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Visiting Bletchley Park and seeing step-by-step telephone switching equipment repurposed for computing re-enforced my appreciation for the brilliance of the telecommunication systems we created in the past 150 years. Packet switching was inevitable and IP everything makes sense in today's world, but something was lost in that transition too. I am glad to see that enthusiasts with the will and means are working to preserve some of that history. -Posted from SC2025-

I wanted to learn more about computer hardware in college so I took a class called "Cybernetics" (taught by D. Huffman). I thought we were going to focus on modern stuff, but instead, it was a tour of information theory- which included various mathematical routing concepts (kissing spheres/spherical code, Karnaugh maps). At the time I thought it was boring, but a couple decades later, when working on Clos topologies, it came in handy.

Other interesting notes: the invention of telegraphy and improvements to the underlying electrical systems really helped me understand communications in the 1800s better. And reading/watching Cuckoo's Egg (with the german relay-based telephones) made me appreciate modern digital transistor-based systems.

Even today, when I work on electrical projects in my garage, I am absolutely blown away with how much people could do with limited understanding and technology 100+ years ago compared to what I'm able to cobble together. I know Newton said he saw farther by standing on the shoulders of giants, but some days I feel like I'm standing on a giant, looking backwards and thinking "I am not worthy".

When the Bell System broke up, the old guys wrote a 3-volume technical history of the Bell System.[1] So all that is well documented.

The history of automatic telephony in the Bell System is roughly:

- Step by step switches. 1920s Very reliable in terms of failure, but about 1% misdirected or failed calls. Totally distributed. You could remove any switch, and all it would do is reduce the capacity of the system slightly. Too much hardware per line.

- Panel. 1930s. Scaled better, to large-city central offices. Less hardware per line. Beginnings of common control. Too complex mechanically. Lots of driveshafts, motors, and clutches.

- Crossbar. 1940s. #5 crossbar was a big dumb switch fabric controlled by a distributed set of microservices, all built from relays. Most elegant architecture. All reliable wire relays, no more motors and gears. If you have to design high-reliability systems, is worth knowing how #5 crossbar worked.

- 1ESS - first US electronic switching. 1960s Two mainframe computers (one spare) controlling a big dumb switch fabric. Worked, but clunky.

- 5ESS - good US electronic switching. Two or more minicomputers controlling a big dumb switch fabric. Very good.

The Museum of Communications in Seattle has step by step, panel, and crossbar systems all working and interconnected.

In the entire history of electromechanical switching in the Bell System, no central office was ever fully down for more than 30 minutes for any reason other than a natural disaster, and in one case a fire in the cable plant. That record has not been maintained in the computer era. It is worth understanding why.

[1] https://archive.org/details/bellsystem_HistoryOfEngineeringA...

Talk about a gargantuan project.. also awesome to bag such a thing. He's lucky to even have the resources to store^W warehouse it

It's not that much space in some parts of the US where properties are measured in acres.

I wonder how many operating 5ESS are left now.

Across the USA? Very likely a few thousand.

a fairly large number - a bigger question is what happens to all the CO buildings once all the copper is turned down.

There is a huge opportunity about 5 years from now for edge datacenters. You have these buildings which have highly reliable power and connectivity, all thats needed is servers which can live in a NEBS environment.

COs are already being used for edge datacenters, its just not been talked about much outside the industry.

The CO closest to me was turned into condos. A friend was the general contractor. It was by all accounts a nightmare.

Most of those CO's are in buildings that don't have all that much space in them, were built in the 40's and 50's, and likely aren't suitable for that kind of thing. Cooling would be a big deal.

I have been in ~15 CO's - there is tons of floor space in them, and the only thing telephone switching equipment has done since the 50's is shrink - beyond that, most existing CO buildings had expansions when electronic switching came about, because they couldnt add the new electronic (1/1A/5 ESS) without additional floor space. Cooling is noted by the requirement for NEBS compliant equipment.

The older ones have lots of tall windows. It's the newer windowless ones that cannot be easily repurposed, unless you want to build a data center.

Central offices are everywhere, too. You've driven or walked by any number of them, and the most you noticed was a Bell System logo. The downtown COs in big cities are on expensive real estate.

(2024), but still a good read!

this date obsession is moronic, especially when we are talking about technology over forty years old. Next time you are tempted to spam the date, wait, and see if conversation still happens without your vital input.

There are many articles missing a (2025) addition, so get to work!