Generally, the most effective way to begin your own Wi-Fi application is to select an example which is similar to your own application, and port the useful part into your project. It is not a MUST but it is strongly recommended that you take some time to read this article first, especially if you want to program a robust Wi-Fi application. This article is supplementary to the Wi-Fi APIs/Examples. It describes the principles of using the Wi-Fi APIs, the limitations of the current Wi-Fi API implementation, and the most common pitfalls in using Wi-Fi. This article also reveals some design details of the Wi-Fi driver. We recommend that you become familiar at least with the following sections: <`ESP32 Wi-Fi API Error Code`_>, <`ESP32 Wi-Fi Programming Model`_>, and <`ESP32 Wi-Fi Event Description`_>.
Generally, it is easy to write code in "sunny-day" scenarios, such as <`SYSTEM_EVENT_STA_START`_>, <`SYSTEM_EVENT_STA_CONNECTED`_> etc. The hard part is to write routines in "rainy-day" scenarios, such as <`SYSTEM_EVENT_STA_DISCONNECTED`_> etc. Good handling of "rainy-day" scenarios is fundamental to robust Wi-Fi applications. Refer to <`ESP32 Wi-Fi Event Description`_>, <`ESP32 Wi-Fi Station General Scenario`_>, <`ESP32 Wi-Fi AP General Scenario`_>
Just like the handling of "rainy-day" scenarios, a good error-recovery routine is also fundamental to robust Wi-Fi applications. Refer to <`ESP32 Wi-Fi API Error Code`_>
ESP32 Wi-Fi API Error Code
---------------------------
All of the ESP32 Wi-Fi APIs have well-defined return values, namely, the error code. The error code can be categorized into:
Whether the error is critical or not depends on the API and the application scenario, and it is defined by the API user.
**The primary principle to write a robust application with Wi-Fi API is to always check the error code and write the error-handling code.** Generally, the error-handling code can be used:
- for recoverable errors, in which case you can write a recoverable-error code. For example, when esp_wifi_start returns ESP_ERR_NO_MEM, the recoverable-error code vTaskDelay can be called, in order to get a microseconds' delay for another try.
- for non-recoverable, yet non-critical, errors, in which case printing the error code is a good method for error handling.
- for non-recoverable, critical errors, in which case "assert" may be a good method for error handling. For example, if esp_wifi_set_mode returns ESP_ERR_WIFI_NOT_INIT, it means that the Wi-Fi driver is not initialized by esp_wifi_init successfully. You can detect this kind of error very quickly in the application development phase.
In esp_err.h, ESP_ERROR_CHECK checks the return values. It is a rather commonplace error-handling code and can be used
as the default error-handling code in the application development phase. However, we strongly recommend that API users write their own error-handling code.
ESP32 Wi-Fi API Parameter Initialization
------------------------------------------
When initializing struct parameters for the API, one of two approaches should be followed:
- explicitly set all fields of the parameter or
- use get API to get current configuration first, then set application specific fields
Initializing or getting the entire structure is very important because most of the time the value 0 indicates the default value is used. More fields may be added to the struct in the future and initializing these to zero ensures the application will still work correctly after IDF is updated to a new release.
The Wi-Fi driver can be considered a black box that knows nothing about high-layer code, such as the
TCPIP stack, application task, event task, etc. All the Wi-Fi driver can do is receive API calls from the high layer,
or post an event-queue to a specified queue which is initialized by API esp_wifi_init().
The event task is a daemon task which receives events from the Wi-Fi driver or from other subsystems, such
as the TCPIP stack. The event task will call the default callback function upon receiving the event. For example,
upon receiving SYSTEM_EVENT_STA_CONNECTED, it will call tcpip_adapter_start() to start the DHCP
client in its default handler.
An application can register its own event callback function by using API esp_event_init. Then, the application callback
function will be called after the default callback. Also, if the application does not want to execute the callback
in the event task, it needs to post the relevant event to the application task in the application callback function.
The application task (code) generally mixes all these things together: it calls APIs to initialize the system/Wi-Fi and
handle the events when necessary.
ESP32 Wi-Fi Event Description
------------------------------------
SYSTEM_EVENT_WIFI_READY
++++++++++++++++++++++++++++++++++++
The Wi-Fi driver will never generate this event, which, as a result, can be ignored by the application event callback. This event may be removed in future releases.
SYSTEM_EVENT_SCAN_DONE
++++++++++++++++++++++++++++++++++++
The scan-done event is triggered by esp_wifi_scan_start() and will arise in the following scenarios:
- The scan is completed, e.g., the target AP is found successfully, or all channels have been scanned.
- The scan is stopped by esp_wifi_scan_stop().
- The esp_wifi_scan_start() is called before the scan is completed. A new scan will override the current scan and a scan-done event will be generated.
The scan-done event will not arise in the following scenarios:
- It is a blocked scan.
- The scan is caused by esp_wifi_connect().
Upon receiving this event, the event task does nothing. The application event callback needs to call esp_wifi_scan_get_ap_num() and esp_wifi_scan_get_ap_records() to fetch the scanned AP list and trigger the Wi-Fi driver to free the internal memory which is allocated during the scan **(do not forget to do this)**!
Refer to 'ESP32 Wi-Fi Scan' for a more detailed description.
If esp_wifi_start() returns ESP_OK and the current Wi-Fi mode is Station or AP+Station, then this event will arise. Upon receiving this event, the event task will initialize the LwIP network interface (netif). Generally, the application event callback needs to call esp_wifi_connect() to connect to the configured AP.
If esp_wifi_stop() returns ESP_OK and the current Wi-Fi mode is Station or AP+Station, then this event will arise. Upon receiving this event, the event task will release the station's IP address, stop the DHCP client, remove TCP/UDP-related connections and clear the LwIP station netif, etc. The application event callback generally does not need to do anything.
If esp_wifi_connect() returns ESP_OK and the station successfully connects to the target AP, the connection event will arise. Upon receiving this event, the event task starts the DHCP client and begins the DHCP process of getting the IP address. Then, the Wi-Fi driver is ready for sending and receiving data. This moment is good for beginning the application work, provided that the application does not depend on LwIP, namely the IP address. However, if the application is LwIP-based, then you need to wait until the *got ip* event comes in.
