Telephone networks use the circuit switching technique. The telephone network had its beginnings in late 1800s. The entire network, which is referred to as plain old telephone system (POTS), was originally an analog system using analog signals to transmit voice. The three major components of the telephone network are local loops, trunks and switching offices. The telephone network has the several levels of the switching offices such as end offices, tandem offices and regional offices. Local loop is a twisted pair cable that connects the subscriber telephone to the nearest end office or the local central office. The local loop when used for the voice has a bandwidth of 4000 Hz (4 KHz). It is very interesting to examine the telephone number associated with the local loop. The first three digits of a local telephone number define office, and the next 4 digits define the local loop number. The trunks are the transmission media that handle the communication between offices. A trunk normally handles hundreds or thousands of connections through the multiplexing. The transmission is usually through the optical fibers or the satellites. To avoid having a permanent physical link between any two subscribers, the telephone company has the switches located in a switching office. A switch connects several local loops or trunks and allows a connection between various subscribers.
