Wi-Fi basics: radio, bands and standards

Why learn radio basics?

A wired link either works or it does not. A wireless link is shared air: walls, people, neighbours' Wi-Fi and even a microwave oven change how well it works. Most "the Wi-Fi is slow" complaints are radio problems, not router problems. A few radio words make those problems easy to understand.

Radio words

• Frequency: how many times the wave repeats per second, measured in hertz (Hz). 2.4 GHz means 2.4 billion cycles per second.
• Wavelength: the length of one cycle. Higher frequency means a shorter wave: about 12.5 cm at 2.4 GHz and about 6 cm at 5 GHz. Shorter waves lose strength faster and pass through walls less well.
• Amplitude: the strength (height) of the wave. More amplitude means a stronger signal.
• dBm: power compared with 1 milliwatt, on a log scale. 0 dBm is 1 mW, 10 dBm is 10 mW, 20 dBm is 100 mW. Adding 3 dB roughly doubles the power; subtracting 10 dB divides it by 10.

Signals your phone receives are tiny, so they are negative numbers in dBm. Around -30 dBm is excellent (next to the AP), -67 dBm is the usual target for voice and video calls, -80 dBm is weak, and near -90 dBm the link barely works. Closer to zero is stronger.

What happens to the signal

• Absorption: materials soak up energy. Concrete, water (and people, who are mostly water) absorb a lot.
• Reflection: metal, glass and mirrors bounce the wave, so copies arrive at slightly different times (multipath).
• Interference: other signals on the same or nearby frequencies: other Wi-Fi networks, Bluetooth, cordless phones and microwave ovens (which leak around 2.4 GHz).

The three bands

The 2.4 GHz band reaches further and passes walls better, but it is crowded. Its channels are only 5 MHz apart while each Wi-Fi signal is about 20 to 22 MHz wide, so neighbouring channels overlap. Only 1, 6 and 11 do not overlap with each other, so a good design uses only those three. (India and most countries allow channels 1 to 13; the USA allows 1 to 11.)

The 5 GHz band has many more non-overlapping channels, so neighbouring APs rarely need to share. Some 5 GHz channels need DFS (Dynamic Frequency Selection): the AP must listen for radar, such as weather or airport radar, and move to another channel if it hears any. The 6 GHz band, used by Wi-Fi 6E and Wi-Fi 7, adds a large clean space with only modern devices on it, and requires WPA3. Which 5 and 6 GHz channels are allowed depends on each country's rules.

The 802.11 standards

SSID, BSS, ESS and friends

• SSID (Service Set Identifier): the network name people see, like "Office-Staff".
• BSS (Basic Service Set): one AP and the clients joined to it. Its area is the basic service area (the cell).
• BSSID: the MAC address of the AP radio that identifies one BSS. One SSID can have many BSSIDs, one per AP radio.
• ESS (Extended Service Set): several APs with the same SSID and security, joined by a wired network (the distribution system), so it feels like one network.
• IBSS (Independent BSS, or ad hoc): devices talk directly with no AP. Rare in companies.
• Roaming: a client moving from one AP to another inside the same ESS, ideally without dropping a call. The client decides when to roam.

CSMA/CA: taking turns in the air

Old half-duplex wired Ethernet used CSMA/CD (Collision Detection): send, and if a collision is heard, stop and retry. A radio cannot listen while it transmits, so Wi-Fi cannot detect collisions. It uses CSMA/CA (Collision Avoidance) instead: listen first; if the channel is busy, wait; then wait a random extra time (backoff) so two waiting stations do not start together; then send. The receiver replies with an ACK. No ACK means the frame is sent again. Optional RTS/CTS messages reserve the air when stations cannot hear each other.