SYSTEM_EVENT_STA_DISCONNECTED
++++++++++++++++++++++++++++++++++++
This event can be generated in the following scenarios:
- When esp_wifi_disconnect(), or esp_wifi_stop(), or esp_wifi_deinit(), or esp_wifi_restart() is called and the station is already connected to the AP.
- When esp_wifi_connect() is called, but the Wi-Fi driver fails to set up a connection with the AP due to certain reasons, e.g. the scan fails to find the target AP, authentication times out, etc. If there are more than one AP with the same SSID, the disconnected event is raised after the station fails to connect all of the found APs.
- When the Wi-Fi connection is disrupted because of specific reasons, e.g., the station continuously loses N beacons, the AP kicks off the station, the AP's authentication mode is changed, etc.
Upon receiving this event, the default behavior of the event task is:
- Shuts down the station's LwIP netif.
- Notifies the LwIP task to clear the UDP/TCP connections which cause the wrong status to all sockets. For socket-based applications, the application callback can choose to close all sockets and re-create them, if necessary, upon receiving this event.
The most common event handle code for this event in application is to call esp_wifi_connect() to reconnect the Wi-Fi. However, if the event is raised because esp_wifi_disconnect() is called, the application should not call esp_wifi_connect() to reconnect. It's application's responsibility to distinguish whether the event is caused by esp_wifi_disconnect() or other reasons. Sometimes a better reconnect strategy is required, refer to <`Wi-Fi Reconnect`_> and <`Scan When Wi-Fi Is Connecting`_>.
Another thing deserves our attention is that the default behavior of LwIP is to abort all TCP socket connections on receiving the disconnect. Most of time it is not a problem. However, for some special application, this may not be what they want, consider following scenarios:
- The application creates a TCP connection to maintain the application-level keep-alive data that is sent out every 60 seconds.
- Due to certain reasons, the Wi-Fi connection is cut off, and the <SYSTEM_EVENT_STA_DISCONNECTED> is raised. According to the current implementation, all TCP connections will be removed and the keep-alive socket will be in a wrong status. However, since the application designer believes that the network layer should NOT care about this error at the Wi-Fi layer, the application does not close the socket.
- Five seconds later, the Wi-Fi connection is restored because esp_wifi_connect() is called in the application event callback function. **Moreover, the station connects to the same AP and gets the same IPV4 address as before**.
- Sixty seconds later, when the application sends out data with the keep-alive socket, the socket returns an error and the application closes the socket and re-creates it when necessary.
In above scenario, ideally, the application sockets and the network layer should not be affected, since the Wi-Fi connection only fails temporarily and recovers very quickly. The application can enable "Keep TCP connections when IP changed" via LwIP menuconfig.
This event arises when the AP to which the station is connected changes its authentication mode, e.g., from no auth to WPA. Upon receiving this event, the event task will do nothing. Generally, the application event callback does not need to handle this either.
This event arises when the DHCP client successfully gets the IPV4 address from the DHCP server, or when the IPV4 address is changed. The event means that everything is ready and the application can begin its tasks (e.g., creating sockets).
The IPV4 may be changed because of the following reasons:
- The DHCP client fails to renew/rebind the IPV4 address, and the station's IPV4 is reset to 0.
- The DHCP client rebinds to a different address.
- The static-configured IPV4 address is changed.
Whether the IPV4 address is changed or NOT is indicated by field "ip_change" of system_event_sta_got_ip_t.
The socket is based on the IPV4 address, which means that, if the IPV4 changes, all sockets relating to this IPV4 will become abnormal. Upon receiving this event, the application needs to close all sockets and recreate the application when the IPV4 changes to a valid one.
This event arises when the IPV6 SLAAC supports auto-configures an address for the ESP32, or when this address changes. The event means that everything is ready and the application can begin its tasks (e.g., creating sockets).
SYSTEM_EVENT_STA_LOST_IP doesn't arise immediately after the WiFi disconnects, instead it starts an IPV4 address lost timer, if the IPV4 address is got before ip lost timer expires, SYSTEM_EVENT_STA_LOST_IP doesn't happen. Otherwise, the event arises when IPV4 address lost timer expires.
Every time a station is connected to ESP32 AP, the <`SYSTEM_EVENT_AP_STACONNECTED`_> will arise. Upon receiving this event, the event task will do nothing, and the application callback can also ignore it. However, you may want to do something, for example, to get the info of the connected STA, etc.
When this event happens, the event task will do nothing, but the application event callback needs to do something, e.g., close the socket which is related to this station, etc.
- s1.1: The main task calls tcpip_adapter_init() to create an LwIP core task and initialize LwIP-related work.
- s1.2: The main task calls esp_event_loop_init() to create a system Event task and initialize an application event's callback function. In the scenario above, the application event's callback function does nothing but relaying the event to the application task.
- s1.3: The main task calls esp_wifi_init() to create the Wi-Fi driver task and initialize the Wi-Fi driver.
- s1.4: The main task calls OS API to create the application task.
Step 1.1~1.4 is a recommended sequence that initializes a Wi-Fi-/LwIP-based application. However, it is **NOT** a must-follow sequence, which means that you can create the application task in step 1.1 and put all other initializations in the application task. Moreover, you may not want to create the application task in the initialization phase if the application task depends on the sockets. Rather, you can defer the task creation until the IP is obtained.
2. Wi-Fi Configuration Phase
+++++++++++++++++++++++++++++++
Once the Wi-Fi driver is initialized, you can start configuring the Wi-Fi driver. In this scenario, the mode is Station, so you may need to call esp_wifi_set_mode(WIFI_MODE_STA) to configure the Wi-Fi mode as Station. You can call other esp_wifi_set_xxx APIs to configure more settings, such as the protocol mode, country code, bandwidth, etc. Refer to <`ESP32 Wi-Fi Configuration`_>.
Generally, we configure the Wi-Fi driver before setting up the Wi-Fi connection, but this is **NOT** mandatory, which means that you can configure the Wi-Fi connection anytime, provided that the Wi-Fi driver is initialized successfully. However, if the configuration does not need to change after the Wi-Fi connection is set up, you should configure the Wi-Fi driver at this stage, because the configuration APIs (such as esp_wifi_set_protocol) will cause the Wi-Fi to reconnect, which may not be desirable.
