The telephone network's structure may be defined as an entire plant of existing connections between telephone exchanges. It consists of three broad categories: local, exchange area, and long-haul networks.
The local network (see Figure 2) links telephones in residences and businesses to a central office serving a particular geographical area. The size of the area may vary from 11.5 sq mi (30 km2) in cities to 123.5 sq mi (320 km2) in the country. The telephone lines connecting a subscriber to the central office are called local lines or loops. Central offices are interconnected through the exchange area network, and all of the above are interconnected with toll (long-distance) exchanges. The telephone lines connecting one telephone exchange with another are called trunks in North America and junctions in Europe.
Each telephone is assigned a number indicating its location in the system. The switching network recognizes which telephone initiates the call and which telephone is to receive the call. From this information, it sets up the circuit connection for a signal path. Modern transmission facilities for conveying the electric analog of speech between telephone stations use diverse transmission paths, such as wire or cable circuits and microwave-radio or infrared-optical channels.
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