In generating a physical layer (PHY) frequency duplication mode data unit for transmission via a communication channel, a preamble of the PHY frequency duplication mode data unit is generated. The preamble includes a signal field, and the preamble is configured so that a receiver can determine that the data unit is a frequency duplication mode-type data unit prior to decoding the signal field of the preamble. A payload of the PHY frequency duplication mode data unit is generated, and the PHY frequency duplication mode data unit is transmitted.