X.25 is a packet-switching WAN technology that exists at the Physical, Data Link, and Network layers of the OSI model. Originally conceived in the 1970s, X.25 is still surprisingly popular, largely a result of its worldwide adoption by most telecommunications carriers and its reputation for reliable data transfer. In fact, in many parts of the world, X.25 still represents the only reliable data transfer technology available. While it originally provided connectivity at rates of 56Kbps (and sometimes much less), the X.25 standard was revised in 1992 to support speeds up to 2Mbps.
Three main types of equipment exist on X.25 networks. These include DTE and DCE devices, as well as X.25 packet-switching equipment (PSE). DTE equipment on an X.25 network would typically be a router, computer or terminal of some sort, both located at the customer premises. DCE devices in the X.25 world are located at the carrier’s facilities, and act as an interface between DTE equipment and PSE. Any given carrier’s X.25 network will consist of many PSEs, including interconnections to other service providers. Ultimately, these devices form the X.25 packet-switched “cloud”, as illustrated below.
Another common piece of equipment found on an X.25 network is known as a packet assembler/disassember, or PAD. A PAD is equipment that connects a DTE device to the X.25 network. It performs three primary functions: packet buffering, assembly, and disassembly. In many cases, companies dial into a PAD service using a traditional modem. In other cases, an operating system will be capable of running X.25 protocols locally and will use what is known as a “smart card” to connect to the X.25 network. A PAD is physically located between a DTE and DCE device on an X.25 network, as illustrated in Figure 11-25. A Cisco router does not require a PAD to connect to an X.25 network, as it is capable of using X.25 encapsulation on serial interfaces.