Wireless Fundamentals and RF Principles
Units and the math exam writers love
dBm is absolute power referenced to 1 mW (0 dBm = 1 mW, 20 dBm = 100 mW, 30 dBm = 1 W). dBi is relative antenna gain. EIRP = AP transmit power + antenna gain − cable/connector loss. Memorize the 3 dB and 10 dB rules: ±3 dB doubles or halves power; ±10 dB multiplies or divides by ten. In free space, every doubling of distance costs about 6 dB - the most useful single number in design.
Standards and bands
802.11b/g live in 2.4 GHz (b uses DSSS/CCK, g uses OFDM); 802.11a is 5 GHz OFDM; 802.11n added MIMO, 40 MHz channels and both bands; 802.11ac is 5 GHz only with 80/160 MHz and downlink MU-MIMO; 802.11ax (Wi-Fi 6/6E) adds OFDMA, uplink MU-MIMO, BSS coloring, TWT, and the 6 GHz band with no legacy clients. Remember that 2.4 GHz has three non-overlapping 20 MHz channels (1, 6, 11), 5 GHz roughly 25, and 6 GHz 59 plus Preferred Scanning Channels every 80 MHz.
Interference and SNR
SNR = signal level minus noise floor; it is not the same as RSSI. Co-channel interference comes from your own cells on the same channel, adjacent-channel interference from partially overlapping spectrum. Wider channels consume more spectrum and reduce reuse, so high-density designs often stay at 20 MHz.
Traps: vendors report “RSSI” on inconsistent scales, so never do dB math on a raw RSSI value; and 2.4 GHz penetrates walls better but carries less capacity - a common distractor.
Worked example: AP at 17 dBm, 4 dBi antenna, 1 dB cable loss → EIRP = 17 + 4 − 1 = 20 dBm = 100 mW. If regulation caps EIRP at 20 dBm, no further reduction is needed.