![History--exchange evolution, C.O. buildings [telecom]](https://forums.cabling-design.com/media/26832/conversions/6034ef41ae8510.94114374-medium.jpg)
The following table shows the types of exchanges in use in the Bell System.
year Manual Panel Step x-bar Ess
1960 715 494 7511 2218 -
1970 11 451 8393 5637 264
1977 1 68 7223 6537 3477
source and more detail:
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The above article provides great detail on the design of Bell System buildings.It notes that years ago buildings were constructed to house many people, primarily operators, but now hold mostly machines and fewer craft people.