Hierarchy of Switching Offices
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.
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