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.