This Recommendation describes the general requirements of NG-PON2 systems supporting a 40-Gbit/s capable aggregate downstream capacity for residential@ business@ mobile backhaul@ and other applications. This Recommendation includes the principal deployment configurations@ migration scenarios from legacy PON systems@ and system requirements. This Recommendation also includes the service and operational requirements to provide for a robust and flexible optical access network supporting all access applications. The NG-PON2 system is capable of meeting the needs of a wide range of networks in diverse markets and is deployable in numerous applications in an efficient manner. As much as possible@ this Recommendation maintains characteristics from legacy PON systems: [ITU-T G.982]@ [ITU-T G.983]@ [ITU-T G.984]@ and [ITU-T G.987] series of Recommendations. This is to promote backward compatibility with existing optical distribution networks (ODNs) that comply with those Recommendations and re-use established technical capabilities as much as possible. This Recommendation also describes smooth migration scenarios from legacy PON systems to NG-PON2 systems. Furthermore@ NG-PON2 systems are is expected to meet bandwidth growth and enable new revenue streams on legacy ODNs as well as supporting greenfield applications over new ODNs.