The Origins of Frame Relay: X.25
Frame Relay has its roots in the X.25 protocol, a robust packet-switched WAN protocol widely deployed in the early 1970s to connect geographically dispersed LANs and WANs. The X.25 protocol works well on noisy transmission mediums, which observe a high rate of errors and packet drops. X.25's windowing, flow, and error control features enable the retransmission and recovery of packets lost because of errors or congestion. As such, the upper layers are relieved of their role of performing error detection and correction. X.25 was very important in the early 1970s because the world, for the most part, was using unreliable public data network circuits for data transfer. X.25 still provides much needed reliability for data transmission over these unreliable and erroneous links.
Figure 1-7 illustrates a simple X.25 network. A basic X.25 network comprises the data terminal equipment (DTE), the data circuit-terminating equipment (DCE), and the X.25 switch. Examples of DTE devices include terminals, file servers, and computers. DCE devices are modems or routers with internal modem interfaces. The DCE connects to a X.25 network to communicate between the DTE devices and the X.25 switches. In Figure 1-7, a router (DTE) is connected to an external modem (DCE), which in turn communicates with an X.25 switch.
Figure 1-7. An Example of a Simple X.25 WAN
Unlike Frame Relay, which is entirely a data-link layer protocol, X.25 works at the first three layers of the OSI model. The Packet Layer Protocol (PLP) is the network layer protocol of the X.25 protocol suite. X.25 involves session establishment, data transfer, and session termination. During data transfer, X.25 performs flow control with windowing and also detects transmission errors and the loss of packets. X.25 ensures that the lost packets are correctly retransmitted by the end stations.
Compared with the Frame Relay protocol, the X.25 protocol involves a larger amount of overheads. Because the quality of digital transmission facility has improved dramatically over the years, there are fewer issues with transmission errors and packet drops. Furthermore, with the advent of TCP/IP and its overwhelming popularity, the robust flow and error control features in X.25 are now generating excessive overheads and are becoming increasingly unnecessary on modern networks. X.25 is considered slow by today's standards.
