3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.5 X.25
X.25 is a very old WAN communications protocol developed by the CCITT (now known as the International Telecommunications Union, or ITU). Telecommunications carriers first offered it as a commercial service in the early 1970s. A sample X.25 network appears in Figure . X.25 network devices fall into three general categories: data terminal equipment (DTE), data circuit-terminating equipment (DCE), and packet switching exchanges (PSE).

X.25 supports the use of both switched and permanent virtual circuits. Switched virtual circuits (SVCs) are established as needed, and are dismantled after the communications session ends. Permanent virtual circuits (PVCs) are predefined, logical connections through a switched network between two points. The advantage of SVCs is that they are flexible and can be used to connect any two points within the X.25 network on demand. Their limitation lies in the call setup time that must be endured prior to exchanging information with another device on the network.

PVCs are not flexible, and they must be defined in advance. Their primary benefit lies in the elimination of a call setup period. Therefore, PVCs are typically used to support communications between devices that need to communicate on a routine and ongoing basis. SVCs are used for ad hoc communications.

An X.25 frame is composed of a series of fields, as shown in Figure . Layer 3 X.25 fields make up an X.25 packet and include a header and user data. Layer 2 X.25 (LAPB) fields include Frame-Level Control and Addressing fields, the embedded Layer 3 packet, and an FCS.

X.25 contains a robust suite of error detection and correction mechanisms that enable it to be a highly reliable protocol having to traverse a noisy electromechanical switching equipment infrastructure. In essence, X.25 sacrificed throughput for reliability. Today, in the era of digital and optical communications, the error detection/correction mechanisms of X.25 result in unnecessary overhead. These functions are now more appropriate at the end devices rather than being embedded in every network device. Applications that still require the use of the X.25 protocol may find better performance in emulating that protocol over a different transmission facility.

Despite the availability of technically superior transmission technologies, X.25 hasn't been completely supplanted. X.25 is used extensively in Europe, and even in the United States! Many organizations either cannot afford to migrate to newer technologies, or are locked into older infrastructures for a variety of reasons. Therefore, it is still widely deployed around the world.