X.25

X.25 is a famous and most extensively used ITU-T protocol suite for the packet switching networks in the networking world, throughout the world by different network administrators. The X.25 protocol suite consists of several things for it to function properly, like an X.25 wide area network or WAN has PSE or packet switching exchange nodes that works as the networking hardware, leased lines and ISDN or plain old telephone lines that work as the physical links between these hardware devices present at the nodes in the network.

This X.25 protocol was extensively famous during the 1980s, when it was being used by nearly all the telecommunication companies made in the world, along with that it was also used in the financial transactions systems such as ATM machines. The X.25 protocol suite was finalized in the publication and released in the year 1976 and the publication was called The Orange Book. In the most recent world the use of this protocol has decreased as the technology has advanced and less complex and sophisticated and simple protocols have been introduced as an alternative to the X.25, which lead to the decrease in the number of the parties who are using this protocol, however, this protocol is far from being called a legacy system as it is still present in the market and the networking world as it is still able to meet the requirements of certain organizations.

This protocol was invented and introduced to the world even before the OSI reference model was released, therefore it is often considered as one of the oldest packet switching protocol service available in the market. This protocol suite is specifically used for three of the seven layers that are present in the OSI model, that is, it is used for layer 1, 2 and 3 in the OSI reference model.