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

Packet Switch Stream

In the United Kingdom, Packet Switch Stream was an X.25-based packet-switched network, provided by the British Post Office Telecommunications and then British Telecom starting in 1980...

 (PSS), an X.25

X.25

X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...

-based packet-switched network, mostly used to provide leased-line connections between local area network

Local area network

A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...

s and the Internet using permanent virtual circuits (PVCs). Today, the term may refer not only to Frame Relay

Frame relay

Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...

 and Asynchronous Transfer Mode

Asynchronous Transfer Mode

Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...

 (ATM), both providing PVCs, but also to Internet Protocol

Internet Protocol

The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...

 (IP), GPRS, and other packet-switching techniques.

Whilst there are several technologies that are superficially similar to the PSDN, such as Integrated Services Digital Network

Integrated Services Digital Network

Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...

 (ISDN) and the Digital Subscriber Line

Digital Subscriber Line

Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...

 (DSL) technologies, they are not examples of it. ISDN utilizes the PSTN circuit-switched network, and DSL uses point-to-point circuit switching

Circuit switching

Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...

communications overlaid on the PSTN local loop (copper wires), usually utilized for access to a packet-switched broadband IP network.