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.
System X was still very much alive and working when I left in ~09, there were boards that started out with a couple of Z80’s in them that had pentiums strapped in there (the pentiums were obsolete by then too, just less obsolete).
The complexity and reliability/redundancy were incredible for the age, definitely one of those “big iron” systems of legend that you just would not see developed these days.
I was working at Plessey in the early “70s at a time when they, along with the other major electronics companies, were tendering for the System X project. Although I was not involved (I was in the modems lab) the talk was all around. The old electromechanical exchanges were inherently self healing – faulty switches would be automatically routed around. Whereas with the central control of System X, if the ‘computer’ went down, the whole exchange went down! So reliability was of paramount importance. I seem to remember the GPO reliability specification was a total downtime of no more than 5 minutes in 30 years! Plessey’s solution (as I believe was everyone else’s) was to have 3 identical processing units simultaneously running each instruction, comparing notes and acting on the majority decision. In the event of disagreement, the majority two would turn on and analyse the the other, attempting to diagnose and report the fault. Plessey didn’t win the contract but managed to sell their system to the military, rebranded as Ptarmigan.
Good timing for the article given that large swathes of the BT network went down yesterday.
So that’s why my Three 5G broadband stopped working yesterday!
“To this day System X remains the backbone of the telephone network in the UK”
That’s almost certainly not true anymore, the UK phone network is transitioning to VoIP for everyone, most people will be on VoIP already by now; the aim is to completely turn off the PSTN in January 2027.
Yup. To quote the wikipedia page linked in the article:
Many of the System X exchanges installed during the 1980s continue in service into the 2020s.
In the first decade of the 21st century, System X was scheduled for replacement with softswitch equipment as part of BT’s 21st Century Network (21CN) programme. Some other users of System X – in particular Jersey Telecom and Kingston Communications – replaced their circuit-switched System X equipment with Marconi XCD5000 softswitches (which were intended as the replacement for System X) and Access Hub multiservice access nodes. However, the omission of Marconi from BT’s 21CN supplier list, and the shift in focus from telephony to broadband, led to much of the System X estate being maintained. The switched telephone network (both PSTN and ISDN) is due to be turned off on 31 January 2027, after customers are moved to voice over IP services.[4][5]
Sadly the majority of domestic customers are still on analogue at the network edge. The 2027 target was originally 2025. The migration hasn’t been well managed. Many services that rely on analogue connections (security alarms etc) haven’t migrated their customers.
Given the rather pathetic FTTP rollout (which is the prerequisite for analog switching to VOIP) “most people will be on VoIP already by now;” is certainly false. As of June 2025 56.03% have FTTP available which means somewhat less (likely a lot less) than 50% will have been connected to FTTP and had their phone service moved to VOIP.
Fibre is not a prerequisite for VoIP.
FTTP is not a precursor to digital voice (ie VoIP). Can work just fine on ADSL or VDSL via SOGEA.
I have FTTC and have switched to VoIP. If you switch supplier now, you will be on VoIP.
I don’t know what nationality the author is, but why is the article written on American English. When the subject is the British telephone system.
Ok, serious question from a non-brit because I’m curious – is there anything, besides using “analogue” instead of “analog”, that you would change to rewrite this in British English?
(Regardless, as far as I know, hackaday authors have always used whatever form of English they prefer, and I don’t see a problem with that. In fact I’d be a bit confused if an author like Jenny List had to write about “color film” instead of “colour film” just because the subject was an American film company like Kodak)
Why are articles about Mars not written in Martian?
I remember as a child standing in the local GPO exchange in Brentwood watching the strowger exchange clacking away, and the ladies that were writing downt the ‘meter readings’ off the back of them for the bills (this would have been around 1975)
Did my weeks work experience at a local exchange in late ’86. There was a mixture of strowger and system X. Mostly got to cut off people who didn’t pay their bill, which was much simpler with system X if my memory serves…
Not all strowger exchanges were replaced with System X. A large amount were replaced with System Y. Swedish Ericsson AXE10. As BT did not want all their eggs in one basket.
Yes erricson were always interested in our system x at trade shows, etc. Often found them creeping around the racks.
I worked with AXE10 for a long while, an amazing system I remember fondly, shame most of my knowledge is completely obsolete these days!
We in the US went with the cheapest route of “let telcos decide and suck our blank check tax money in the process”.
Results are such – while the initial switch to all digital (I’d say early 1980s throughout early 1990s – THAT LONG), the secondary push to “everything is cloud” that had complete fairly recently (maybe 2020) saw the inevitable gotcha happen more often than not – a meager regional netsplit would disable ALL phones in the area, including emergency/police/fire/hospitals. Literally. That’s because for-profit entities will NOT voluntarily spill over their traffic to their competitors, actually, they will proaggressively resist such nonsense at each and every turn.
Where I live we have witnessed such a thing – ALL phones gone dead for something like 4 hours at the time – at least three times within one year alone.
Translation, regulators/schmegulators, FCC already showed just how corrupt it can be (net neutrality? anyone? anyone?), so it is no longer capable of enforcing stable critical communications – in comparison, it is quite reliable with receiving lobby money in exchange for looking elsewhere.
(as a side note, one of the oldest – in the area – automated local exchanges was gutted some time early 2000; the building itself, a drab two-story windowless disaster of architecture, sporting ugly bare basics warehouse kind of aesthetics, stands to this day right on a main street, occupying prime real estate island; it is too small to do amazon warehousing, no windows means no office want it; thick solid walls and massive basement that used to house batteries, means nobody wants to invest into cleaning the thing; it probably will stay around for another decade, surrounded by the quickly growing mushrooms of “prime condos” and “high-priced rentals” of unclear demographics – the “job growth” has been moderate around maybe 1% annually).
The BT network no longer uses system X backbone but the name of escapes me!
It was charging 10 years ago completed last year with interface fitted to existing System X.
I went to one of the launches on a University campus as the new way forward. Was going to be world leader but got cut back with the cutbacks in the mid to late 80’s!
Got to look at my first system frame in 84.
Even bolted on Vax machines to do the heavy lifting & covert over to a digital network faster.
The Vax machines was my first introduction to MUD gaming!
I worked on System X starting in the esrly 80’s.. Quite the forefront of technology.
Worked on quite a number of processors and Microcontrollers such as 6809, 8051, 68008, 68010, 68020 then onto powerquicc.. Mostly assembler and C.. All this good groundwork has meant I’ve been relatively successful in the software industry my whole career. I do feel though that we’ll never see great engineers that I had the privilege to work with again.. Today’s young upstarts wouldn’t have a clue or don’t even show any interest in what happens behind the scenes. Times change I guess.
My dad used to work for STC, remember him talking about them doing work as part of the System X changeover.
The prototype System X DSSS 1 (digital switching sub system) aka System X MK 1 is on exhibition at the National Museum Of Computing, Bletchley Park, Milton Keynes (home of the WW2 codebreakers).
See:
https://www.tnmoc.org/events/2023/2/25/flowers-to-fibre-exhibition-opening
https://www.communicationsmuseum.org.uk/flowerstofibre/
https://www.tnmoc.org/flowers-to-fibre