X.25

X.25

X.25 is a packet-switching network standard developed by the International Telegraph and Telephone Consultative Committee (CCITT), which has been renamed the International Telecommunications Union (ITU). The standard, referred to as Recommendation X.25, was introduced in 1974 and is now implemented most commonly in WANs.

As shown in Figure 7.7, X.25 is one level of a three-level stack that spans the Network, Data Link, and Physical layers. The middle layer, Link Access Procedures-Balanced (LAPB), is a bit-oriented, full-duplex, synchronous Data Link layer LLC protocol. Physical layer connectivity is provided by a variety of standards, including X.21, X.21bis, and V.32.

X.25 packet-switching networks provide the options of permanent or switched virtual circuits. Although a datagram (unreliable) protocol was supported until 1984, X.25 now is required to provide reliable service and end-to-end flow control. Because each device on a network can operate more than one virtual circuit, X.25 must provide error and flow control for each virtual circuit.

At the time X.25 was developed, this flow control and error checking was essential because X.25 was developed around relatively unreliable telephone line communications. The drawback is that error checking and flow control slow down X.25. Generally, X.25 networks are implemented with line speeds up to 64 Kbps. These speeds are suitable for the file transfer and terminal activity that comprised the bulk of network traffic when X.25 was defined. Such speeds, however, are inadequate to provide LAN-speed services, which typically require speeds of 1 Mbps or better. X.25 networks, therefore, are poor choices for providing LAN application services in a WAN environment. One advantage of X.25, however, is that it is an established standard that is used internationally.

Figure 7.8 shows a typical X.25 configuration. In X.25 parlance, a computer or terminal is called data terminal equipment (DTE). A DTE could also be a gateway providing access to a local network. Data communications equipment (DCE) provides access to the packet-switched network (PSN). A PSE is a packet-switching exchange, also called a switch or switching node.

The X.25 protocol oversees the communication between the DTE and the DCE. A device called a packet assembler/disassembler (PAD) translates asynchronous input from the DTE into packets suitable for the PDN.