"A History of Engineering & Science in the Bell System: Communications Sciences, 1925-1980".
Here's probably the simplest introduction.[1] 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.
Here's probably the simplest introduction.[1] 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.
[1] http://wedophones.com/TheBellSystem/pdf/no5crossbar.pdf