public switched network (PSN)

Any switching system that provides a circuit switched to many customers. In the USA, there are four: Telex, TWX, telephone, and Broadband Exchange.; 提供线路交换给许多客户的交换系统。在美国有Telex(电传用户电报)、TWX(电传打字)、电话 和宽频带交换系统等四类。