Fiber in the loop is a system implementing or upgrading portions of the PSTN network Local loop with Optical Fiber technology from the Telephone Exchange of a telephone carrier to a remote Serving Area Interface (SAI) located in a neighborhood or to an Optical Network Unit located at the customer premises (residential or business). Generally, fiber is used in either all or part of the local loop distribution network. FITL can be implemented with any FTTx architecture, such as fiber to the cabinet (FTTC), fiber to the node (FTTN), and fiber to the premises (FTTP).
Residential areas already served by balanced pair distribution plant call for a trade-off between cost and capacity. The closer the fiber head, the higher the cost of construction and the higher the channel capacity. In places not served by metallic facilities, little cost is saved by not running fiber to the home.
A similar network called a Hybrid fibre-coaxial (HFC) network is used by Cable Television operators but is usually not synonymous with "fiber In the loop", although similar advanced services are provided by the HFC networks.