A method, in a communication system utilizing channels for transmitting first PHY mode data units, includes generating first and second data units conforming to first and second PHY modes, respectively, causing the first data unit to be transmitted via a channel, determining a frequency band for transmitting the second data unit, and causing the second data unit to be transmitted via the frequency band. Generating the first and second data units includes generating first and second series of OFDM symbols, respectively. At least a portion of the second OFDM symbols includes more upper-edge than lower-edge guard tones, or vice versa. The frequency band has a bandwidth equal to the channel bandwidth divided by n≧2, and either a lowest or highest sub-band of one or more channels is excluded from the frequency band when the second OFDM symbols include more upper-edge or more lower-edge guard tones, respectively.