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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. Whilst there are several technologies that are superficially similar to the PSDN, such as Integrated Services Digital Network (ISDN) and the 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 communications overlaid on the PSTN local loop (copper wires), usually utilized for access to a packet-switched broadband IP network. Short for packet-switched data network, a data communications network that is based on the principles of packet switching, as opposed to circuit switching that is used in public telephone networks. Packet-switched networks do not establish physical communication channels between communicating devices like circuit-switched networks do. Instead, signals are formed into fixed-length packets that are affixed with a source and destination address and packet ordering details. The packets then rely on network routers to read the address data and route the packets through the network to their destinations. When the packets arrive at the receiving device, the packet ordering data is used to reassemble the original signal. One advantage of packet-switched networks is that packets from different sources going to different destinations can share common data pathways. ==See also== * Packet switching 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Public switched data network」の詳細全文を読む スポンサード リンク
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