What is dark fiber monitoring?
Active dark fiber monitoring involves injecting a low-power measurement signal into the unused fiber, allowing the condition of the glass path to be continuously monitored without putting the fiber into service.
The principle is similar to that of an OTDR (Optical Time Domain Reflectometer): a signal is sent into the fiber, the reflected light is analyzed, and reflections and loss peaks reveal where a connector is misaligned, where there is a kink, or where the fiber is damaged. An OTDR is a manual measuring instrument that technicians use during
installation or after a malfunction. An ALM applies the same principle, but in an automated and continuous manner, without a technician having to initiate the measurement.
The Adtran ALM
The ALM (Active Line Monitor) is a product in the Adtran portfolio, resulting from the merger of Adtran and ADVA Optical Networking in 2022. It is designed to monitor dark fiber and fiber-optic cables that are already carrying traffic.
The ALM injects a measurement signal at a wavelength outside the traffic band in use, typically around 1625 nm or 1650 nm. This allows the measurement to run in parallel with active services at adjacent wavelengths, without causing interference.
The system continuously monitors the optical loss value along the entire fiber path and sets alarm thresholds. If the loss value exceeds a preset limit, the system generates an alert. In addition to loss monitoring, the ALM provides fault localization: based on reflection measurements, the system calculates the location of a problem in the fiber path. This reduces the time needed to dispatch a technician to the correct location.
What does an ALM see that you would miss without measurement?
A connector that consistently exhibits higher loss indicates contamination or mechanical wear. A connection that
deviates at low temperatures may indicate a microbending problem: a microcrack in the fiber-protective
layer that expands in the cold and causes additional attenuation.
Without continuous monitoring, this remains undetected until it becomes critical. And in a network with dozens or
hundreds of fiber links, manual OTDR testing is not a realistic alternative. That is precisely the gap that an
ALM fills: not reacting to a failure, but detecting it before it occurs.
When is an ALM useful?
Any organization that manages or leases dark fiber, and where the availability of the fiber path is critical to service delivery—stands to benefit from active monitoring. Service providers that offer dark fiber as a product with an SLA are an obvious example. But enterprises with their own inter-data-center connections over leased or proprietary fiber also face the same risk.
Dark fiber that runs through public spaces, such as parking garages, utility rooms, or shared cable ducts,
adds an extra dimension. Physical interference with the fiber, construction work in the vicinity, or simply a door slamming shut too hard quickly become apparent in the measurement data. The ALM detects this. Without measurement, it wouldn’t.
