A first preamble portion of a physical layer (PHY) data unit is generated according to a first clock rate, wherein the first preamble portion is formatted according to a first PHY mode of a wireless communication protocol when the PHY data unit is to be transmitted according to the first PHY mode, and the first preamble portion is formatted according to a second PHY mode of the wireless communication protocol when the PHY data unit is to be transmitted according to the second PHY mode. An OFDM portion of the PHY data unit is generated, wherein the OFDM portion follows the first preamble portion, includes a second preamble portion having one or more long training fields, is clocked at the first clock rate when the PHY data unit is to be transmitted according to a first PHY mode, and is clocked at a second clock rate different than the first clock rate when the PHY data unit is to be transmitted according to a second PHY mode.