The purpose of this paper is to give an introduction into the new standard base of Next-Generation Passive Optical Network (NG-PON). Study and analysis of future trends in the development of next-generation fixed broadband optical network is performed. The main intention of this paper is to describe migration from Gigabit-capable Passive Optical Network (GPON) to Ten-Gigabit-Capable Passive Optical Networks (XG-PON). Paper answers the question of what extent active and passive GPON elements need to be replaced and what needs to be added when migrating to XG-PON. Special focus is also pointed on the coexistence of GPON and XG-PON.