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 to support the reduction of the transmit PSD can be found in Recommendation ITU-T G.9901. Recommendation G.9903 (2012) used material from Recommendations ITU-T G.9955 and ITU T G.9956; specifically from Annexes A and D of ITU-T G.9955 and Annex A of ITU-T G.9956. This revised Recommendation integrates G.9903 (2012) with its Amd.1 (2013) and further adds several improvements to the PHY, the MAC and the routing parts of the specification.