A communication device determines a format according to which a physical layer (PHY) data unit is to be generated. The communication device selects i) length information to be included in a field in a legacy portion of a PHY preamble of the PHY data unit, and ii) a phase of modulation of an orthogonal frequency division modulation (OFDM) symbol in a non-legacy portion of the PHY preamble, wherein the length information and the phase of modulation are selected to indicate the format from a set of multiple PHY formats defined by a communication protocol. The communication device generates the PHY data unit to include the selected length information in the field in the legacy portion of the PHY preamble and to modulate the OFDM symbol in the non-legacy portion of the PHY preamble with the selected phase, and transmits the PHY data unit.