If the Wi-Fi NVS flash is enabled by menuconfig, all Wi-Fi configuration in this phase, or later phases, will be stored into flash. When the board powers on/reboots, you do not need to configure the Wi-Fi driver from scratch. You only need to call esp_wifi_get_xxx APIs to fetch the configuration stored in flash previously. You can also configure the Wi-Fi driver if the previous configuration is not what you want.
3. Wi-Fi Start Phase
++++++++++++++++++++++++++++++++
- s3.1: Call esp_wifi_start to start the Wi-Fi driver.
- s3.2: The Wi-Fi driver posts <`SYSTEM_EVENT_STA_START`_> to the event task; then, the event task will do some common things and will call the application event callback function.
- s3.3: The application event callback function relays the <`SYSTEM_EVENT_STA_START`_> to the application task. We recommend that you call esp_wifi_connect(). However, you can also call esp_wifi_connect() in other phrases after the <`SYSTEM_EVENT_STA_START`_> arises.
4. Wi-Fi Connect Phase
+++++++++++++++++++++++++++++++++
- s4.1: Once esp_wifi_connect() is called, the Wi-Fi driver will start the internal scan/connection process.
- s4.2: If the internal scan/connection process is successful, the <`SYSTEM_EVENT_STA_CONNECTED`_> will be generated. In the event task, it starts the DHCP client, which will finally trigger the DHCP process.
- s4.3: In the above-mentioned scenario, the application event callback will relay the event to the application task. Generally, the application needs to do nothing, and you can do whatever you want, e.g., print a log, etc.
In step 4.2, the Wi-Fi connection may fail because, for example, the password is wrong, the AP is not found, etc. In a case like this, <`SYSTEM_EVENT_STA_DISCONNECTED`_> will arise and the reason for such a failure will be provided. For handling events that disrupt Wi-Fi connection, please refer to phase 6.
5. Wi-Fi 'Got IP' Phase
+++++++++++++++++++++++++++++++++
- s5.1: Once the DHCP client is initialized in step 4.2, the *got IP* phase will begin.
- s5.2: If the IP address is successfully received from the DHCP server, then <`SYSTEM_EVENT_STA_GOT_IP`_> will arise and the event task will perform common handling.
- s5.3: In the application event callback, <`SYSTEM_EVENT_STA_GOT_IP`_> is relayed to the application task. For LwIP-based applications, this event is very special and means that everything is ready for the application to begin its tasks, e.g. creating the TCP/UDP socket, etc. A very common mistake is to initialize the socket before <`SYSTEM_EVENT_STA_GOT_IP`_> is received. **DO NOT start the socket-related work before the IP is received.**
6. Wi-Fi Disconnect Phase
+++++++++++++++++++++++++++++++++
- s6.1: When the Wi-Fi connection is disrupted, e.g. because the AP is powered off, the RSSI is poor, etc., <`SYSTEM_EVENT_STA_DISCONNECTED`_> will arise. This event may also arise in phase 3. Here, the event task will notify the LwIP task to clear/remove all UDP/TCP connections. Then, all application sockets will be in a wrong status. In other words, no socket can work properly when this event happens.
- s6.2: In the scenario described above, the application event callback function relays <`SYSTEM_EVENT_STA_DISCONNECTED`_> to the application task. We recommend that esp_wifi_connect() be called to reconnect the Wi-Fi, close all sockets and re-create them if necessary. Refer to <`SYSTEM_EVENT_STA_DISCONNECTED`_>.
The scan type and other per-scan attributes are configured by esp_wifi_scan_start. The table below provides a detailed description of wifi_scan_config_t.
The scenario above describes an all-channel, foreground scan. The foreground scan can only occur in Station mode where the station does not connect to any AP. Whether it is a foreground or background scan is totally determined by the Wi-Fi driver, and cannot be configured by the application.
- s1.2: Call esp_wifi_scan_start() to configure the scan. To do so, you can refer to <`Scan Configuration`_>. Since this is an all-channel scan, just set the SSID/BSSID/channel to 0.
Wi-Fi Driver's Internal Scan Phase
**************************************
- s2.1: The Wi-Fi driver switches to channel 1, in case the scan type is WIFI_SCAN_TYPE_ACTIVE, and broadcasts a probe request. Otherwise, the Wi-Fi will wait for a beacon from the APs. The Wi-Fi driver will stay in channel 1 for some time. The dwell time is configured in min/max time, with default value being 120 ms.
- s2.2: The Wi-Fi driver switches to channel 2 and performs the same operation as in step 2.1.
- s2.3: The Wi-Fi driver scans the last channel N, where N is determined by the country code which is configured in step 1.1.
Scan-Done Event Handling Phase
*********************************
- s3.1: When all channels are scanned, <`SYSTEM_EVENT_SCAN_DONE`_> will arise.
- s3.2: The application's event callback function notifies the application task that <`SYSTEM_EVENT_SCAN_DONE`_> is received. esp_wifi_scan_get_ap_num() is called to get the number of APs that have been found in this scan. Then, it allocates enough entries and calls esp_wifi_scan_get_ap_records() to get the AP records. Please note that the AP records in the Wi-Fi driver will be freed, once esp_wifi_scan_get_ap_records() is called. Do not call esp_wifi_scan_get_ap_records() twice for a single scan-done event. If esp_wifi_scan_get_ap_records() is not called when the scan-done event occurs, the AP records allocated by the Wi-Fi driver will not be freed. So, make sure you call esp_wifi_scan_get_ap_records(), yet only once.
The scenario above is an all-channel background scan. Compared to `Scan All APs In All Channels(foreground)`_ , the difference in the all-channel background scan is that the Wi-Fi driver will scan the back-to-home channel for 30 ms before it switches to the next channel to give the Wi-Fi connection a chance to transmit/receive data.
- s2.1~s2.N: Each time the Wi-Fi driver scans an AP, it will check whether it is a target AP or not. If the scan is WIFI_FAST_SCAN scan and the target AP is found, then the scan-done event will arise and scanning will end; otherwise, the scan will continue. Please note that the first scanned channel may not be channel 1, because the Wi-Fi driver optimizes the scanning sequence.
