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Free space optic (FSO) communication systems have the great potential of serving as high bandwidth links. However, their popularity and widespread use is limited by the fact, that this technology is dependent upon an unobstructed line of sight more than any other wireless communication system. Especially weather conditions affecting visibility, such as fog or heavy rain, often cause unacceptable link failures. To increase reliability, a variety of hybrid FSO/RF systems has been introduced, among them a great number of switch-over systems. These systems require an accurate and timely detection of link failure and an appropriate algorithm to react. Generally, the received signal strength is considered as an indicator for link quality. Fluctuations in this indicator prohibit the use of simple threshold comparison techniques. In this paper, a set of switch-over algorithms is introduced and analyzed, using attenuation measurement data of fog events.