A first communication device generates a physical layer (PHY) preamble for a PHY data unit that conforms to a first communication protocol. A first portion of the PHY preamble is generated to include a first signal field having a length subfield that indicates a length of the PHY data unit, A second signal field is generated, and the second signal field and a duplicate of the second signal field are included in the PHY preamble. The PHY preamble is formatted such that the first portion of the PHY preamble is decodable by any second communication device that conforms to a second communication protocol, but does not conform to the first communication protocol, to determine a duration of the PHY data unit based on the length subfield in the first portion of the PHY preamble. The first communication device generates the PHY data unit to include the PHY preamble.