X.25 was developed by common carriers in the early 1970s and approved in 1976 by the CCITT, the precursor of the International Telecommunication Union (ITU), and was designed as a global standard for a packet-switching network. X.25 was originally designed to connect remote character-based terminals to mainframe hosts.
The original X.25 standard operated only at 19.2 Kbps, but this was generally sufficient for character-based communication between mainframes and terminals. An X.25 WAN consists of packet-switching exchange (PSE) nodes as the networking hardware, and leased lines, plain old telephone service connections, or ISDN connections as physical links.
The packet assembler/disassembler (PAD) is a device commonly found in X.25 etworks. PADs are used when a DTE device, such as a character-mode terminal, is too simple to implement the full X.25 functionality. The PAD acts as a point-to-point 56 Kbps statistical multiplexer and uses buffers to send packets to an X.25 switch.
X.25 is still used (e.g., as of 2012 in the credit card payment industry) and available in niche applications such as Retronet that allows vintage computers to use the internet.
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