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AIXLink/X.25 1.1 for AIX: Guide and Reference

Chapter 1. X.25 Network Communications Overview

X.25 can be a cost-effective means of networking systems in a wide geographical area, compared to traditional dial-up (circuit switched) connections, or remote-bridged local area networks (LANs) connected by leased lines. It provides worldwide interconnection for international corporations.

X.25 provides the ability to transmit data between remote machines. It is a set of recommendations from the International Telegraph and Telephone Consultative Committee (CCITT). These recommendations define a standard network access protocol for attaching different types of computer equipment to a packet-switched data network (PSDN) . A PSDN is an interconnecting set of switching nodes that enables subscribers to exchange data using a standard protocol and packet-switching technology. This protocol is particularly useful for communication between different types of computer systems and for accessing public databases.

Both public and private PSDNs can be based on the X.25 protocol. Public networks are usually provided nationally by the Post, Telegraph, and Telecommunications (PTT) authority. Private networks are operated by individual corporations. Many of the corporations using X.25 networks have a requirement for communication between themselves and other companies, such as dealers and agents. An example of such an X.25 network use is an airline reservation system.

Although some corporations have created private networks, most companies subscribe to a public PSDN. Such a network carries messages divided into packets over circuits that are shared by many network users. A single physical line into an office can handle many concurrent connections. A packet consists of a sequence of data and control elements in a format that is always transmitted as a whole. The network packet size is commonly 128 bytes, but can vary from 16 to 4096 bytes. In X.25, a byte is called an octet.

You can use X.25 communications to provide a network service for higher-level protocols , such as System Network Architecture (SNA) and Transmission Control Protocol/Internet Protocol (TCP/IP) . Or you can use an X.25 network directly, either by using the xtalk command , or by using an application programming interface (API) to write your own applications.

X.25 Equipment Terminology

The terms DTE, DCE, and DSE are used with both the X.25 protocol and modems, with slightly different meanings. The CCITT has defined these terms as follows for X.25 protocol:

DTE (Data-Terminal Equipment.) A computer that uses a network for communications.
DCE (Data Circuit-Terminating Equipment.) A device at the point of access to a network.
DSE (Data-Switching Equipment.) A switching node in a packet-switched data network.
  1. Every DTE must have an associated DCE.
  2. DTE and DCE are functional definitions; they need not correspond to specific items of equipment. For example, a single device may be a DSE and may also provide multiple DCE interfaces.

X.25 is not an end-to-end protocol. CCITT Recommendation X.25 defines a standard protocol for information exchange in packet mode between a DTE and a DCE (that is, between an individual user's equipment and the network provider's equipment).

The network is composed of DCEs and DSEs that route the packets of data through the network to the intended destination. The path that a user's data takes might vary with every packet. In most cases the DTE is connected to a DCE in some form of network. In a few cases, two systems might be attached more directly, without an intermediate network. When this is the case, one system has to act as a DCE at the Data Link layer. This usage is not common because it restricts the flexibility of the protocol and degrades performance.

Operation and maintenance of DCEs and DSEs are the responsibilities of the network provider. If a link between two DSEs goes down, the provider must reroute traffic. X.25 does not define the route through the network or the protocols employed within it.

The X.25 Network figure shows the elements of a packet-switched data network.

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