Wi-Fi is shared radio
A wired switch gives every port its own collision-free link. Wi-Fi is different: every device near an access point (AP) shares the same radio channel, and only one can transmit at a time. Wi-Fi is half duplex and uses CSMA/CA (collision avoidance): listen first, wait a random time, then send, and expect an acknowledgement for every frame.
Bands
| Band | Range | Channels | Notes |
|---|---|---|---|
| 2.4 GHz | Longest; goes through walls best | 1-11 (1-13 in Europe), each 22 MHz wide, 5 MHz apart | Only 1, 6 and 11 don't overlap. Crowded: microwaves, Bluetooth |
| 5 GHz | Shorter | Many 20 MHz channels (36, 40, 44, 48 ... 165), none overlap | Can bond channels to 40/80/160 MHz for speed |
| 6 GHz (Wi-Fi 6E/7) | Shortest | Many more channels | WPA3 only |
Signal and interference
Signal strength is measured in dBm (-30 is excellent, -67 is the usual minimum for voice, -80 is barely usable) and drops with distance and walls. Two APs near each other on the same or overlapping channels interfere: frames collide and are resent, and everything slows down. Neighbouring APs should use different non-overlapping channels.
802.11 standards
| Standard | Name | Band(s) | Max speed (theory) |
|---|---|---|---|
| 802.11b | - | 2.4 GHz | 11 Mb/s |
| 802.11a | - | 5 GHz | 54 Mb/s |
| 802.11g | - | 2.4 GHz | 54 Mb/s |
| 802.11n | Wi-Fi 4 | 2.4 and 5 GHz | 600 Mb/s |
| 802.11ac | Wi-Fi 5 | 5 GHz | 6.9 Gb/s |
| 802.11ax | Wi-Fi 6 / 6E | 2.4, 5 (and 6) GHz | 9.6 Gb/s |
| 802.11be | Wi-Fi 7 | 2.4, 5 and 6 GHz | 46 Gb/s |
In NetLab a laptop sees signal as a percentage (from the distance to the AP on the diagram), and an access point warns about overlapping neighbours:
In NetLab
▶ LAP1 NetLab Windows-style command prompt. Type help to see the commands. C:\Users\student>netsh wlan show networks Interface name : Wi-Fi There are 2 networks currently visible. SSID 1 : Staff Network type : Infrastructure Authentication : WPA2-Personal Encryption : CCMP Signal : 79% Band : 2.4 GHz Channel : 1 SSID 2 : Staff Network type : Infrastructure Authentication : WPA2-Personal Encryption : CCMP Signal : 40% Band : 2.4 GHz Channel : 3 ▶ AP1 NetLab access point. Type help to see the commands. AP1> show wlan Radio : on SSID : Staff Security : WPA2-Personal Passphrase : set (12 characters) Band : 2.4 GHz Channel : 1 (non-overlapping 2.4 GHz channels are 1, 6 and 11) Interference : AP2 on channel 3
Remember: 2.4 GHz: further, slower, only three non-overlapping channels (1, 6, 11). 5 GHz: shorter range, faster, many non-overlapping channels.