Fiber in the loop
（重定向自IFITL）
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).