In WLAN network, which of the following descriptions about Symbol and GI is wrong?

Question

Options

• AThe role of GI is to reduce the interference between symbols
• BSymbol's unit is microseconds, which indicates the time required to transmit the modulated data
• CGI is in microseconds, indicating a protection interval
• DThe larger the GI, the higher the transmission efficiency

• B
  13% (2)
• C
  6% (1)
• D
  81% (13)

Explanation

Option D is wrong because GI (Guard Interval) is overhead - it carries no data. A larger GI means a greater proportion of transmission time is wasted on protection gaps, which reduces efficiency. In practice, 802.11n/ac introduced a "Short GI" (400ns vs. the standard 800ns) specifically to increase throughput by about 11%.

Why the other options are correct:

• A is correct - GI's purpose is to absorb multipath echoes so they don't bleed into the next symbol, directly reducing Inter-Symbol Interference (ISI).
• B is correct - symbol duration is indeed measured in microseconds and represents how long one modulated unit of data occupies the channel.
• C is correct - GI is measured in microseconds (standard: 0.8μs, short: 0.4μs) and functions as a protective guard between symbols.

Memory tip: Think of GI like a buffer zone between lanes on a highway - the wider the buffer, the fewer lanes of traffic (data) you can fit. Bigger guard = less room for data = lower efficiency. The word "guard" should remind you it protects, not transmits.

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