Editing Carrier system (section)

== History ==
Carrier systems increase economic efficiency by carrying more traffic on comparable cost of [[Infrastructure#Communications| communication infrastructure]]. 19th century telephone systems, operating by direct [[baseband]] transmission, could only carry one telephone call on each wire pair, hence routes with heavy traffic needed many wire pairs.

In the 1920s, frequency-division multiplexing could carry several circuits on the same [[balanced pair|balanced wires]], and by the 1930s [[L-carrier]] and similar systems carried hundreds of calls simultaneously on [[coaxial cable]]s. The capacity of these systems increased in the middle of the century.

In the 1950s, research began into further increasing the throughput of terminal equipment by using digital signals with [[time-division multiplexing]] (TDM). This work led to [[T-carrier]], [[E-carrier]] and other similar digital systems.

Due to the shorter [[repeater]] spacings required by digital systems, long-distance transmission still used [[Frequency-division multiplexing|FDM]] until the late 1970s when [[optical fiber]] was improved to the point that digital connections became the cheapest ones for all distances, short and long. By the end of the century, analog connections between and within [[telephone exchange]]s became rare.