For the stored program control exchanges of the early 1970s, many relays were made with steel cores that remained magnetized after current ceased to flow in the winding.
12.
Meanwhile, Stored Program Control came to live up to its promise of versatility, and Centrex customers were among the first in the middle 1970s to be removed from 5XB to 1ESS switch.
13.
During the 1970s, common control exchanges became stored program control exchanges, using in the 1980s common-channel signaling in which the channels that are used for signaling are not used for message traffic.
14.
The first generation of electronic switching systems in the 1960s were not entirely digital in nature, but used reed relay-operated metallic paths or crossbar switches operated by stored program control ( SPC ) systems.
15.
During the middle 20th Century markers in Bell System exchanges, being complex common control circuits with short holding time, acquired other functions that were only needed once or twice per telephone call, including outgoing digit translation and enforcement of different policies upon different Stored Program Control became mature.
16.
After the spread of stored program control switching, many services of Class 1 through 3 could be delegated to newer switches in the class 4 and 5 offices, and that portion of the network became obsolete, although it was partially replaced by the establishment of multiple long distance carrier networks, connected to the local networks through their points of presence.
