This is one fiber optic network converter, or fiber optic Ethernet converter. It is the HTB-3100, available from many vendors on Ebay and Aliexpress. We added some layers of aluminum tape to the sides, to cover the grid of ventilation holes because there is too much EM radiation that escapes from there. This should not be a problem if the device is kept in a fresh room. Of course we need two of these for each fiber optic link.