A GEPON system usually consists of an OLT (Optical Line Terminal) at the service provider's central office and multiple ONU (Optical Network Units) or ONT (Optical Network Terminals) close to the end user as optical splitters. It is a passive device connecting OLT and ONU. The optical splitter has one upstream optical interface and several downstream optical interfaces. It forms the backbone of the PON architecture. The ODN can typically cover distances up to 20 km or more, depending on the network design.