Still common in the networking world; as an example, Ciena optical gear can do Optical Time Domain Reflectometer(try) (OTDR) periodically to measure the length

Still common in the networking world; as an example, Ciena optical gear can do Optical Time Domain Reflectometer(try) (OTDR) periodically to measure the length of the physical circuit, and the nature of the test also reveals where cables are joined based on reflections. It's also how you measure where a break is, at which point you dig out the geo-coords for the path the cable takes to work out where someone just ran an optical cable finder (backhoe/JCB digger/whatever you call it where you are).