Recommendation ITU-T G.988 specifies the optical network unit (ONU) management and control interface (OMCI) for optical access networks. Recommendation ITU-T G.988 specifies the managed entities (MEs) of a protocol-independent management information base (MIB) that models the exchange of information between an optical line termination (OLT) and an ONU. In addition, it covers the ONU management and control channel, protocol and detailed messages. Amendment 1 contains various updates to ITU-T G.988 (2017). This amendment contains editorial corrections and clarifications along with the following substantive changes and extensions to PON OMCI related to bonded ONUs, filtering on DHCP for admission control purposes, synchronization alarm support, ONU timezone offset and ONU manufacturing data. Amendment 2 adds support for DC voltage-based visual message indicators in ITU-T G.988 voice over IP (VOIP) application service profile ME. Amendment 3 makes editorial changes on extended virtual local area network (VLAN) and adds: -Support for the discovery of Extended VLAN tagging operation configuration data ME enhanced mode through the ONU3-G ME. -Support for Extended VLAN ME tagging operation configuration data ME enhanced mode. -Corrections to Table 9.1.5-1 "Plug-in unit types" This amendment makes editorial changes to TWDM channel tuning performance history data part 1, TWDM channel managed entity and clause 9.2.21. Amendment 4 also adds: -Support of RFC2543 call hold (with connection address 0.0.0.0) -Support of dial plan alarms -Support of DHCP performance monitoring " Support of ONU operational performance monitoring " Supports additional VCD voice alarms " Adds clarifications to FEC seconds definition