Data communications started with leased analog lines and modems, then later used digital services that ran at fixed bit rates. Mainframe computers supported thousands of terminals on shared lines that seldom exceeded 9600 bit/s—for 30 or more terminals per multidrop line. Packet switching brought greater flexibility to data transmission, and provided easier provisioning and troubleshooting.
The ITU started in the 1970s to define protocols and packet formats for accessing a packet data service. Under the umbrella Recommendation X.25, they defined the service interface between customer and network. How the network operated internally was left to each switch vendor. As was the custom before open standards became the norm, each make of X.25 switch added its own proprietary methods to register terminals, route connection requests, and create billing records. Frame relay standards followed a similar path, defining the user network interface (UNI) but not the network internals.
By the 1990s, X.25 service covered the world and was available on almost every computer made. It remained popular until after Internet access became widely available. The similarity between X.25 and SS7 allowed phone companies to offer a digital X.25 interface on ISDN lines, both in a DS-0 channel for user data and in the D channel (D stands for data, signaling not voice).