These used to be big, expensive, standalone devices (I'd occasionally use one in a previous job). Nowadays, there are portable, handheld units specifically for identifying where a break has occurred and the same, basic functionality is often even built into the gear used at either end of connections (e.g., even the "enterprise" switches -- from Brocade and Cisco -- that I use at home).
These used to be big, expensive, standalone devices (I'd occasionally use one in a previous job). Nowadays, there are portable, handheld units specifically for identifying where a break has occurred and the same, basic functionality is often even built into the gear used at either end of connections (e.g., even the "enterprise" switches -- from Brocade and Cisco -- that I use at home).
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[0]: https://en.wikipedia.org/wiki/Optical_time-domain_reflectome...