7. Installed systems: network construction and evolution

7.1 The first systems

The first fiber-optic transmission systems appeared in the early 1980s in what was then known as the inter-city network, i.e. the network connecting national or regional hubs, as opposed to the distribution network, which is the terminal part serving the subscriber. These are digital systems, i.e. optoelectronic repeater-regenerators distributed along the transmission line regenerate the optical signal when it reaches a certain level of distortion. The optical signal is then transformed into an electrical signal in a photodiode (this is known as direct detection), the information is extracted, and the data remodels an optical wave sent on to the next step. These early systems offered speeds of 34 or 140 Mbit/s per fiber, with a regeneration step of around 40...