The main types include GPON OLT, EPON OLT, and XGS-PON OLT, each designed for specific network configurations and requirements. An OLT (Optical Line Terminal) is the core device in a Passive Optical Network (PON) — the interface between the core network and the subscriber's optical access network. It aggregates multiple ONUs/ONTs through optical splitters and handles data distribution, management, and synchronization.