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Wireless — Cheatsheet

Wi-Fi, radio frequencies, antennas and security — 2025–2026

Table of Contents

Radio Waves and Units

c = f × λ

c is roughly 3 × 10^8 m/s; a higher frequency means a shorter wavelength.

Quantity Unit Meaning
frequency Hz cycles per second
wavelength m distance of one cycle
power W, mW, dBm transmitted/received energy
antenna gain dBi directional effect relative to an isotropic antenna
loss dB attenuation

dBm and dB

dBm = 10 × log10(PmW)
PmW = 10^(dBm/10)
Power dBm
1 mW 0 dBm
10 mW 10 dBm
100 mW 20 dBm
1 W 30 dBm

Rules of thumb: +3 dB ≈ ×2, -3 dB ≈ ÷2, +10 dB = ×10.

Adding in dB

You may add powers, gains and losses in a link budget algebraically; don't add mW values as if they were dB.

Antenna Pattern Use
omnidirectional all around in the horizontal plane general access point
directional/panel broad focused beam coverage of a zone
Yagi narrower focused beam point-to-point/distance
parabolic very narrow, high gain long point-to-point link

Gain doesn't create extra energy; it concentrates energy in certain directions. More gain generally means a narrower beamwidth.

EIRP = transmit power dBm + antenna gain dBi − cable/connector loss dB
received signal = EIRP − path loss + receive antenna gain − receive losses

Free-space loss:

FSPL(dB) = 32.44 + 20 log10(distance in km) + 20 log10(frequency in MHz)

The fade margin is the difference between the expected received signal and the minimum receiver sensitivity. Allow margin for rain, obstacles, interference and a changing environment.

Propagation

  • Reflection: bouncing off large/smooth surfaces.
  • Refraction: direction changes when crossing between media.
  • Diffraction: a wave bends around edges.
  • Scattering: dispersion caused by small/rough objects.
  • Absorption: energy is absorbed; water and people attenuate strongly.
  • Multipath: multiple copies arrive with different path/phase.

The first Fresnel zone around a point-to-point line of sight must remain largely clear. Visual line of sight alone is not sufficient.

Doppler effect

Motion between transmitter and receiver shifts the perceived frequency. The effect increases with higher speed and higher carrier frequency.

Frequencies and Channels

Band Pros Cons
2.4 GHz greater range, better penetration few channels, lots of interference
5 GHz more/wider channels, higher capacity shorter range, DFS possible
6 GHz lots of clean spectrum, modern Wi-Fi shorter range, compatibility/regulation

At 2.4 GHz, 20/22 MHz channels overlap heavily; in many regions a three-cell plan uses channels 1, 6 and 11. A channel width of 40/80/160 MHz raises potential speed but uses more spectrum and increases the chance of interference.

Capacity over peak speed

In a busy network, more cells on narrower, reused channels are often better than 80 or 160 MHz everywhere.

DFS channels share spectrum with radar. An AP must detect radar and switch channels if needed.

Wi-Fi MAC and Hidden Nodes

Wi-Fi is half-duplex and uses CSMA/CA: listen, random backoff, transmit and acknowledge. Ethernet could detect collisions; radio cannot reliably listen while transmitting.

Hidden node

Client A and B can't hear each other, but both hear the same AP. They can transmit at the same time and collide at the AP. RTS/CTS can reserve the medium, but adds overhead.

Frames

Type Examples
Management beacon, probe, authentication, association
Control ACK, RTS, CTS
Data payload/QoS data

A client discovers BSSs via beacons or probes, authenticates and associates. The SSID is the network name; the BSSID typically identifies the radio/MAC of a specific cell.

Monitor mode lets an adapter capture 802.11 frames on a channel without being normally associated.

Wi-Fi Security

Mechanism Assessment
Open no encryption
WEP broken; never use
WPA/TKIP outdated
WPA2-AES/CCMP common, secure baseline
WPA3-SAE better password protection and forward secrecy
802.1X/Enterprise individual accounts via RADIUS/EAP

Protected management frames (802.11w/PMF) limit spoofed deauthentication/disassociation. WPA3 requires PMF.

Strong protocol, weak password

A short or predictable WPA2-PSK remains vulnerable to offline guessing after capturing the relevant handshake data. Use a long random passphrase or Enterprise authentication.

Enterprise

supplicant (client) → authenticator (AP/switch) → authentication server (RADIUS)

With EAP, always validate the server certificate; otherwise an evil twin can intercept credentials.

MIMO and Modern Wi-Fi

  • MIMO uses multiple antennas/radio chains.
  • Spatial streams increase throughput via multipath.
  • Beamforming steers energy/phase toward the client.
  • MU-MIMO serves multiple clients simultaneously in the spatial domain.
  • OFDMA splits a channel into resource units for efficient multi-user transmission.

Wi-Fi 7 (802.11be) builds further with, among other things, wider channels, higher modulation and Multi-Link Operation, where a device can combine or flexibly use multiple bands/links. Actual speed remains limited by signal quality, client capability, airtime, backhaul and interference.

Site Survey and Troubleshooting

Measure at least:

  • RSSI/signal strength;
  • noise floor and SNR;
  • channel utilization and co-channel interference;
  • retry rate and data rates;
  • coverage, roaming and capacity per location.
SNR = received signal level − noise floor
example: -60 dBm − (-92 dBm) = 32 dB

An RSSI closer to 0 is stronger (-55 dBm is stronger than -75 dBm). A strong signal doesn't guarantee quality if the noise floor or airtime occupancy is high.

Problem-solving order:

  1. Radio/band/channel and client compatibility.
  2. RSSI, SNR and obstacles.
  3. Interference and channel utilization.
  4. Authentication/association/DHCP.
  5. Gateway, DNS and application.
  6. Correlate controller/AP logs and packet capture.

UniFi practice

  • Adopt APs in the controller and check firmware.
  • Create the SSID, security policy, network/VLAN and any RADIUS link.
  • Avoid automatic maximum transmit power on every AP.
  • Plan non-overlapping channels and tune cell size.
  • Check the uplink, trunk and native/tagged VLANs when clients associate but don't get an IP.

Other Wireless Systems

Satellite

Orbit Characteristic
LEO low latency, many moving satellites/handovers
MEO middle ground
GEO fixed position relative to earth, wide coverage, high latency

Mobile Networks

Cellular networks reuse spectrum across cells. Mobility requires registration, authentication and handovers. Modern generations separate the radio access network from the core; capacity depends on spectrum, cell size, modulation, MIMO and load.