Recommendation ITU T G.9903 contains the physical layer (PHY) and data link layer (DLL) specification for the G3 PLC narrowband orthogonal frequency division multiplexing (OFDM) power line communication transceivers, for communications via alternating current and direct current electric power lines over frequencies below 500 kHz. The control parameters that determine spectral content, power spectral density (PSD) mask requirements, and the set of tools and procedures to support the measurement and reduction of the transmit PSD can be found in Recommendation ITU-T G.9901. This Recommendation adds several corrections of and improvements to the PHY, the DLL, and the routing parts of the specification. It also integrates Amendment 1 to Recommendation ITU-T G.9903 (2014), adding support for coexistence with other narrowband PLC technologies via the preamble-based coexistence mechanism specified in clause 10 of IEEE 1901.2.