How many subcarriers can be used to transmit data in 802.11a in 20M mode?

Question

Options

• A48
• B52
• C54
• D56

• A
  84% (47)
• B
  2% (1)
• C
  9% (5)
• D
  5% (3)

Explanation

In 802.11a, the OFDM channel uses 64 total subcarrier slots (matching the 64-point IFFT), but only 52 of those are active - 48 carry actual data and 4 are reserved as pilot subcarriers used for phase and frequency synchronization. Since the question asks specifically about subcarriers used to transmit data, the answer is 48 (option A).

Option B (52) is a common trap: it is the total number of active subcarriers, but it bundles in the 4 pilot tones that carry no user data. Option C (54) has no basis in the 802.11a subcarrier plan and likely serves as a distractor by echoing the 54 Mbps maximum data rate of both 802.11a and 802.11g. Option D (56) is simply not part of the standard's OFDM allocation.

Memory tip: Think "52 active, 4 are pilots, 48 carry data." The 4 pilot subcarriers are the gatekeepers that keep the signal locked - they steer, they don't carry cargo.

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