Fiber-In-The-Loop (FITL) systems require an Optical Network Unit (ONU) which operates near user living units, typically in an outdoor environment. Enclosures housing ONUs are required to meet Bellcore GR-950-CORE “Generic Requirements for Optical Network Unit (ONU) Enclosures”. Among other things, this specification requires that the enclosure’s internal ambient temperature does not exceed the lowest maximum rated operating temperature of the optics housed in the enclosure under certain thermal conditions. This requirement provides a reasonable level of assurance towards the reliability of the system.

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