If there are multiple APs which match the target AP info, for example, if we happen to scan two APs whose SSID is "ap". If the scan is WIFI_FAST_SCAN, then only the first scanned "ap" will be found, if the scan is WIFI_ALL_CHANNEL_SCAN, both "ap" will be found and the station will connect the "ap" according to the configured strategy, refer to `Station Basic Configuration`_.
You can scan a specific AP, or all of them, in any given channel. These two scenarios are very similar.
Scan in Wi-Fi Connect
+++++++++++++++++++++++++
When esp_wifi_connect() is called, then the Wi-Fi driver will try to scan the configured AP first. The scan in "Wi-Fi Connect" is the same as `Scan for a Specific AP In All Channels`_, except that no scan-done event will be generated when the scan is completed. If the target AP is found, then the Wi-Fi driver will start the Wi-Fi connection; otherwise, <`SYSTEM_EVENT_STA_DISCONNECTED`_> will be generated. Refer to `Scan for a Specific AP in All Channels`_
Scan In Blocked Mode
++++++++++++++++++++
If the block parameter of esp_wifi_scan_start() is true, then the scan is a blocked one, and the application task will be blocked until the scan is done. The blocked scan is similar to an unblocked one, except that no scan-done event will arise when the blocked scan is completed.
Two application tasks may call esp_wifi_scan_start() at the same time, or the same application task calls esp_wifi_scan_start() before it gets a scan-done event. Both scenarios can happen. **However, the Wi-Fi driver does not support multiple concurrent scans adequately. As a result, concurrent scans should be avoided.** Support for concurrent scan will be enhanced in future releases, as the ESP32's Wi-Fi functionality improves continuously.
The esp_wifi_scan_start() fails immediately if the Wi-Fi is in connecting process because the connecting has higher priority than the scan. If scan fails because of connecting, the recommended strategy is to delay sometime and retry scan again, the scan will succeed once the connecting is completed.
However, the retry/delay strategy may not work all the time. Considering following scenario:
- The station is connecting a non-existed AP or if the station connects the existed AP with a wrong password, it always raises the event <`SYSTEM_EVENT_STA_DISCONNECTED`_>.
- The application call esp_wifi_connect() to do reconnection on receiving the disconnect event.
- Another application task, e.g. the console task, call esp_wifi_scan_start() to do scan, the scan always fails immediately because the station is keeping connecting.
- When scan fails, the application simply delay sometime and retry the scan.
In above scenario the scan will never succeed because the connecting is in process. So if the application supports similar scenario, it needs to implement a better reconnect strategy. E.g.
- The application can choose to define a maximum continuous reconnect counter, stop reconnect once the reconnect reaches the max counter.
- The application can choose to do reconnect immediately in the first N continous reconnect, then give a delay sometime and reconnect again.
The application can define its own reconnect strategy to avoid the scan starve to death. Refer to <`Wi-Fi Reconnect`_>.
This scenario only depicts the case when there is only one target AP are found in scan phase, for the scenario that more than one AP with the same SSID are found, refer to <`ESP32 Wi-Fi Station Connecting When Multiple APs Are Found`_>.
Generally, the application does not need to care about the connecting process. Below is a brief introduction to the process for those who are really interested.
- s1.1, The Wi-Fi driver begins scanning in "Wi-Fi Connect". Refer to <`Scan in Wi-Fi Connect`_> for more details.
- s1.2, If the scan fails to find the target AP, <`SYSTEM_EVENT_STA_DISCONNECTED`_> will arise and the reason-code will be WIFI_REASON_NO_AP_FOUND. Refer to <`Wi-Fi Reason Code`_>.
Auth Phase
+++++++++++++++++++++
- s2.1, The authentication request packet is sent and the auth timer is enabled.
- s2.2, If the authentication response packet is not received before the authentication timer times out, <`SYSTEM_EVENT_STA_DISCONNECTED`_> will arise and the reason-code will be WIFI_REASON_AUTH_EXPIRE. Refer to <`Wi-Fi Reason Code`_>.
- s2.3, The auth-response packet is received and the auth-timer is stopped.
- s2.4, The AP rejects authentication in the response and <`SYSTEM_EVENT_STA_DISCONNECTED`_> arises, while the reason-code is WIFI_REASON_AUTH_FAIL or the reasons specified by the AP. Refer to <`Wi-Fi Reason Code`_>.
- s3.1, The association request is sent and the association timer is enabled.
- s3.2, If the association response is not received before the association timer times out, <`SYSTEM_EVENT_STA_DISCONNECTED`_> will arise and the reason-code will be WIFI_REASON_ASSOC_EXPIRE. Refer to <`Wi-Fi Reason Code`_>.
- s3.3, The association response is received and the association timer is stopped.
- s3.4, The AP rejects the association in the response and <`SYSTEM_EVENT_STA_DISCONNECTED`_> arises, while the reason-code is the one specified in the association response. Refer to <`Wi-Fi Reason Code`_>.
Four-way Handshake Phase
++++++++++++++++++++++++++
- s4.1, The four-way handshake is sent out and the association timer is enabled.
- s4.2, If the association response is not received before the association timer times out, <`SYSTEM_EVENT_STA_DISCONNECTED`_> will arise and the reason-code will be WIFI_REASON_ASSOC_EXPIRE. Refer to <`Wi-Fi Reason Code`_>.
- s4.3, The association response is received and the association timer is stopped.
- s4.4, The AP rejects the association in the response and <`SYSTEM_EVENT_STA_DISCONNECTED`_> arises and the reason-code will be the one specified in the association response. Refer to <`Wi-Fi Reason Code`_>.
Wi-Fi Reason Code
+++++++++++++++++++++
The table below shows the reason-code defined in ESP32. The first column is the macro name defined in esp_wifi_types.h. The common prefix *WIFI_REASON* is removed, which means that *UNSPECIFIED* actually stands for *WIFI_REASON_UNSPECIFIED* and so on. The second column is the value of the reason. The third column is the standard value to which this reason is mapped in section 8.4.1.7 of ieee802.11-2012. (For more information, refer to the standard mentioned above.) The last column is a description of the reason.
This scenario is similar as <`ESP32 Wi-Fi Station Connecting Scenario`_>, the difference is the station will not raise the event <`SYSTEM_EVENT_STA_DISCONNECTED`_> unless it fails to connect all of the found APs.
