Like Frame Relay, X.25 uses virtual circuits to define a path across a packet-switched network. In cases where connections should be available at all time, PVCs are the better choice, as they eliminate the overhead associated with call setup and teardown. For less frequent connections, SVCs can also be used.
What X.25 lacks in speed, it more than makes up for in reliability. In fact, X.25 was primarily designed with reliability in mind, since the analog circuitry over which X.25 originally ran tended to be rather error-prone, thus requiring a high degree of error checking. The reliability of today’s digital networks is part of the reason why other packet-switching technologies like Frame Relay have grown so popular. Both availability and reliability will dictate whether a company needs to consider X.25 as a WAN solution in some parts of the world.
Communication between DTE devices over an X.25 network is subject to delays because of its reliability features. For example, X.25 networks use a store-and-forward method, where intermediate devices buffer packets as they cross the X.25 network. Not only does this buffering ensure that the receiving device (such as the next PSE in the path) is ready and able to receive data, but it also provides an opportunity for frames to be checked for errors. Additionally, as a frame is forwarded between switches across the point-to-point links of an X.25 network, acknowledgements must be sent back to the device that forwarded the frame, in order to be sure that it arrived. In cases where an acknowledgement is not received, the frame will be retransmitted. To make the acknowledgement process more efficient, a windowing mechanism is used that allows multiple packets to be sent before an acknowledgement must be received.
Consider this figure of an X.25 network. In it, one DTE device is sending data to another across the X.25 network. For illustration purposes, only a single frame is shown traversing the 3 PSEs between the routers using X.25. At each step along the way, an acknowledgement has to be sent back to the sender by the recipient. For example, when the first X.25 switch receives a frame from the sending router, it will send an acknowledgement. After forwarding the frame to the next switch, this second switch will forward an acknowledgement back to the first switch, and so on. This provides a great example of the amount of error checking involved with making X.25 a reliable WAN technology.
