Public switched data network
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PSDN is an acronym for public switched data network, a publicly-available network supporting packet-switched data, separate from the PSTN.
In the UK, this term refers only to PSS (Packet Switch Stream), an X.25-based packet-switched network, originally used to provide leased-line Internet connections.
Although superficially similar to the PSDN, ISDN, ADSL, SDSL, VDSL are not examples of it. ISDN is a circuit-switched digital network, and DSL variants are packet-switched data services overlaid on the PSTN.