The only central office downtime from a major fire was in 1975 in New York City, resulting in 23 days of downtime. All the incoming cables and the main distributing frame had to be replaced. The switching equipment, on upper floors, survived and just needed some maintenance and cleaning. That was still a crossbar office; it hadn't been converted to electronic switching yet. Worst disaster in the history of the Bell System. Bell switched to less flammable cable coverings, like plenum cable, after that.
Widespread failure of 911 service suggests an overcentralized architecture. 911 requires a phone number to address lookup, so there's a database involved. Widespread failure indicates this was implemented as a remote query service ("in the cloud") rather than read-only database copies of the directory at each central office.
(And even during the cutover, they're careful not to interrupt any emergency calls.)
Where were the signals going after the cables have been cut?
And then again in 2015.
Also, there was that kid who put together areport of all infra cable runs for a thesis and the FBI confiscated it...
Here's probably the simplest introduction. When reading this, note phrases like "ORIGINATING REGISTER SEIZES AN IDLE MARKER". Think of that as "originating register asks for an idle marker from the pool of markers". Very little equipment is dedicated to specific lines. Everything is done by requesting a service from one of several identical units. If one of those units fails, the system capacity is reduced, but the switch does not go down as long as at least one of each unit type is still up.
The switch fabric, the actual crossbars, is dumb. It just makes the connections it's told to make.
Shared resources include:
- Originating registers. These provide dial tone and record dialed digits. They parse the incoming number to the limited extent needed to decide when it's finished.
- Markers. The smart part of the system. When an originating register has a full set of digits, it finds an idle marker and sends it the call info. The marker figures out what to do next, in about half a second, and then it's free for another call. Markers tell the switch fabric what connections to make. They're duplicated, and the two halves check each other. If the halves disagree, the marking aborts. If a marker aborts, the originating register tries again with another marker. One retry only. Marker failures also cause data to be sent to a "trouble recorder". As usual, there's more than one of those, and they're "seized" as needed.
- Senders. These send digits from one exchange to the next. They're primitive modems.
- Trunks. Lines between exchanges. Full duplex, four wires.
- Terminating senders. The receive side of senders.
There are also units associated with accounting, coin telephones, routing tables, and other auxiliary functions.
The key takeway here is that there's no single point of failure.
Can you recommend such a book?
/edit: i see you answered this for another poster
Thank you for sharing, that was an enjoyable watch!