Public switched data network
（重定向自PSDN）
A public switched data network (PSDN) is a publicly available packet-switched network, distinct from the PSTN.
Originally this term referred only to Packet Switch Stream (PSS), an X.25-based packet-switched network, mostly used to provide leased-line connections between local area networks and the Internet using permanent virtual circuits (PVCs). Today, the term may refer not only to Frame Relay and Asynchronous Transfer Mode (ATM), both providing PVCs, but also to Internet Protocol (IP), GPRS, and other packet-switching techniques.