Wi-Fi Fundamentals
Wi-Fi Fundamentals
This domain tests the RF and 802.11 mechanics that Mist exposes in its dashboards: how signal, noise, channel width, and security handshakes become the coverage, capacity, and connect metrics you troubleshoot later.
RF rules to memorize
- Power is logarithmic. +3 dB doubles power, +10 dB is ten times, −3 dB halves it. dBm is absolute; dBi and dB are relative.
- EIRP = AP transmit power − cable/connector loss + antenna gain. Regulators cap EIRP, so a 20 dBm radio with 3 dB loss and 5 dBi gain produces 22 dBm EIRP.
- SNR = RSSI − noise floor. A −65 dBm signal over a −92 dBm floor is 27 dB SNR. Voice generally needs ≥25 dB; data ≥20 dB.
- Channels: 2.4 GHz uses 1/6/11 for non-overlapping 20 MHz. 5 GHz adds UNII bands and 40/80/160 MHz. 6 GHz (Wi-Fi 6E) has no legacy 802.11a/b/g clients, requires WPA3 or OWE, and defines Preferred Scanning Channels every 80 MHz.
802.11ax and security
Wi-Fi 6 (802.11ax) brings OFDMA, uplink and downlink MU-MIMO, BSS Coloring, and Target Wake Time. WPA2-PSK uses the 4-way handshake to derive the PTK; WPA2-Enterprise wraps 802.1X/EAP inside that exchange. WPA3-SAE replaces the PSK exchange, and Protected Management Frames (802.11w) are mandatory in WPA3 and 6 GHz. Fast roaming uses 802.11r (FT), assisted by 802.11k neighbor reports and 802.11v BSS transition.
Worked example
An AP transmits at 18 dBm into 2 dB cable loss and a 6 dBi antenna: 18 − 2 + 6 = 22 dBm EIRP. A client reports −70 dBm and the AP hears −93 dBm noise: SNR = 23 dB, adequate for data, marginal for voice. Raising AP power does not fix the uplink, because the client's transmit power is unchanged.
Traps
Do not add dBm values together; a 160 MHz channel is not twice as fast when the spectrum is congested; 6 GHz does not support WPA2 or legacy rates; and a strong RSSI with a raised noise floor still yields a poor SNR.