A real-life situation
A user says the Wi-Fi in the meeting room is "bad". Their laptop shows two bars. The survey tool shows an RSSI of −74 dBm and a noise floor of −90 dBm. Is that good enough for a video call? To answer, you need to read those numbers, and that means understanding decibels.
From milliwatts to dBm
Radio power is measured in milliwatts (mW). An access point might transmit 25 mW; the signal arriving at a laptop across the room might be 0.000001 mW. Numbers that far apart are awkward, so engineers use a logarithmic scale: dBm = 10 × log₁₀(power in mW).
You rarely need the formula. Two rules cover almost everything:
- Rule of 3: +3 dB doubles the power, −3 dB halves it.
- Rule of 10: +10 dB multiplies the power by 10, −10 dB divides it by 10.
Example: what is 50 mW in dBm? Start at 100 mW = 20 dBm and halve once: 20 − 3 = 17 dBm. And 4 mW? 1 mW is 0 dBm; double twice: 0 + 3 + 3 = 6 dBm.
EIRP: the power that actually leaves the antenna
The radio's transmit power is not the whole story. The cable to the antenna loses some, and the antenna concentrates the rest in certain directions (its gain). Regulators limit the result, called EIRP:
EIRP = transmit power − cable loss + antenna gain
Example: a radio transmits 17 dBm, the cable loses 2 dB and the antenna has 4 dBi of gain. EIRP = 17 − 2 + 4 = 19 dBm (about 80 mW). Because the values are in dB, they are just added and subtracted.
Try it: 20 dBm radio, 3 dB cable loss, 6 dBi antenna
EIRP = 20 − 3 + 6 = 23 dBm, which is 200 mW (20 dBm = 100 mW, +3 dB doubles it).
Measuring the received signal: RSSI, noise and SNR
At the receiver, the useful number is RSSI (received signal strength), shown in dBm. Every radio also hears a background of noise from electronics and other radio sources, the noise floor, often around −90 to −95 dBm. What matters is how far the signal stands above the noise: the signal-to-noise ratio (SNR).
SNR = signal (dBm) − noise floor (dBm)
| RSSI | Typical meaning |
|---|---|
| −30 to −50 dBm | Excellent; next to the access point |
| −50 to −67 dBm | Good; enough for voice and video (−67 dBm is a common design target) |
| −67 to −75 dBm | Fair; web and email work, real-time traffic may suffer |
| below −80 dBm | Poor; low data rates and disconnections |
Back to the meeting room: SNR = −74 − (−90) = 16 dB. Data works, but a common target for voice and video is RSSI −67 dBm or better and SNR 25 dB or more. The room needs better coverage, for example an access point closer to it.
Why it works this way
Radio signals lose power by a fraction for every doubling of distance, not by a fixed amount. A logarithmic scale turns those fractions into steps you can add: a wall might cost 6 dB, a cable 2 dB, an antenna add 5 dB. A receiver decodes a signal by telling it apart from the noise, so SNR, not raw strength, decides which data rate works.
Common mistakes
- Reading −80 dBm as "bigger" than −60 dBm. With negative numbers, closer to zero is stronger.
- Adding milliwatts and decibels together. Convert first, then add dB to dBm.
- Judging by RSSI alone. A strong signal in a noisy room can still have a poor SNR.
- Thinking antenna gain adds energy. It only focuses the same energy into a narrower area.
💡 Exam tip: remember +3 dB = ×2, +10 dB = ×10, 0 dBm = 1 mW, and EIRP = Tx power − cable loss + antenna gain.
Key takeaways
- dBm is power relative to 1 mW; dB is a relative change. +3 dB doubles, +10 dB multiplies by 10.
- EIRP = transmit power − cable loss + antenna gain.
- RSSI is the received signal in dBm; around −67 dBm or better is a typical voice target.
- SNR = signal − noise floor; 25 dB or more suits voice and video.
Check yourself
An AP's power is raised from 10 mW to 20 mW. How much is that in dB?
A radio sends 15 dBm through a cable that loses 3 dB to an antenna with 8 dBi gain. What is the EIRP?
RSSI is −62 dBm and the noise floor is −92 dBm. What is the SNR?
Which received signal is strongest?