Can ESP32 execute a scan while a client is connected to its AP?

Projects Networking, Protocols, and Devices
1

Here I learnt how to make a reliable scan of the available WiFi network in the nearby.

In my real application, I put the ESP32 in WIFI_AP_STA mode and enable the hotspot:

WiFi.mode(WIFI_AP_STA);
WiFi.softAPConfig(apIP, apIP, netMsk);
WiFi.softAP(ssid, password);
WiFi.softAPsetHostname(HOSTNAME);

in this way the user can navigate to a web page served by AsyncWebServer where the available networks will be shown.

Every time I call WiFi.scanNetworks(true); the client is disconnected from the ESP32'AP:

Starting Network Scan...
....[1314334][W][AsyncTCP.cpp:950] _poll(): rx timeout 4
..[1316616][V][WiFiGeneric.cpp:383] _arduino_event_cb(): SCAN Done: ID: 166, Status: 0, Results: 12
[1316617][D][WiFiGeneric.cpp:1039] _eventCallback(): Arduino Event: 1 - SCAN_DONE
Found 12 networks
[1323366][V][WiFiGeneric.cpp:414] _arduino_event_cb(): AP Station Disconnected: MAC: 8c:b8:7e:10:39:7f, AID: 1
[1323368][D][WiFiGeneric.cpp:1039] _eventCallback(): Arduino Event: 13 - AP_STADISCONNECTED
[1330335][V][WiFiGeneric.cpp:407] _arduino_event_cb(): AP Station Connected: MAC: 8c:b8:7e:10:39:7f, AID: 1
[1330336][D][WiFiGeneric.cpp:1039] _eventCallback(): Arduino Event: 12 - AP_STACONNECTED
[1330421][V][WiFiGeneric.cpp:421] _arduino_event_cb(): AP Station IP Assigned:192.168.4.2
[1330422][D][WiFiGeneric.cpp:1039] _eventCallback(): Arduino Event: 14 - AP_STAIPASSIGNED
[1333679][W][AsyncTCP.cpp:930] _poll(): pcb is NULL
[1333680][W][AsyncTCP.cpp:930] _poll(): pcb is NULL
Starting Network Scan...
......[1346865][V][WiFiGeneric.cpp:383] _arduino_event_cb(): SCAN Done: ID: 167, Status: 0, Results: 14
[1346866][D][WiFiGeneric.cpp:1039] _eventCallback(): Arduino Event: 1 - SCAN_DONE
Found 14 networks
[1352230][V][WiFiGeneric.cpp:414] _arduino_event_cb(): AP Station Disconnected: MAC: 8c:b8:7e:10:39:7f, AID: 1
[1352231][D][WiFiGeneric.cpp:1039] _eventCallback(): Arduino Event: 13 - AP_STADISCONNECTED
[1357445][V][WiFiGeneric.cpp:407] _arduino_event_cb(): AP Station Connected: MAC: 8c:b8:7e:10:39:7f, AID: 1
[1357446][D][WiFiGeneric.cpp:1039] _eventCallback(): Arduino Event: 12 - AP_STACONNECTED
[1357517][V][WiFiGeneric.cpp:421] _arduino_event_cb(): AP Station IP Assigned:192.168.4.2
[1357517][D][WiFiGeneric.cpp:1039] _eventCallback(): Arduino Event: 14 - AP_STAIPASSIGNED

I also have a captive portal to automatically bring the user to the provisioning page when he connects to the AP and the behavior above leads to re-open the captive portal every time I run a scan.

I'm aware that the ESP32 must be in STA mode to perform a scan, but looking at the code of scanNetworks I see:

int16_t WiFiScanClass::scanNetworks(bool async, bool show_hidden, bool passive, uint32_t max_ms_per_chan, uint8_t channel, const char * ssid, const uint8_t * bssid)
{
    if(WiFiGenericClass::getStatusBits() & WIFI_SCANNING_BIT) {
        return WIFI_SCAN_RUNNING;
    }

    WiFiScanClass::_scanTimeout = max_ms_per_chan * 20;
    WiFiScanClass::_scanAsync = async;

    WiFi.enableSTA(true);

So it just enables the STA mode, it does not turns off the AP mode. And I set it to WIFI_AP_STA hence it should keep the AP mode on while scanning.

Bottom line: is the ESP32 capable to perform a WiFi scan while a client remains connected to its AP (and the AsyncWebServer still answers to the AsyncWebServerRequests) ?

If "yes": I need to fix my code...

2

My guess (never tried) is that it can do a scan and maintain the AP live but you should expect disruption for AsyncWebServerRequest as I think both activities would run on the same core

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3

scan goes thru all channels so it will turn of the SoftAP and disconnect the stations from it.

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4

From a user-experience point of view, what you would suggest then?

I mean, when the user opens the provisioning page he would expect periodic scans to update the available networks. But since this leads to a disconnection, how to make a usable UI?

5

you can't update the visible networks. scan them before starting the SoftAP.
btw: why not use the WiFiManager library?

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6

Most IoT devices allow a refresh of the available networks while provisioning. I'm trying to mimic that behavior.

I tried the WiFiManager library in the past, but now I want to use my own code to better understand how it works under the hood and provide a custom page for provisioning (along with the credentials I need to set few others options).

7

ESP32 has only one radio which can run only at one channel at time.
you could try if the connected station can stay connected with AP lost for a moment.

1 Like
8

you can dot that with WiFiManager

but that's a good reason :slight_smile:

1 Like
9

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