Hi deepika
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.
if u want more u mail me at this id:daarunkumar0@lycos.com
Thanks
Arun