Optical fibers carry huge amounts of bandwidth, representing thousands or even millions of concurrent end-customer services. Any disruption of a fiber link can have tremendous impact. And yet fibers are frequently not monitored at all, with service providers relying entirely on service layer alarms to detect failures. This results in time-consuming and expensive fault isolation. What's more, service unavailability creates frustrated customers. This can easily be avoided by fiber assurance solutions. In-service fiber monitoring is an efficient way to instantly detect any fiber degradation, triggering countermeasures even before services are affected.