Recommendation ITU-T G.9805 presents three methods for the coexistence of multiple PON generations on a common ODN: Coexistence element (CE), multi-PON module (MPM), and splitter-based. These allow the reuse of already deployed fibre and splitters when evolving a legacy PON to a higher capacity. Methods for calculating required isolation for Coexistence element, filter considerations for HSP and XG(S)-PON OLT, and optical interface parameters for GPON/XG(S)-PON MPM supporting Class B+, C+ and D OPL are also described. Amendment 1 includes additional 3-gen PON systems coexistence methods, and Crosstalk analysis between PON systems. Amendment 2 includes Class C+ and D OPL specifications for GPON/XG(S)-PON 2-generation MPM and GPON/XG(S)-PON/50G-PON 3-generation MPM, and specifications for 10G-EPON/50G-PON 2-generation MPM and EPON/10G-EPON/50G-PON 3-generation MPM. The 12.5Gb/s upstream line rate option for 50G-PON is deprecated.