Wireless and Mobility
The Wireless and Mobility domain tests RF reasoning as much as configuration: coverage is easy, capacity and interference are not.
Spectrum and channels
- 2.4 GHz (2400–2483.5 MHz) offers only three non-overlapping 20 MHz channels - 1, 6 and 11 - so reuse planning is forced. Do not use 40 MHz here.
- 5 GHz: UNII-1 (36–48, non-DFS), UNII-2A (52–64, DFS), UNII-2C (100–144, DFS), UNII-3 (149–165). DFS channels must vacate on radar detection. 6 GHz (5925–7125 MHz) adds many 20 MHz channels and preferred scanning channels for discovery.
- Wider channels (40/80/160 MHz) raise peak rate but reduce the number of reusable channels; in dense areas 20 or 40 MHz wins on aggregate capacity.
Power and RF math
- 0 dBm = 1 mW; +3 dB doubles power, +10 dB is tenfold, 30 dBm = 1 W.
- EIRP = radio power + antenna gain − cable loss and must stay within the regulatory limit for band and country; fitting a higher-gain antenna means cutting radio power to stay legal.
- Free-space path loss rises about 6 dB per doubling of distance, so doubling range costs 6 dB, and 5 GHz attenuates more through walls than 2.4 GHz.
Security and roaming
- WPA2-Enterprise uses 802.1X with EAP; WPA2-PSK uses a shared key. WPA3 replaces the PSK handshake with SAE, makes protected management frames mandatory, and adds a 192-bit enterprise mode.
- 802.11r speeds re-association, 11k supplies neighbour reports, 11v manages transition - but the roaming decision always belongs to the client. CAPWAP uses UDP 5246 and 5247 between AP and controller.
Worked example
A client sees RSSI −67 dBm against a noise floor of −92 dBm, giving 25 dB SNR and a high MCS. A nearby AP is then set to maximum power, the floor rises to −80 dBm and SNR falls to 13 dB, forcing a lower data rate. The heat map looks better while throughput drops. The fix is lower power plus additional APs on separate channels, not a louder radio.
Traps that cost marks
Boosting power to fix coverage, which creates sticky clients and co-channel interference. Designing for coverage instead of capacity. Assuming WPA3-only mode accepts WPA2 clients. Ignoring DFS radar events when planning channels.