| Yes. Electromechanical switching systems were substantially more reliable than their components. How this was done should be understood by anybody designing high-reliability systems today. "A History of Science and Engineering in the Bell System - Switching Technology 1925-1975" is a readable reference. The Internet Archive has it.[1] More hardcore: "No. 5 Crossbar"[2] The Connections Museum in Seattle still has a #5 Crossbar working.[3] Long distance used toll switches, "#4 Crossbar", and there were 202 of them. #4 and #5 Crossbar machines are collections of stateless microservices, implemented from electromechanical components. The terminology is used in the old books is completely different, but that's what they are. Each service always has at least two servers. The parts that do have state are distributed. The crossbar switches that make actual connections have state, but are dumb - they are told what to do by "markers", which are stateless but can read the state of the crossbars and of other components. Failure of a single crossbar unit can take down less than a hundred lines at most. Other than the crossbars to external lines, everything had alternate routes. Everything has fault detection, with lights and alarm bells. Error rates were fairly high. In the previous "step by step" system, a good central office misdirected about 1% of calls. With bad maintenance (and those things were high maintenance) that could get much worse. Crossbar was better, maybe 0.1% misdirected calls. Routing tables in crossbar were mostly static ROMs of one kind or another. Routing consisted of trying a predetermined set of routes, in order. Clunky, but reliable. Modern systems need a backdown to that mode. [1] https://archive.org/details/historyofenginee0000unse_q0d8 [2] https://archive.org/details/bellsystem-no-5-crossbar-blr [3] http://www.telcomhistory.org/connections-museum-seattle-exhi... |