The telephone system is an arrangement of electrically interconnected transmitters and receivers, by means of which speech is communicated between two points, either close together or remote.
In the telephone system the two-way communication is carried' either by a two wire or a four-wire system. The two wire systems are less costly. A general description of the automatic telephone system is given below and is illustrated with the help of a block diagram.
A two-wire line from the telephone set of the subscriber terminates in the Distribution Point (D.P.), which consists of ten pairs for connecting ten subscribers. These ten pairs are extended in the form of underground cable to the cabinet. The underground cable runs from the cabinet to the exchange where it terminates in the Main Distribution Frame (M.D.F-.). The M.D.F. connects the external and internal parts of the system. From the M.D.F. every pair of the individual subscriber is extended to the Subscriber Line Circuit (S.L.C.) inside the switch room. The S.L.C. with the help of control set seizes the free line finder when the subscriber lifts his handset. The Line Finder (L.F) seizes the free outlet in the 1st Group Selector (G.S) and the subscriber gels dial tone, which means that the system is now ready for dialing. When the subscriber dials the 1st digit 1st G.S. is followed by the 2nd G.S the 2nd G.S is followed by the 3rd G.S. and so on up to Final Selector (F.S). The number of G.S. depends upon the number of digits. From the F.S. the line is extended to the S.L.C. of the called subscribers. The loop is completed and the calling subscriber receives ring from the F.S. while the called subscriber is ring up. In Below figure a block diagram of Automatic telephone Exchange is shown.
The sketch shown in the figure is a very limited system. In modern system many paths are available to allow connection of any given telephone to another selected from the hundred of millions of telephone available throughout the world. The transmission links in modern telephone systems may take several different forms, such as overhead wire, buried cable, microwave links, satellite links and so on. All these systems of transmission are shown in the figure.
Several different forms of transmission links in modern telephone systems