In a method for generating a data unit for transmission via a communication channel, a first preamble portion of the data unit is generated based on whether a physical layer (PHY) mode is a first PHY mode or a second PHY mode. An OFDM portion of the data unit is generated. The OFDM portion follows the first preamble portion, includes a second preamble portion including one or more long training fields, is clocked at a first clock rate when the PHY mode is the first PHY mode, and is clocked at a second clock rate lower than the first clock rate when the PHY mode is the second PHY mode.