WLAN RF Fundamentals and Channel Planning
RF quantities
RSSI is received signal power in dBm; the noise floor is typically -90 to -95 dBm in a quiet 5 GHz channel. SNR = RSSI - noise floor. Higher SNR supports higher MCS and wider channels, but wider channels raise the noise floor and reduce reuse. Free-space path loss rises with frequency and distance; walls add attenuation that varies by material. EIRP = transmit power + antenna gain - cable loss, and regulatory limits cap EIRP.
Channel planning
2.4 GHz has three non-overlapping 20 MHz channels (1, 6, 11 in North America). 5 GHz has many more, but UNII-2 and UNII-2 Extended include DFS channels that must vacate for radar; UNII-1 and UNII-3 are non-DFS. 6 GHz adds clean spectrum with no DFS. Use 20 MHz in high-density and 2.4 GHz; reserve 40/80 MHz for low-density 5 GHz. Co-channel interference comes from reuse on the same channel; adjacent-channel interference from overlapping widths.
Rule of thumb
Each +3 dB doubles power; +10 dB is tenfold. A 20 dB SNR is a common floor for stable higher-rate data; voice designs often target about -65 dBm RSSI or better. Aruba ARM/AirMatch automates channel and power, but a manual plan should still avoid stacking APs on one 5 GHz channel.
Worked example
20 dBm transmit + 3 dBi antenna - 1 dB cable = 22 dBm EIRP. If the regulatory cap is 23 dBm, the setting is legal; if it were 20 dBm, reduce transmit power by 2 dB.