I think you’re talking about telephone company switches. A “Class 5 Office” is a local telephone company switch which handles local station loops, etc. The “Class 4 Office” is the major long distance switch that processes calls to and from Class 5 offices, and would be a long distance company’s switch. This hierarchy of switches was, I believe, made up by AT&T prior to divestiture. It goes up to Class 1 which is a regional switching center.
Its Hierarchy of switching system. It consists of Five class of switch. Class1 to Class5.
Class1 to Class4 handls trunk lines it does not supports direct subscribs interface.
class5 switch support both trunk and subscribers interface the local loop.
Class4 is called Tandem office. It interconnects all the class5 switchs.
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