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CWT: RF Fundamentals

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RF Fundamentals

RF Fundamentals

This domain tests the physical layer: how RF energy behaves, how it is measured, and how 802.11 uses it. Expect unit conversions, channel plans, propagation effects, and the relationship between modulation, bandwidth, and data rate.

Units and link budget

  • dBm is absolute power referenced to 1 mW; dB is a relative change. 0 dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W. A 3 dB increase doubles power; 10 dB is ten times.
  • EIRP = radio output (dBm) + antenna gain (dBi) − cable and connector loss (dB). Keep absolute and relative units separate.
  • RSSI is a received power indicator; SNR = RSSI − noise floor. Noise floor rises with interference and wider channels.

Bands and channels

  • 2.4 GHz uses 20 MHz channels with only three non-overlapping plans: 1, 6, and 11. 40 MHz in 2.4 GHz consumes most of the band and raises co-channel interference.
  • 5 GHz has UNII-1 (36–48), UNII-2A (52–64), UNII-2C (100–144), and UNII-3 (149–165). UNII-2A and UNII-2C require DFS and TPC; radar detection can force channel changes.
  • Wavelength at 2.4 GHz is about 12.5 cm and at 5 GHz about 6 cm, so 5 GHz has higher free-space path loss at the same distance.

Propagation and PHY

  • Absorption, reflection, refraction, diffraction, and scattering create multipath; multipath causes fading and intersymbol interference. Fresnel zone clearance matters for long point-to-point links.
  • 802.11b uses DSSS; 802.11a/g/n/ac/ax use OFDM, and 802.11ax adds OFDMA and 1024-QAM. Higher MCS values need better SNR.

Worked example

A 5 GHz AP transmits 17 dBm into a cable with 2.5 dB loss and a 9 dBi antenna. EIRP = 17 − 2.5 + 9 = 23.5 dBm. Convert to mW: 10^(23.5/10) ≈ 224 mW. If the receiver was at −55 dBm and the distance doubles, free-space path loss rises about 6 dB, so the new level is about −61 dBm before obstructions.

Traps

  • Adding dBm values; 20 dBm + 3 dBm is not 23 dBm. Add dB to dBm.
  • Treating dBi as absolute power; it is relative gain over an isotropic radiator.
  • Assuming every 5 GHz channel is usable everywhere; regulatory limits, DFS, and client capabilities vary.
  • Confusing channel width with data rate; wider channels raise throughput but also noise and interference.