[jakedata]

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-

[dekhn]

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".

[Animats]

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...