Recommendation ITU-T G.989.3 specifies the transmission convergence layer of 40 Gigabit-capable passive optical network (NG-PON2) systems providing optical access for residential, business, mobile backhaul, and other applications. An NG-PON2 system supports multiple wavelength channels and enables flexibility to add capacity as the demand grows to 100 Gbit/s and beyond. An NG-PON2 system may contain a set of TWDM channels, or a set of PtP WDM channels, or both. The TWDM channels operate at the nominal line rates of 9.95328 and 2.48832 Gbit/s in both downstream and upstream directions. The PtP WDM channels support nominal line rate classes of 1.25 Gbit/s, 2.5 Gbit/s, and 10 Gbit/s, depending on the PtP WDM client. The transmission convergence (TC) layer is the protocol layer of the NG-PON2 system that is positioned between the physical media dependent (PMD) layer and service clients. It builds on [ITU-T G.987.3], with modifications for NG-PON2 specific features. This Recommendation forms an integral part of the ITU-T G.989 series of ITU-T Recommendations [ITU-T G.989], [ITU-T G.989.1], [ITU-T G.989.2] that, together with the ONU management and control interface (OMCI) [ITU-T G.988], specifies a coherent set of access transmission systems. The NG-PON2 system is also a member of the [b-ITU-T G.9802] family.