"History feeds history", and, in the process, the digital subscriber line (DSL) technology stems directly from the "saga of the modems". From 1956, when the first modems were built and standardized, until 1995 which saw the birth of the last telephone-line modem, we saw the gradual development of data transmission techniques for short and long distances, pushed by the needs of industrial and commercial activities in every part of the world. Every aspect of this saga of the modems was carefully studied in this handbook in order to resolve the problems arising from the confrontation between transmission theory and the common difficulties due to the basic characteristics of equipment in use in access networks and in the backbone networks (copper wires, the length of lines, spurious signals, impulse noise, etc.). Step by step and year after year, problems were overcome and bit rates were gradually increased so that new office applications, such as the facsimile service, could be offered worldwide.