We review the advancements in Rx DSP-based transmission-link monitoring methods, which reveal fiber-longitudinal distributions of various physical parameters (e., signal power profile, gain spectra, and filter responses) along a multi-span link without analog testing instruments. In fiber-optic communication systems, it is crucial for operators to accurately monitor various physical parameters along optical links to fully leverage the potential transmission capacity and conduct fault analysis. Digital longitudinal monitoring (DLM) has been intensively studied for its. Abstract—Optical transmission links are generally composed of optical fibers, optical amplifiers, and optical filters. In this paper, we present a channel reconstruction method (CRM) that extracts physical characteristics of multiple link components such as longitudinal fiber losses, chromatic. We review recent advancements in the digital longitudinal monitoring (DLM) of fiber-optic links. Dense wavelength-division-multiplexed (WDM) systems with 10 Gb/s per channel have been widely deployed all over the world. 40-Gb/s systems are moving from lab demos. T., Technical Digest Series (Optica Publishing Group, 2022), paper Th1D.