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 * Introduction**


 * Background**

The use of this protocol started in the 1970's. X.25 protocol started its way due to the need to provide users with wide area network (WAN) connectivity across public data networks (PDNs). It was felt that improved equipment compatibility and lower cost would increase subscription to PDNs. When started its way, the X.25 protocol was the most popular one among others that were developed at that time. X.25 was developed by the common carriers, rather than any single commercial enterprise. Because of that, the specifications of the protocol are well suited to various system types. One of the big advantages of the X.25 is the fact that its carry an international nature. X.25 is managed by the an agency of the United Nations called the International Telecommunications Union (ITU). That is why X.25 is known as a global standard.
 * What is X.25?**

X.25 has been created in order to exchange data over WAN (Wide Area Network). The X.25 is a protocol for packet switching. The first version of the protocol was intended to work on a network with lower speed, end-to-end error detection and correction over noisy analog lines. The X.25 protocol has been developed due to the need to connect between lots of computer sites. the main role was to assure interoperability among various vendor products. Another goal was to maximize the sharing of network resources. X.25 protocol has methodologies for the three lowest layers: physical, data-link and network. The defines an "open" system connection by creating a standard interface for each layer. therefore, it enables compatibility of multi-vendor hardware and software systems.


 * How does X.25 work?**

In order to transmit the data over the network, the data is divided into data packets. In addition to the data, each packet contains information regarding the setup and managing the data flow. Standardized addresses are used in order to send exchange data between one end and another. Virtual circuits, which are logical connections between source and destination, allow the establishment of calls between two devices. In order to start a connection, the source device signals the destination address with a request call. The destination has three options of how to handle the request. It can accept the call, clear it or redirect it to a third party. For example, if someone wants to withdraw money from an Automatic Teller Machine. When inserting the credit card, a request to the control server is done. After checking validity of the card, the control server forwards the call to the server that hosts the account database, to authorize cash delivery.

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