The big dumb switch fabric of #5 Crossbar has no processing power at all, but it has persistent state. The units that have processing power all go down to their ground state at the end of each call processing event, and have no state that persists over transactions. The various processing units (markers, junctors, senders, originating registers, etc.) are all at least duplicated, and usually there's a pool of them. Requests "seize" a unit at random from a pool, the unit does its thing, and the unit is quickly released.
Units have self-checking, and if they fail, they drop out of their pool and raise an alarm. The call capacity or connection speed of the exchange is reduced but it keeps working. Everything has short hardware stall timers which will prevent some unit failure from hanging the exchange.
#5 Crossbar has almost no persistent memory. End offices (for connecting subscriber lines) did not log call info. Toll offices did, but that used an output-only paper tape punch. There's so little state in the switch that matching up call start and call end events was done later in a billing office where the paper tape was read.
The combination of statelessness and resource pools prevented total failure. Errors and unit failures happened occasionally but could not take down the whole switch.
There's plenty of info about #5 Crossbar on line, but 1950s telephony jargon is so different from 2020s server jargon that it's not obvious that #5 Crossbar is a microservices architecture.
The big dumb switch fabric of #5 Crossbar has no processing power at all, but it has persistent state. The units that have processing power all go down to their ground state at the end of each call processing event, and have no state that persists over transactions. The various processing units (markers, junctors, senders, originating registers, etc.) are all at least duplicated, and usually there's a pool of them. Requests "seize" a unit at random from a pool, the unit does its thing, and the unit is quickly released.
Units have self-checking, and if they fail, they drop out of their pool and raise an alarm. The call capacity or connection speed of the exchange is reduced but it keeps working. Everything has short hardware stall timers which will prevent some unit failure from hanging the exchange.
#5 Crossbar has almost no persistent memory. End offices (for connecting subscriber lines) did not log call info. Toll offices did, but that used an output-only paper tape punch. There's so little state in the switch that matching up call start and call end events was done later in a billing office where the paper tape was read.
The combination of statelessness and resource pools prevented total failure. Errors and unit failures happened occasionally but could not take down the whole switch.
There's plenty of info about #5 Crossbar on line, but 1950s telephony jargon is so different from 2020s server jargon that it's not obvious that #5 Crossbar is a microservices architecture.