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There are two main types of public network offices through which a call is routed. These are the local offices and the toll offices. Subscribers are connected to local offices. The switching offices used to interconnect the local offices are called toll offices. The toll offices in tum can be at different levels.

To route a call to a specific region or a specific country, a numbering plan is used. Depending on the digits dialed, the switching system identifies whether the call is a local call, an inter-regional call or an inter-country call, and routes the call accordingly. The numbering plan is as follows. Prefix-comprises one or more digits, that allow the selection of different numbering formats; for example, local, national or international; transit networks or a service.

International prefix-allows a calling subscriber to obtain access to outgoing international equipment if the called person is in another country. National (trunk) prefix-allows a subscriber to make a call to a subscriber within the same country, but outside the local area. Escape code-is used to indicate that the digits following are from a different numbering plan. Escape codes are currently being used in the numbering plan for Public Data Networks to interconnect with ISDN and telex number plans.

An inter-exchange call is switched between two switching systems. There are five main stages of processing an inter-exchange call. Call Origination begins when the caller first picks up the telephone handset and completes a circuit to the local exchange. Digit Analysis is done during this stage where dialed digits are received and the originating exchange determines how to route the call to the local exchange of the called line.

Ringing during this stage, a path is established between the originating and terminating exchanges. A ringing signal is sent to the called line and an audible ring is sent to the calling party. Talking begins when the called line is answered. A talking connection is established between two telephone lines so that conversation can take place. Disconnection happens when one of the lines changes to on-hook. The talking path between the two lines is released.

In 1876, Alexander Graham Bell invented the first telephone. The early telephone system was simply a wired connection between two points. Today there are over 650 million telephones all over the world. The world’s telephones must be capable of being interconnected. These telephones cannot be interlinked with each other as laying a separate line is not feasible for so many connections. A switching system is needed to switch calls between various points.

A switching system controls the flow of information being transmitted over telephone lines. The switching centers are of two types-those that form part of a public network and those that are private and installed for use within an organization (called private board exchanges). A switching system controls the way it connects calls by performing three functions

Monitoring function determines when an action is needed, that is, when a service is requested by a subscriber and when it is no longer required. Logic function determines what action is needed, that is, what lines must be connected and what is the best route to complete the call. Commands function initiates the needed action. They operate those parts of the system which complete the connection and release them when the connection is no longer required.