If the noise floor is -90 dBm and wireless client is receiving a signal of -75 dBm, what is the SNR?

Question

Options

• A15
• B1.2
• C-165
• D.83

• A
  83% (24)
• B
  3% (1)
• C
  10% (3)
• D
  3% (1)

Explanation

Option A (15 dB) is correct because SNR (Signal-to-Noise Ratio) is calculated by subtracting the noise floor from the received signal strength: -75 dBm − (-90 dBm) = 15 dB. The two negatives cancel correctly, giving a positive SNR value that indicates how much stronger the signal is compared to the noise.

Option C (-165) is wrong because it incorrectly adds the two values (-75 + -90 = -165) rather than subtracting them. Option B (1.2) is wrong because it appears to divide the values (-75 ÷ -90 ≈ 0.83), and Option D (0.83) is that same incorrect division result - both options confuse dBm math with linear ratio math, which is not how SNR is calculated in wireless networking.

Memory Tip: Think of SNR as "Signal minus Noise" - just remember the phrase "S-N = SNR" and always subtract the noise floor from the signal level. A higher positive number means a cleaner, stronger signal (15 dB is generally considered a usable SNR in Wi-Fi environments).

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