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

Question

Options

• A48
• B52
• C54
• D56

• A
  3% (1)
• B
  83% (29)
• C
  11% (4)
• D
  3% (1)

Explanation

In 802.11n 20 MHz mode, 52 of the 64 OFDM subcarrier positions are allocated for data transmission, making B correct. The 64-subcarrier structure reserves 8 positions for guard bands and a DC null, and 4 additional subcarriers serve as pilot tones used for channel estimation and synchronization - leaving 52 for actual data payload.

Option A (48) is the data subcarrier count for legacy 802.11a/g, which is a common source of confusion since those standards use a similar OFDM scheme but with fewer data subcarriers. Option D (56) is a trap: it represents the total number of active (non-null) subcarriers in 802.11n 20 MHz mode, combining the 52 data and 4 pilot subcarriers - but pilot subcarriers carry control information, not user data. Option C (54) does not correspond to any standard subcarrier allocation.

Memory tip: Think "52 for n, 48 for a/g" - 802.11n added 4 more data subcarriers compared to its predecessors, and the full active count of 56 minus the 4 pilots gives you the data count of 52.

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