  We report on methods to monitor the transmission path in optical networks
using a correlation-based OTDR technique with direct and coherent detection. A
high probing symbol rate can provide picosecond-accuracy of the fiber
propagation delay, while a sensitive phase detection with a high repetition
rate allows the monitoring of dynamic effects in the vicinity of the fiber. We
discuss various approaches to evaluate the measured traces and show the results
of a few monitoring applications.