Wi-Fi Reconnect
---------------------------
The station may disconnect due to many reasons, e.g. the connected AP is restarted etc. It's the application's responsibility to do the reconnect. The recommended reconnect strategy is to call esp_wifi_connect() on receiving event <`SYSTEM_EVENT_STA_DISCONNECTED`_>.
Sometimes the application needs more complex reconnect strategy:
- If the disconnect event is raised because the esp_wifi_disconnect() is called, the application may not want to do reconnect.
- If the esp_wifi_scan_start() may be called at anytime, a better reconnect strategy is necessary, refer to <`Scan When Wi-Fi Is Connecting`_>.
Another thing we need to consider is the reconnect may not connect the same AP if there are more than one APs with the same SSID. The reconnect always select current best APs to connect.
Wi-Fi Beacon Timeout
---------------------------
The beacon timeout mechanism is used by ESP32 station to detect whether the AP is alive or not. If the station continuously loses 60 beacons of the connected AP, the beacon timeout happens.
After the beacon timeout happens, the station sends 5 probe requests to AP, it disconnects the AP and raises the event <`SYSTEM_EVENT_STA_DISCONNECTED`_> if still no probe response or beacon is received from AP.
Long Range (LR) mode is an Espressif-patented Wi-Fi mode which can achieve a one-kilometer line of sight range. It has better reception sensitivity, stronger anti-interference ability and longer transmission distance than the traditional 802.11B mode.
Since LR is Espressif unique Wi-Fi mode, only ESP32 devices can transmit and receive the LR data. In other words, the ESP32 device should NOT transmit the data in LR data rate if the connected device doesn't support LR. The application can achieve this by configuring suitable Wi-Fi mode. If the negotiated mode supports LR, the ESP32 may transmit data in LR rate, otherwise, ESP32 will transmit all data in traditional Wi-Fi data rate.
Following table depicts the Wi-Fi mode negotiation:
In above table, the row is the Wi-Fi mode of AP and the column is the Wi-Fi mode of station. The "-" indicates Wi-Fi mode of the AP and station are not compatible.
According to the table, we can conclude that:
- For LR enabled in ESP32 AP, it's incompatible with traditional 802.11 mode because the beacon is sent in LR mode.
- For LR enabled in ESP32 station and the mode is NOT LR only mode, it's compatible with traditional 802.11 mode.
- If both station and AP are ESP32 devices and both of them enable LR mode, the negotiated mode supports LR.
If the negotiated Wi-Fi mode supports both traditional 802.11 mode and LR mode, it's the WiFi driver's responsibility to automatically select the best data rate in different Wi-Fi mode and the application don't need to care about it.
LR Impacts to Traditional Wi-Fi device
***************************************
The data transmission in LR rate has no impacts on the traditional Wi-Fi device because:
- The CCA and backoff process in LR mode are consistent with 802.11 specification.
- The traditional Wi-Fi device can detect the LR signal via CCA and do backoff.
In other words, the impact transmission in LR mode is similar as the impact in 802.11B mode.
LR Transmission Distance
*************************
The reception sensitivity of LR has about 4 dB gain than the traditional 802.11 B mode, theoretically the transmission distance is about 2 to 2.5 times the distance of 11B.
The default country info is {.cc="CN", .schan=1, .nchan=13, policy=WIFI_COUNTRY_POLICY_AUTO}, if the WiFi Mode is station/AP coexist mode, they share the same configured country info. Sometimes, the country info of AP, to which the station is connected, is different from the country info of configured. For example, the configured station has country info {.cc="JP", .schan=1, .nchan=14, policy=WIFI_COUNTRY_POLICY_AUTO}, but the connected AP has country info {.cc="CN", .schan=1, .nchan=13}, then country info of connected AP's is used.
Following table depicts which country info is used in different WiFi Mode and different country policy, also describe the impact to active scan.
In AP mode, the home channel is defined as that of the AP channel. In Station mode, the home channel is defined as the channel of the AP to which the station is connected. In Station+AP mode, the home channel of AP and station must be the same. If the home channels of Station and AP are different, the station's home channel is always in priority. Take the following as an example: at the beginning, the AP is on channel 6, then the station connects to an AP whose channel is 9. Since the station's home channel has a higher priority, the AP needs to switch its channel from 6 to 9 to make sure that both station and AP have the same home channel.
By default, all Wi-Fi management frames are processed by the Wi-Fi driver, and the application does not need to care about them. Some applications, however, may have to handle the beacon, probe request, probe response and other management frames. For example, if you insert some vendor-specific IE into the management frames, it is only the management frames which contain this vendor-specific IE that will be processed. In ESP32, esp_wifi_set_vendor_ie() and esp_wifi_set_vendor_ie_cb() are responsible for this kind of tasks.
Currently, ESP32 Wi-Fi supports the Modem-sleep mode which refers to the legacy power-saving mode in the IEEE 802.11 protocol. Modem-sleep mode works in Station-only mode and the station must connect to the AP first. If the Modem-sleep mode is enabled, station will switch between active and sleep state periodically. In sleep state, RF, PHY and BB are turned off in order to reduce power consumption. Station can keep connection with AP in modem-sleep mode.
Modem-sleep mode includes minimum and maximum power save modes. In minimum power save mode, station wakes up every DTIM to receive beacon. Broadcast data will not be lost because it is transmitted after DTIM. However, it can not save much more power if DTIM is short for DTIM is determined by AP.
In maximum power save mode, station wakes up every listen interval to receive beacon. This listen interval can be set longer than the AP DTIM period. Broadcast data may be lost because station may be in sleep state at DTIM time. If listen interval is longer, more power is saved but broadcast data is more easy to lose. Listen interval can be configured by calling API :cpp:func:`esp_wifi_set_config` before connecting to AP.
Call ``esp_wifi_set_ps(WIFI_PS_MIN_MODEM)`` to enable Modem-sleep minimum power save mode or ``esp_wifi_set_ps(WIFI_PS_MAX_MODEM)`` to enable Modem-sleep maximum power save mode after calling :cpp:func:`esp_wifi_init`. When station connects to AP, Modem-sleep will start. When station disconnects from AP, Modem-sleep will stop.
Call `esp_wifi_set_ps(WIFI_PS_MIN_MODEM)` to disable modem sleep entirely. This has much higher power consumption, but provides minimum latency for receiving Wi-Fi data in real time. When modem sleep is enabled, received Wi-Fi data can be delayed for as long as the DTIM period (minimum power save mode) or the listen interval (maximum power save mode).
