Physical layer frame format for WLAN

A first communication device generates a PHY preamble of a PHY data unit to include a first orthogonal frequency division multiplexing (OFDM) symbol corresponding to a legacy signal field. The legacy signal field includes i) a length subfield, and ii) a rate subfield. The length subfield and the rate subfield indicate a duration of the PHY data unit, and the legacy signal field is formatted according to a legacy second communication protocol. The first communication device generates the PHY preamble of a PHY data unit to include a second OFDM symbol corresponding to a duplicate of the legacy signal field, and a plurality of additional OFDM symbols corresponding to a non-legacy signal field. The first communication device sets the length subfield of the legacy signal field to a length value such that a remainder value resulting from dividing the length value by three, indicates that the PHY data unit conforms to the first communication protocol.