Espressif Systems ESP Faq page 148

Chapter 4. Software framework
4.10.23 [Connect] What does bcn_timeout, ap_probe_send_start mean？
The STA does not receive the Beacon frame within the specified time (6 s by default for ESP32, equals
to 60 Beacon Intervals). - The reason could be:
• Insufficient memory. "ESP32_WIFI_MGMT_SBUF_NUM"is not enough (there will be errors
like "esf_buf: t=8, l=beacon_len, ..."in the log). You can check this by typing the heap size
when received a Disconnect event.
• The AP did not send a beacon. This can be checked by capturing beacons from AP.
• Rssi too low. When the Rssi value is too low in complex environments, the STA may not receive the
beacon. This can be checked by retrieving Rssi values via esp_wifi_sta_get_ap_info.
• Hardware related issues. Bad package capturing performance.
When there is a bcn_timeout, the STA will try to send Probe Request for five times. If a Probe Response
is received from the AP, the connection will be kept, otherwise, the STA will send a Disconnect event
and the connection will fail.
4.10.24 [Connect] How to reconnect Wi-Fi after it disconnected?
Call esp_wifi_connect
4.10.25 [Connect] When does ESP32 disconnect from SoftAP in station mode？
By default, the ESP32 will disconnect from the AP if it does not receive any beacon for 6 s. This time
can be modified via esp_wifi_set_inactive_time.
4.10.26 [Scan] Why does the STA cannot find any AP sometimes during the scanning?
There are many possible reasons why ESP32 and ESP8266 cannot scan any AP, and some common
reasons and solutions are listed below.
• The AP is too far away or the signal is too weak, while Wi-Fi of ESP32 and ESP8266 can only
work within a certain range. If the AP is too far away or the Wi-Fi signal is too weak, ESP32 and
ESP8266 may not be able to scan the AP. You can move ESP32 or ESP8266 closer to the AP or
use a signal amplifier to enhance the signal strength.
• SSID of the AP is hidden. SSID of some APs may be hidden, so they will not be broadcasted to
nearby devices. In this case, ESP32 and ESP8266 cannot scan these APs. You can connect these
APs by inputting their SSID and passwords manually.
• The AP is overloaded or malfunctioning. If the AP is overloaded or malfunctioning, it may not be
able to handle new connection requests, and thus ESP32 and ESP8266 cannot connect to the AP.
You can try to wait for a while and then scan the AP again.
• ESP32 or ESP8266 has some software issues. Sometimes, software issues of ESP32 or ESP8266
may cause problems with scanning for APs. In this case, you can try to reset ESP32 or ESP8266
and restart Wi-Fi. If this method does not work, you may need to update the firmware of ESP32
or ESP8266.
• Other reasons include wireless interference, security settings, and network configuration. These
reasons may also affect Wi-Fi of ESP32 or ESP8266. In this case, you need to carefully check the
Wi-Fi environment and implement corresponding settings.
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after received the WIFI_EVENT_STA_DISCONNECTED event.
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