“802.11a/g utilize 20 MHz channels, and tones on the side of each channel are not used to protect the main carrier.” What does tones mean, in the previous sentence?

“802.11a/g utilize  20 MHz channels, and tones on the side of each channel are not used to  protect the main carrier.” What does tones mean, in the previous sentence?


See for more context: “This paper presents an overview of discrete multitone (DMT) modulation/demodulation systems with particular emphasis on channel equalization techniques. Three popular time-domain equalization techniques are discussed, including the minimum mean-squared error, the shortening signal-to-noise ratio (SNR) and the geometric SNR method. As an example of the frequency-domain equalization technique, the per tone equalization technique, commonly used in the conventional DMT systems, is discussed. This technique equalizes each tone separately such that a higher channel capacity is reached. These equalization techniques are subsequently compared in terms of their computational complexity and capability for bit-rate maximization through the channel. This is followed by a discussion of other multitone modulation/demodulation systems, such as discrete wavelet multitone and filtered multitone. The latter systems may be designed to give rise to better subchannel isolation and robustness in noisy environments.” From: https://ieeexplore.ieee.org/document/986875