You say that, but telephony used to be obsessed with uptime. These are the people who invented live patching of a running system, after all. Case in point, the #1 ESS telephone switch was designed for less than 2 hours of cumulative downtime over a 40-year service life. In practice, most achieved less than 1 hour. (How many nines is that?)

Which is to say, individual subscriber lines can have all sorts of faults, but they only affect that subscriber. Trunks between offices can fail, but they only affect a certain number of circuits. The central call processing ability, of the switch to react appropriately to lines changing state and numbers being dialed, for calls to be completed if both stations are available, is what had to meet that target.

I was skeptical when I first heard this, so next time I was in an office that still had a #1 (actually a 1A, it had been upgraded in the late 80s), I talked to the switchman for a bit, and he showed me the downtime counter. It's a mechanical thing like an odometer, and once a second when the switch is executing its main loop, it touches a register that keeps the counter from incrementing. If the processor halts or isn't processing calls for some reason, the counter starts counting.

The switch was installed in the mid-70s, and from the moment it took over for its crossbar predecessor, including an in-place processor upgrade, it had logged less than an hour. Most of that came a few seconds at a time when swapping between active processors during software upgrades, he said. At the time (this was around 2002 I think) it was slated to be replaced with a DMS100, but the replacement activity hadn't commenced yet. I don't know what sort of reliability numbers the DMS machines achieve, but they'd do well to match their predecessors.