When a Gigabit link needs to leave the cabinet — across a plant floor, between buildings on a campus riser, or through an electrically hostile environment — fiber is the answer, and the PR4DU22K is Teltonika's short-reach module for the job. It is an SFP transceiver running 1.25 Gb/s Gigabit Ethernet over multimode fiber, designed for the SFP cages on Teltonika devices such as the RUTX-series routers and TSW-series switches that provide fiber ports. Short-reach multimode optics are the workhorse of in-building fiber: they pair with the OM-grade multimode cable already pulled through most commercial risers and use the standard LC duplex interface. Compared with copper, the fiber link is immune to ground loops, EMI from motor drives and welders, and lightning-induced surges between buildings — often the real reason industrial sites choose fiber even over short distances. The module is hot-pluggable, so it can be inserted and swapped without powering down the host. That, plus the SFP format's ubiquity, makes deployment simple: one module in the Teltonika device, a compatible 1.25 Gb/s SR module at the far end, and a multimode patch between them. As with all optics, match both ends: SR multimode modules must talk to SR multimode modules over multimode cable. For single-fiber or long-haul runs, look instead at the PR4LC21K BiDi module; for copper Ethernet out of an SFP cage, the PR4JR20K RJ45 module completes the family.