The invention pertains to optical fiber transmission systems, and is particularly relevant to transmission of high volume of data and voice traffic among different locations. The detection unit includes a sleeve body, a moisture detector, a casing, a processor, a wireless transmitter, and a battery. A convex-row-meshing portion of the convex row is inserted through a side of the slider so as to be engaged with a concave-row-meshing portion of the concave row.