The default Modem-sleep mode is WIFI_PS_MIN_MODEM.
AP Sleep
+++++++++++++++++++++++++++++++
Currently ESP32 AP doesn't support all of the power save feature defined in Wi-Fi specification. To be specific, the AP only caches unicast data for the stations connect to this AP, but doesn't cache the multicast data for the stations. If stations connected to the ESP32 AP are power save enabled, they may experience multicast packet loss.
In future, all power save features will be supported on ESP32 AP.
2. The optimized functions is in the folder components/wpa_supplicant/src/fast_crypto, these function used the hardware crypto to make it faster than origin one, the type of function's name add `fast_` to distinguish with the original one. For example, the API aes_wrap() is used to encrypt frame information when do 4 way handshake, the fast_aes_wrap() has the same result but can be faster.
Two groups of crypto function can be used when register in the wpa_crypto_funcs_t, wpa2_crypto_funcs_t and wps_crypto_funcs_t structure, also we have given the recommend functions to register in the
fast_crypto_ops.c, you can register the function as the way you need, however what should make action is that the crypto_hash_xxx function and crypto_cipher_xxx function need to register with the same function to operation. For example, if you register crypto_hash_init() function to initialize the esp_crypto_hash structure, you need use the crypto_hash_update() and crypto_hash_finish() function to finish the operation, rather than fast_crypto_hash_update() or fast_crypto_hash_finish().
- Either esp_wifi_set_promiscuous(true), or esp_wifi_start(), or both of these APIs return ESP_OK. This is because we need to make sure that Wi-Fi hardware is initialized before esp_wifi_80211_tx() is called. In ESP32, both esp_wifi_set_promiscuous(true) and esp_wifi_start() can trigger the initialization of Wi-Fi hardware.
- If there is WiFi connection and the packet is from station to AP or from AP to station, the data rate is same as the WiFi connection. Otherwise the data rate is 1Mbps.
Theoretically, if we do not consider the side-effects the API imposes on the Wi-Fi driver or other stations/APs, we can send a raw 802.11 packet over the air, with any destination MAC, any source MAC, any BSSID, or any other type of packet. However,robust/useful applications should avoid such side-effects. The table below provides some tips/recommendations on how to avoid the side-effects of esp_wifi_80211_tx in different scenarios.
The Wi-Fi sniffer mode can be enabled by esp_wifi_set_promiscuous(). If the sniffer mode is enabled, the following packets **can** be dumped to the application:
- 802.11 Management frame
- 802.11 Data frame, including MPDU, AMPDU, AMSDU, etc.
- 802.11 MIMO frame, for MIMO frame, the sniffer only dumps the length of the frame.
For frames that the sniffer **can** dump, the application can additionally decide which specific type of packets can be filtered to the application by using esp_wifi_set_promiscuous_filter() and esp_wifi_set_promiscuous_ctrl_filter(). By default, it will filter all 802.11 data and management frames to the application.
The Wi-Fi sniffer mode can be enabled in the Wi-Fi mode of WIFI_MODE_NULL, or WIFI_MODE_STA, or WIFI_MODE_AP, or WIFI_MODE_APSTA. In other words, the sniffer mode is active when the station is connected to the AP, or when the AP has a Wi-Fi connection. Please note that the sniffer has a **great impact** on the throughput of the station or AP Wi-Fi connection. Generally, we should **NOT** enable the sniffer, when the station/AP Wi-Fi connection experiences heavy traffic unless we have special reasons.
Another noteworthy issue about the sniffer is the callback wifi_promiscuous_cb_t. The callback will be called directly in the Wi-Fi driver task, so if the application has a lot of work to do for each filtered packet, the recommendation is to post an event to the application task in the callback and defer the real work to the application task.
ESP32 supports up to sixteen antennas through external antenna switch. The antenna switch can be controlled by up to four address pins - antenna_select[0:3]. Different input value of antenna_select[0:3] means selecting different antenna. E.g. the value '0b1011' means the antenna 11 is selected. The default value of antenna_select[3:0] is '0b0000', it means the antenna 0 is selected by default.
Up to four GPIOs are connected to the four active high antenna_select pins. ESP32 can select the antenna by control the GPIO[0:3]. The API :cpp:func:`esp_wifi_set_ant_gpio()` is used to configure which GPIOs are connected to antenna_selects. If GPIO[x] is connected to antenna_select[x], then gpio_config->gpio_cfg[x].gpio_select should be set to 1 and gpio_config->gpio_cfg[x].gpio_num should be provided.
Although up to sixteen anteenas are supported, only one or two antennas can be simultaneously enabled for RX/TX. The API :cpp:func:`esp_wifi_set_ant()` is used to configure which antennas are enabled.
The enabled antennas selecting algorithm is also configured by :cpp:func:`esp_wifi_set_ant()`. The RX/TX antenna mode can be WIFI_ANT_MODE_ANT0, WIFI_ANT_MODE_ANT1 or WIFI_ANT_MODE_AUTO. If the antenna mode is WIFI_ANT_MODE_ANT0, the enabled antenna 0 is selected for RX/TX data. If the antenna mode is WIFI_ANT_MODE_ANT1, the enabled antenna 1 is selected for RX/TX data. Otherwise, WiFi automatically selects the antenna that has better signal from the enabled antennas.
If the RX antenna mode is WIFI_ANT_MODE_AUTO, the default antenna mode also needs to be set. Because the RX antenna switching only happens when some conditions are met, e.g. the RX antenna starts to switch if the RSSI is lower than -65dBm and if another antenna has better signal etc, RX uses the default antenna if the conditions are not met. If the default antenna mode is WIFI_ANT_MODE_ANT1, the enabled antenna 1 is used as the default RX antenna, otherwise the enabled antenna 0 is used as the default RX antenna.
Some limitations need to be considered:
- The TX antenna can be set to WIFI_ANT_MODE_AUTO only if the RX antenna mode is WIFI_ANT_MODE_AUTO because TX antenna selecting algorithm is based on RX antenna in WIFI_ANT_MODE_AUTO type.
- Currently BT doesn't support the multiple antennas feature, please don't use multiple antennas related APIs.
