred to as "Plain Old Telephone Service," or POTS. While there is still a need for POTS as a service, its delivery via the PSTN will end.

Evolution to Digital

The Main Network, connecting COs,?originally used a 4kHz analogue audio channel for each call with multiple calls multiplexed together on one link. By the late 20th century the Main Network was, in many countries, built using 64KBit/s digital channels—known as the Integrated Services Digital Network (ISDN).

Moving to IP

In parallel, the IT world focused on packet switched technology using the then new Internet Protocol, or IP. The delivery of internet to end users?originally used dial-up modems over the PSTN. This evolved to ADSL, where internet was piggybacked onto the existing copper pair telephone service. Broadband internet delivery started sharing cable TV delivery systems.

Years later, the cost of copper drove the replacement of copper cables by fibre to a curbside "cabinet" or directly to end users, making fibre broadband delivery the latest technology.

Deregulation

The telco monopoly evolved in many countries into a free-market system. It rapidly became apparent that IP was significantly cheaper than ISDN and offered more evolution options. Business phone systems started to move from digital circuit-based systems to Voice over IP (VoIP) systems, enabling the sharing of office networks and ending the days of separate voice and data wiring.

Internet service was available from many companies and PSTN providers lost high volume customers. Regulators had to levy cross-subsidies where the new entrants had to pay to support the old PSTN providers and their legacy services. Pressure is on to remove that subsidy and effectively kill the PSTN service where adequate broadband IP service exists.

Non Speech Use

The biggest non-voice PSTN service is fax. There are IP-based fax services and document scanning is now commonplace but PSTN fax remains simple and verifiable. VoIP technology is not fax-friendly because of its lower coding bandwidth and variable latency. It is possible to compensate, but most users have traditionally installed stand-alone PSTN lines for their fax machines, alarm lines, etc.

What Should You Do?

The future of the analog PSTN is limited—in many locations it is already unavailable. Your services MUST migrate to delivery on an IP platform. Simply using PSTN-over-IP boxes will frequently not work unless those boxes are an end-to-end solution that compensates for latency drift and other imperfections. Why go to that expense when every service can be built better on a native IP platform?

If you’d like to explore these ideas further or comment on this article, contact me at?[email protected].

This article is reproduced from the January 2023 edition of The TMC Advisor.?advdoc.php (tmcconsulting.ca)