X.25 is a packet-switching protocol standard that was developed in the 1970s by the ITU-T (formerly known as CCITT). Its primary purpose is to facilitate the transmission of large volumes of data over public data networks. Subscribers typically connect to this data network through leased lines, which are dedicated communication lines established for their exclusive use.

The end-user data terminals are referred to as Data Terminal Equipment (DTE). These DTEs can include various devices such as terminals, personal computers, or network hosts, and they are usually located on the premises of individual subscribers. To establish a connection between the DTE and the network, Data Circuit-Terminating Equipment (DCE) is employed. DCE devices, such as modems, serve as communication interfaces between the DTE and the network.

In cases where a DTE is too simple, such as character-mode terminals that lack the capability to support the full functionality of X.25, a Packet Assembler/Disassembler (PAD) is used. The PAD is positioned between the DTE and the DCE and plays a crucial role in buffering data and assembling/disassembling packets sent to and from the end devices.

Within the X.25 network, nodes responsible for routing and switching data packets through the network are known as Packet Switching Exchanges (PSEs). These PSEs ensure that data packets find their way from the source to the destination within the network.

One key feature of X.25 is its use of virtual connections. Before the actual data transfer begins, X.25 establishes a dedicated path from the source to the destination. All data packets traveling between these two points follow the same predetermined route through the network. Once the data transfer is completed, the virtual connection is terminated or “cleared.” This approach ensures that data is reliably delivered from one end to the other while maintaining a predictable route for all packets during the transmission process.