Following is the recommended scenarios to use the multiple antennas:
- In Wi-Fi mode WIFI_MODE_STA, both RX/TX antenna modes are configured to WIFI_ANT_MODE_AUTO. The WiFi driver selects the better RX/TX antenna automatically.
- The RX antenna mode is configured to WIFI_ANT_MODE_AUTO. The TX antenna mode is configured to WIFI_ANT_MODE_ANT0 or WIFI_ANT_MODE_ANT1. The applications can choose to always select a specified antenna for TX, or implement their own TX antenna selecting algorithm, e.g. selecting the TX antenna mode based on the channel switch information etc.
- Both RX/TX antenna modes are configured to WIFI_ANT_MODE_ANT0 or WIFI_ANT_MODE_ANT1.
Wi-Fi Multiple Antennas Configuration
+++++++++++++++++++++++++++++++++++++
Generally, following steps can be taken to configure the multiple antennas:
- Configure which GPIOs are connected to the antenna_selects, for example, if four antennas are supported and GPIO20/GPIO21 are connected to antenna_select[0]/antenna_select[1], the configurations look like::
wifi_ant_gpio_config_t config = {
{ .gpio_select = 1, .gpio_num = 20 },
{ .gpio_select = 1, .gpio_num = 21 }
};
- Configure which antennas are enabled and how RX/TX use the enabled antennas, for example, if antenna1 and antenna3 are enabled, the RX needs to select the better antenna automatically and uses antenna1 as its default antenna, the TX always selects the antenna3. The configuration looks like::
Channel state information (CSI) refers to the channel information of a Wi-Fi connection. In ESP32, this information consists of channel frequency responses of sub-carriers and is estimated when packets are received from the transmitter. Each channel frequency response of sub-carrier is recorded by two bytes of signed characters. The first one is imaginary part and the second one is real part. There are up to three fields of channel frequency responses according to the type of received packet. They are legacy long training field (LLTF), high throughput LTF (HT-LTF) and space time block code HT-LTF (STBC-HT-LTF). For different types of packets which are received on channels with different state, the sub-carrier index and total bytes of signed characters of CSI is shown in the following table.
- The CSI data corresponding to each Long Training Field(LTF) type is stored in a buffer starting from the buf field. Each item is stored as two bytes: imaginary part followed by real part. The order of each item is the same as the sub-carrier in the table. The order of LTF is: LLTF, HT-LTF, STBC-HT-LTF. However all 3 LTFs may not be present, depending on the channel and packet information (see above).
- If first_word_invalid field of wifi_csi_info_t is true, it means that the first four bytes of CSI data is invalid due to a hardware limitation in ESP32.
- More information like RSSI, noise floor of RF, receiving time and antenna is in the rx_ctrl field.
..note::
- For STBC packet, CSI is provided for every space-time stream without CSD (cyclic shift delay). As each cyclic shift on the additional chains shall be -200ns, only the CSD angle of first space-time stream is recorded in sub-carrier 0 of HT-LTF and STBC-HT-LTF for there is no channel frequency response in sub-carrier 0. CSD[10:0] is 11 bits, ranging from -pi to pi.
- If LLTF, HT-LTF or STBC-HT-LTF is not enabled by calling API :cpp:func:`esp_wifi_set_csi_config`, the total bytes of CSI data will be fewer than that in the table. For example, if LLTF and HT-LTF is not enabled and STBC-HT-LTF is enabled, when a packet is received with the condition above/HT/40MHz/STBC, the total bytes of CSI data is 244 ((61 + 60) * 2 + 2 = 244, the result is aligned to four bytes and the last two bytes is invalid).
To use Wi-Fi CSI, the following steps need to be done.
- Select Wi-Fi CSI in menuconfig. It is "Menuconfig --> Components config --> Wi-Fi --> WiFi CSI(Channel State Information)".
- Set CSI receiving callback function by calling API :cpp:func:`esp_wifi_set_csi_rx_cb`.
- Configure CSI by calling API :cpp:func:`esp_wifi_set_csi_config`.
- Enable CSI by calling API :cpp:func:`esp_wifi_set_csi`.
The CSI receiving callback function runs from Wi-Fi task. So, do not do lengthy operations in the callback function. Instead, post necessary data to a queue and handle it from a lower priority task. Because station does not receive any packet when it is disconnected and only receives packets from AP when it is connected, it is suggested to enable sniffer mode to receive more CSI data by calling :cpp:func:`esp_wifi_set_promiscuous`.
ESP32 supports Wi-Fi bandwidth HT20 or HT40, it doesn't support HT20/40 coexist. `esp_wifi_set_bandwidth` can be used to change the default bandwidth of station or AP. The default bandwidth for ESP32 station and AP is HT40.
In station mode, the actual bandwidth is firstly negotiated during the Wi-Fi connection. It is HT40 only if both the station and the connected AP support HT40, otherwise it's HT20. If the bandwidth of connected AP is changes, the actual bandwidth is negotiated again without Wi-Fi disconnecting.
Similarly, in AP mode, the actual bandwidth is negotiated between AP and the stations that connect to the AP. It's HT40 only if the AP and all the stations support HT40, otherwise it's HT40.
In station/AP coexist mode, the station/AP can configure HT20/40 seperately. If both station and AP are negotiated to HT40, the HT40 channel should be the channel of station because the station always has higher priority than AP in ESP32. E.g. the configured bandwidth of AP is HT40, the configured primary channel is 6 and the configured secondary channel is 10. The station is connected to an router whose primary channel is 6 and secondary channel is 2, then the actual channel of AP is changed to primary 6 and secondary 2 automatically.
Theoretically the HT40 can gain better throughput because the maximum raw physicial (PHY) data rate for HT40 is 150Mbps while it's 72Mbps for HT20. However, if the device is used in some special environment, e.g. there are too many other Wi-Fi devices around the ESP32 device, the performance of HT40 may be degraded. So if the applications need to support same or similar scenarios, it's recommended that the bandwidth is always configured to HT20.
Wi-Fi QoS
-------------------------
ESP32 supports all the mandatory features required in WFA Wi-Fi QoS Certification.
Four ACs(Access Category) are defined in Wi-Fi specification, each AC has a its own priority to access the Wi-Fi channel. Moreover a map rule is defined to map the QoS priority of other protocol, such as 802.11D or TCP/IP precedence to Wi-Fi AC.
