7.1 X.25
7.1.2 X.25 protocol stack
The X.25 packet-switching protocol suite compares to the lower three layers of the Open System Interconnection (OSI) model, as shown in the Figure.

In general, X.25 is used as an over-engineered data link in the internetworking world. Both X.25 at Layer 3 and Link Access Procedure, Balanced (LAPB) at Layer 2 provide reliability and sliding windows. Layers 2 and 3 were designed with strong flow control and error checking to reduce the requirement for these functions external to X.25.

X.25 evolved in the days of analog circuits, when error rates were much higher than today. For analog circuit technology at Layer 1, it is more efficient to build more reliability into the network at the hardware level. With digital or fiber-optic technologies, the error rates have dropped dramatically. Newer technologies, such as Frame Relay, have taken advantage of drops in error rates by providing a stripped-down, “unreliable” data link.

X.25 was designed in the days of alphanumeric terminals and computing on central time-sharing computers. Demands on the packet switch were lower than today. Today’s complex applications on desktop workstations demand more bandwidth and speed. Newer technologies such as ISDN and X.25 over Frame Relay add packet-switching capability.