Telephone switching - Public network automatic exchanges

Telephone switching is the set of techniques used to select, establish, maintain and, ultimately, release telephone paths between pairs of users subscribing to the network. Telephone transmission ensures the exchange or transport of useful information (conversation signals or data) on the routes thus established. The nodes used to establish connections between users according to their requests are the switching centers, commonly known as automatic exchanges, because their operation is now always automatic. Switches are used to interconnect, in pairs, the transmission paths that lead to their accesses in large numbers, whether these be subscriber lines or circuits (trunks) linking the switches together. To establish routes between subscribers and their home exchanges, or between exchanges, the telephone network uses exchanges of control information known as signalling.