Below is a table describes how the IP Precedences are mapped to Wi-Fi ACs in ESP32, it also indicates whether the AMPDU is supported for this AC. The table is sorted with priority descending order, namely, the AC_VO has highest priority.
The application can make use of the QoS feature by configuring the IP precedence via socket option IP_TOS. Here is an example to make the socket to use VI queue::
const int ip_precedence_vi = 4;
const int ip_precedence_offset = 5;
int priority = (ip_precedence_vi << ip_precedence_offset);
Theoretically the higher priority AC has better performance than the low priority AC, however, it's not always be true, here are some suggestions about how to use the Wi-Fi QoS:
- For some really important application traffic, can put it into AC_VO queue. Avoid sending big traffic via AC_VO queue. On one hand, the AC_VO queue doesn't support AMPDU and it can't get better performance than other queue if the traffic is big, on the other hand, it may impact the the management frames that also use AC_VO queue.
- Avoid using more than two different AMPDU supported precedences, e.g. socket A uses precedence 0, socket B uses precedence 1, socket C uses precedence 2, this is a bad design because it may need much more memory. To be detailed, the Wi-Fi driver may generate a Block Ack session for each precedence and it needs more memory if the Block Ack session is setup.
Wi-Fi AMSDU
-------------------------
ESP32 supports receiving AMSDU but doesn't support transmitting AMSDU. The transmitting AMSDU is not necessary since ESP32 has transmitting AMPDU.
Wi-Fi Fragment
-------------------------
ESP32 supports Wi-Fi receiving fragment, but doesn't support Wi-Fi transmitting fragment. The Wi-Fi transmitting fragment will be supported in future release.
WPS Enrolle
-------------------------
ESP32 supports WPS enrollee feature in Wi-Fi mode WIFI_MODE_STA or WIFI_MODE_APSTA. Currently ESP32 supports WPS enrollee type PBC and PIN.
This section is only about the dynamic buffer configuration.
Why Buffer Configuration Is Important
+++++++++++++++++++++++++++++++++++++++
In order to get a robust, high-performance system, we need to consider the memory usage/configuration very carefully, because:
- the available memory in ESP32 is limited.
- currently, the default type of buffer in LwIP and Wi-Fi drivers is "dynamic", **which means that both the LwIP and Wi-Fi share memory with the application**. Programmers should always keep this in mind; otherwise, they will face a memory issue, such as "running out of heap memory".
- it is very dangerous to run out of heap memory, as this will cause ESP32 an "undefined behavior". Thus, enough heap memory should be reserved for the application, so that it never runs out of it.
- the peak heap memory that the ESP32 LwIP/Wi-Fi may consume depends on a number of factors, such as the maximum TCP/UDP connections that the application may have, etc.
- the total memory that the application requires is also an important factor when considering memory configuration.
Due to these reasons, there is not a good-for-all application configuration. Rather, we have to consider memory configurations separately for every different application.
Dynamic vs. Static Buffer
++++++++++++++++++++++++++++++
The default type of buffer in LwIP and Wi-Fi drivers is "dynamic". Most of the time the dynamic buffer can significantly save memory. However, it makes the application programming a little more difficult, because in this case the application needs to consider memory usage in LwIP/Wi-Fi.
Peak LwIP Dynamic Buffer
++++++++++++++++++++++++++++++
The default type of LwIP buffer is "dynamic", and this section considers the dynamic buffer only.
The peak heap memory that LwIP consumes is the **theoretically-maximum memory** that the LwIP driver consumes. Generally, the peak heap memory that the LwIP consumes depends on:
- the memory required to create a UDP connection: lwip_udp_conn
- the memory required to create a TCP connection: lwip_tcp_conn
- the number of UDP connections that the application has: lwip_udp_con_num
- the number of TCP connections that the application has: lwip_tcp_con_num
- the TCP TX window size: lwip_tcp_tx_win_size
- the TCP RX window size: lwip_tcp_rx_win_size
**So, the peak heap memory that the LwIP consumes can be calculated with the following formula:**
Some TCP-based applications need only one TCP connection. However, they may choose to close this TCP connection and create a new one when an error (such as a sending failure) occurs. This may result in multiple TCP connections existing in the system simultaneously, because it may take a long time for a TCP connection to close, according to the TCP state machine (refer to RFC793).
Peak Wi-Fi Dynamic Buffer
++++++++++++++++++++++++++++++
The Wi-Fi driver supports several types of buffer (refer to `Wi-Fi Buffer Configure`_). However, this section is about the usage of the dynamic Wi-Fi buffer only.
The peak heap memory that Wi-Fi consumes is the **theoretically-maximum memory** that the Wi-Fi driver consumes. Generally, the peak memory depends on:
- the number of dynamic rx buffers that are configured: wifi_rx_dynamic_buf_num
- the number of dynamic tx buffers that are configured: wifi_tx_dynamic_buf_num
- the maximum packet size that the Wi-Fi driver can receive: wifi_rx_pkt_size_max
- the maximum packet size that the Wi-Fi driver can send: wifi_tx_pkt_size_max
So, the peak memory that the Wi-Fi driver consumes can be calculated with the following formula:
Generally, we do not need to care about the dynamic tx long buffers and dynamic tx long long buffers, because they are management frames which only have a small impact on the system.
If you are going to modify the default number or type of buffer, it would be helpful to also have an overview of how the buffer is allocated/freed in the data path. The following diagram shows this process in the TX direction:
- The application allocates the data which needs to be sent out.
- The application calls TCPIP-/Socket-related APIs to send the user data. These APIs will allocate a PBUF used in LwIP, and make a copy of the user data.
- When LwIP calls a Wi-Fi API to send the PBUF, the Wi-Fi API will allocate a "Dynamic Tx Buffer" or "Static Tx Buffer", make a copy of the LwIP PBUF, and finally send the data.
If the Wi-Fi NVS flash is enabled, all Wi-Fi configurations set via the Wi-Fi APIs will be stored into flash, and the Wi-Fi driver will start up with these configurations next time it powers on/reboots. However, the application can choose to disable the Wi-Fi NVS flash if it does not need to store the configurations into persistent memory, or has its own persistent storage, or simply due to debugging reasons, etc.
