XPON stands for Gigabit Passive Optical Networks. XPON can transport not only Ethernet but also ATM and TDM (PSTN, ISDN, E1 and E3) traffic. So, we can say that gigabit passive optical network (GPON) is a point-to-multipoint access mechanism providing end-users with the ability to consolidate multiple services onto a single fiber transport network. ONU (Optical Network Unit) , ONU provides services such as data, IPTV (i.e. interactive network television), voice (using IAD, i.e. Integrated Access Device), and truly realizes "triple-play" applications.