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.
|