How a network is found

The beacon

An access point announces its network with a beacon, a short frame sent at a fixed interval. The interval is counted in time units of 1024 microseconds, and the common default is 100 of them: 102.4 ms, about ten beacons a second. That is a default, not a rule.

A beacon carries the network's name (the SSID), the rates the access point supports, its channel, the security it offers, and a traffic map for stations that are asleep. It also carries the access point's own identifier, the BSSID, written like a MAC address: six bytes, usually the address of the access point's radio.

The SSID is up to 32 bytes, not 32 characters: an emoji takes four.

One name, several access points

The SSID names the network; the BSSID names one access point. A dual-band router, a mesh system or an office with an access point in every room shows one name with several BSSIDs, often on different channels. The BSSID and the channel tell them apart.

Passive and active scans

A radio decodes frames only on the channel it is tuned to, so a scan visits the channels one at a time. In a passive scan it listens on each and waits for beacons. In an active scan it sends a probe request and the access points on that channel answer with probe responses, without waiting for their next beacon. ESP-IDF scans actively by default, about 120 ms on each channel.

Which channels it visits is the country setting's decision. The default setting covers channels 1 to 11, so a router on channel 12 or 13 may be missed. If the router is on 12 or 13, set the board's country.

A hidden name

An access point set to hide its name still sends beacons, with the SSID left empty, and its BSSID still shows in every scan. A phone that has joined it before asks for it by name, in probe and association frames anyone nearby can read. Hiding a name is not security. In ESP-IDF the access point's setting is ssid_hidden, and a scan reports hidden networks only with show_hidden.

The code for scanning and joining is in the ESP32 book, in station mode and several networks with WiFiMulti.

Common mistakes

• Counting characters in the SSID. The limit is 32 bytes. Count the bytes of the encoded name.
• Expecting channels 12 and 13 in the scan. The default country setting covers 1 to 11, so a router on 12 or 13 may be missed; set the board's country.
• Hiding the network to secure it. The beacons go on, the BSSID shows, and every phone that knows the name says it. Security is the password and WPA2 or WPA3 (lesson 11).