A wireless network interface device selects a guard interval from a set of guard intervals including a first guard interval, a second guard interval, and a third guard interval. The first guard interval has a length that is different than a length of the second guard interval and a length of the third guard interval, and the length of the second guard interval is different than the length of the third guard interval. The wireless network interface device generates a preamble of a data unit to include: a legacy signal field, a repetition of the legacy field, and a non-legacy field that includes a field that indicates the selected guard interval. The wireless network interface device generates a data portion of the data unit, including generating orthogonal frequency division multiplexing (OFDM) symbols of the data portion using the selected guard interval.