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I think the confusion comes in with 802.11a and OFDM. In this 802.11a defines in the lower and middle bands as having 8 channels (4 each) that do not overlap. Each of these channels is devided into 52 sub carriers. The channels are 20 MHz wide and each sub carrier is about 300 kHz wide.

Of the 52 sub carriers only 48 carry data. the other 4 are used to carry
pilot tones. There are more non-overlapping channels with 802.11a and OFDM than existed in the earlier DSSS. When a radio transmits in 802.11a all of the sub-carriers are used simultaneously. All of this is testable information. If you are working on the certs please read pages 197-201 in the CWNA study guide 3rd ed. In fact, you should read very carefully the entire chapter. There are some good figures that help to make this more clear.

Good luck is nice Good Study is better.