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CWSS: Define Basic RF Characteristics

CWSS · Work through your exam, one topic at a time.

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Define Basic RF Characteristics

Domain 1, Define Basic RF Characteristics, tests your ability to explain how radio signals travel, how transmit power and antenna gain combine, and how measured signal quality drives security and compliance decisions. Expect calculation-style items, not just vocabulary.

Power and the dB scale

  • RF power is referenced to 1 milliwatt: 0 dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W.
  • A 3 dB change doubles or halves power; 10 dB changes it tenfold. Antenna gain (dBi) and cable loss (dB) are relative values, not absolute power.
  • EIRP = conducted transmit power + antenna gain − cable/connector loss. Regulatory limits are written as EIRP; U.S. FCC rules, for example, cap 2.4 GHz point-to-multipoint EIRP at 36 dBm.

Propagation and impairment

  • Free-space path loss rises about 6 dB each time distance doubles and about 6 dB when frequency doubles, so a 5 GHz link has roughly 6–7 dB more loss than 2.4 GHz at the same distance.
  • Reflection, refraction, diffraction, scattering, and absorption cause multipath, nulls, and reduced range. Fresnel-zone clearance matters more than visual line of sight.

Signal quality

  • RSSI is a relative received level; noise floor is the ambient energy. SNR = RSSI − noise floor, and it predicts usable throughput better than RSSI alone.
  • A commonly used planning target is 25 dB SNR or better for high-throughput data; voice and real-time applications need more headroom.

Worked example

An AP transmits 20 dBm into a cable with 3 dB loss and a 9 dBi antenna. EIRP = 20 + 9 − 3 = 26 dBm, or about 400 mW (26 dBm is 6 dB above 20 dBm, and 6 dB is a fourfold increase over 100 mW). If the FCC 36 dBm ceiling applies, the design has 10 dB of regulatory headroom. If the client then moves to twice the distance, expect about −6 dB: a −60 dBm RSSI becomes −66 dBm. With a −90 dBm noise floor, SNR falls from 30 dB to 24 dB, which may still work for data but is marginal for voice.

Traps that cost marks

  • Adding antenna gain without subtracting cable loss.
  • Treating dBi as dBm or reporting conducted power when the question asks for EIRP.
  • Forgetting that 6 dB is four times the power and 10 dB is ten times.
  • Assuming 5 GHz always has less range only because of higher frequency; channel width and regulatory power also matter.