X.25 is a packet-switching protocol standard that is developed in the 1970's by ITU-T (formerly CCITT). It is used to carry large amounts of data over public data networks. Subscribers are usually connected to this data network with a leased line.
The data terminal from end users are called Data Terminal Equipment (DTE). They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers. To connect DTE to the network, data circuit-terminating equipment (DCE) is used. DCE devices are communications devices, such as modems. A PAD (packet assembler/disassembler) is used if a DTE is to simple (like character-mode terminals) to support the full X.25 functionality. The PAD is connected between the DTE and the DCE. The PADs buffers data and assembles and disassembles packets sent to the end devices. The nodes within the network that are responsible for the switching of the packets through the network are called a packet switching exchange (PSE).
X.25 uses virtual connections. A path from source to destination is set up before the data transfer begins. All data packets follow the same route through the network. The call is cleared after the data transfer is completed.