A PSTN network topology is the switching network topology of a telephone network connected to the public switched telephone network (PSTN). In the United States and Canada, the Bell System network topology was the switching system hierarchy implemented and operated from the late 1940s to the 1980s for the purpose of integrating the diverse array of local telephone companies and telephone numbering plans to achieve nationwide Direct Distance Dialing (DDD) by telephone subscribers. It was the precursor of the world-wide interconnected public switched telephone network (PSTN).