Number One Crossbar Switching System
https://en.wikipedia.org/wiki/Number_One_Crossbar_Switching_SystemHow does a No. 1 Crossbar work? - Connections Museum
How does a No. 1 Crossbar Marker work? - Connections Museum
How does MF signaling work? - Connections Museum
Crossbar's Hidden Superpower: Alternate Routing! - Connections Museum
8 minutes of No. 1 Crossbar - Connections Museum