A method for generating a physical layer, PHY, data unit for transmission via a communication channel, the method comprising: generating a first set of orthogonal frequency division multiplexing, OFDM, symbols for a legacy portion of the PHY data unit, wherein OFDM symbols of the first set of OFDM symbols are generated with a first OFDM tone spacing; generating a second set of OFDM symbols for a non-legacy portion of the PHY data unit, wherein at least some OFDM symbols of the second set of OFDM symbols are generated with a second tone spacing different from the first tone spacing; determining, based on the first tone spacing and the second tone spacing, a value for a length indicator indicative of a duration of the PHY data unit, wherein determining the value for the length indicator includes determining a time duration corresponding to transmission of the second set of OFDM symbols as a function of a number of OFDM symbols in the second set of OFDM symbols and a duration of each OFDM symbol in the second set of OFDM symbols, wherein the duration of each OFDM symbol in the second set of OFDM symbols corresponds to the second tone spacing, and generating the value for the length indicator by applying, to the determined time duration, a conversion factor that corresponds to a predetermined data rate, and a ceiling function; generating the legacy portion of the PHY data unit to include the first set of OFDM symbols and the length indicator set to the determined value, wherein the legacy portion of the PHY data unit is generated such that the length indicator is included in a legacy preamble in the legacy portion of the PHY data unit; generating the non-legacy portion of the PHY data unit to include the second set of OFDM symbols; and generating the PHY data unit to include the legacy portion and the non-legacy portion. generating a first set of orthogonal frequency division multiplexing, OFDM, symbols for a legacy portion of the PHY data unit, wherein OFDM symbols of the first set of OFDM symbols are generated with a first OFDM tone spacing; generating a second set of OFDM symbols for a non-legacy portion of the PHY data unit, wherein at least some OFDM symbols of the second set of OFDM symbols are generated with a second tone spacing different from the first tone spacing; determining, based on the first tone spacing and the second tone spacing, a value for a length indicator indicative of a duration of the PHY data unit, wherein determining the value for the length indicator includes determining a time duration corresponding to transmission of the second set of OFDM symbols as a function of a number of OFDM symbols in the second set of OFDM symbols and a duration of each OFDM symbol in the second set of OFDM symbols, wherein the duration of each OFDM symbol in the second set of OFDM symbols corresponds to the second tone spacing, and generating the value for the length indicator by applying, to the determined time duration, a conversion factor that corresponds to a predetermined data rate, and a ceiling function; determining a time duration corresponding to transmission of the second set of OFDM symbols as a function of a number of OFDM symbols in the second set of OFDM symbols and a duration of each OFDM symbol in the second set of OFDM symbols, wherein the duration of each OFDM symbol in the second set of OFDM symbols corresponds to the second tone spacing, and generating the value for the length indicator by applying, to the determined time duration, a conversion factor that corresponds to a predetermined data rate, and a ceiling function; generating the legacy portion of the PHY data unit to include the first set of OFDM symbols and the length indicator set to the determined value, wherein the legacy portion of the PHY data unit is generated such that the length indicator is included in a legacy preamble in the legacy portion of the PHY data unit; generating the non-legacy portion of the PHY data unit to include the second set of OFDM symbols; and generating the PHY data unit to include the legacy portion and the non-legacy portion.