telephone exchange

6 Articles

When The UK’s Telephone Network Went Digital With System X

July 24, 2025 by 16 Comments

The switch from analog telephone exchanges to a purely digital network meant a revolution in just about any way imaginable. Gone were the bulky physical switches and associated system limitations. In the UK this change happened in the early 1980s, with what the Post Office Telecommunications (later British Telecom) and associated companies called System X. Along with the system’s rollout, promotional videos like this 1983 one were meant to educate the public and likely any investors on what a smashing idea the whole system was.

Although for the average person in the UK the introduction of the new digital telephone network probably didn’t mean a major change beyond a few new features like group calls, the same wasn’t true for the network operator whose exchanges and networks got much smaller and more efficient, as explained in the video. To this day System X remains the backbone of the telephone network in the UK.

To get an idea of the immense scale of the old analog system, this 1982 video (also embedded below) shows the system as it existed before System X began to replace it. The latter part of the video provides significant detail of System X and its implementation at the time, although when this video was produced much of the system was still being developed.

Thanks to [James Bowman] for the tip.

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A Complete Exchange From Scratch For Your Rotary Dial Phones

January 29, 2024 by 12 Comments

Such has been the success of the mobile phone that in many places they have removed the need for wired connections, for example where this is being written the old copper connection can only be made via an emulated phone line on an internet router. That doesn’t mean that wired phones are no longer of interest to a hardware hacker though, and many of us have at times experimented with these obsolete instruments. At the recent 37C3 event in Germany, [Hans Gelke] gave a talk on the analog exchange he’s created from scratch.

The basic form of the circuit is built around a crosspoint switch array, with interfaces for each line and a Raspberry Pi to control it all. But that simple description doesn’t fully express its awesomeness, rather than hooking up a set of off-the-shelf modules he’s designed everything himself from scratch. His subscriber line interface circuit uses a motor controller to generate the bell signal, his analogue splitter has an op-amp and a transistor, and his crosspoint array is a collection of JFETs. Having dabbled in these matters ourselves, it’s fascinating to see someone else making this work. Video below the break.

Have an analogue phone but nowhere to use it? Bring it to a hacker camp!

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Stack Trace From The 1950s Punches Again

December 2, 2022 by 4 Comments

This repair/tutorial video by the telephone Connections Museum of Seattle features an amazing piece of electro-mechanical technology from the 1950s — the 5XB trouble recorder. Museum volunteer Sarah the “Switch Witch” has a deep passion for old phone equipment, and gives an excellent description of the trouble recorder, the problems it solved, and how it works, and how they went about fixing it.

As central office switching became more complex and more dense, the manual methods of hunting down faults became unmanageable. Semi-automatic approaches using trouble lamps, but even that had its limits. This “stack trace”, which could have hundreds of indicators, had to be frozen while the technician recorded the status on a form. If another fault came along during this time, it was lost. The solution, using the available technology of the day, was a mind-boggling punched card apparatus that punches over a thousand bits of information when an switching error is detected or when various watchdog timers expire.

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Two landline phones connected to a set of wires and boards

How To Build Your Own Analog Phone Network

November 23, 2022 by 18 Comments

Analog phones may be nearly obsolete today, but having served humanity for well over a century they’re quite likely to pop up in drawers or attics now and then. If you’ve got a few of them lying around and you think it’d be cool to hook them up and make your own local telephone system, check out [Gadget Reboot]’s latest work. His video series shows all the steps towards making a fully-functional wired phone system.

Of course, dedicated phone exchanges for home or small business use are not hard to find, but [Gadget Reboot] decided it would be way more interesting to design his own system from the ground up. To begin with, he used off-the-shelf subscriber line interface circuits (SLICs) to implement the correct voltages, currents and impedances to drive analog phones. He then added a DTMF decoder chip to allow the phone to dial a number, and hooked up both systems to an ESP8266 which controls the entire system. It implements the different states of picking up, dialing, ringing and hanging up, and also generates the corresponding audio signals.

The system becomes even more interesting through the implementation of a multi-exchange layout, just like in large-scale phone systems: when a number is dialled that’s connected to a different exchange, then a connection must be made between two exchanges in order to complete the call. Large-scale systems use dedicated protocols like SS7, but [Gadget Reboot] preferred to keep things simple and used an RS-485 connection. The two ESPs check each others status and if everything’s in order, a relay connects the two lines and the circuit is completed.

The current system is a bit of a mess of wires, but it works, and [Gadget Reboot] plans to make a cleaner setup based on custom circuit boards, possibly expanding it with functions like modem support. In any case it’s already way more advanced than a simple electromechanical system. Want to know more about classic phone networks? We’ve got you covered.

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Several relays and switches mounted on a metal frame

The Simplest Electro-Mechanical Telephone Exchange That Actually Works

April 14, 2022 by 34 Comments

While rarely seen by users, the technology behind telephone exchanges is actually quite interesting. In the first hundred or so years of their existence they evolved from manually-operated switchboards to computer-controlled systems, but in between those two stages was a time when dialling and switching was performed electromechanically. This was made possible by the invention of the stepping switch, a type of pulse-operated relay that can connect a single incoming wire to one of many outgoing wires.

Public telephone exchanges contained hundreds of these switches, but as [dearuserhron] shows, it’s possible to make a smaller system with way fewer components: the Cadr-o-station is built around one single stepper switch. Although it looks rather complicated, the only other components are a bunch of ordinary 24 V relays and a few power supplies. Together they make up a minimal telephone exchange that connects up to ten handsets.

It doesn’t have all the functionality of a larger system however, as only a single voice circuit is made to which all phones are automatically connected. Still, it does allow users to dial a number and let the other phone ring, which might be good enough for a home or indeed the hackerspace where it’s currently sitting. It’s also a fine demonstration of how relatively simple technology can be applied to make a surprisingly complex system.

[dearuserhron] wrote an in-depth article on the workings of electromechanical telephone exchanges, which might come in helpful to anyone who’d like to design such a system for their own home. For a more general introduction into analog phone technology, check out our analysis of a 1970s rotary telephone.

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Rotary Phones And The Birth Of A Network

July 26, 2017 by 85 Comments

I can’t help but wonder how long it will be before the movie title  “Dial M for Murder” becomes mysterious to most of the population. After all, who has seen a dial phone lately? Sure, there are a few retro phones, but they aren’t in widespread use. It may not be murder, but it turns out that the dial telephone has its roots in death — or at least the business of death. But to understand why that’s true, you need to go back to the early days of the telephone.

Did you ever make a tin can phone with a string when you were a kid? That dates back to at least 1667. Prior to the invention of what we think of as the telephone, these acoustic phones were actually used for specialized purposes.

We all know that [Alexander Graham Bell] made a working telephone over a wire, drawing inspiration from the telegraph system. However, there’s a lot of dispute and many others about the same time were working on similar devices. It is probably more accurate to say that [Bell] was the first to successfully patent the telephone (in 1876, to be exact).

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