ESP32-S3 WiFi in STA Mode

“ This article focuses on the Wi-Fi architecture and TCP/IP protocol implementation of the ESP32-S3. It analyzes the core modules integrated into its 40nm chip, including the RF front end and baseband, details the STA/AP working modes, dissects the STA connection process and data path, and compares the OSI and TCP/IP protocol stack implementations.”

ESP32-S3 WiFi in STA Mode

01

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ESP32-S3 Chip-Level Wi-Fi Architecture

The ESP32-S3 integrates a complete 2.4 GHz Wi-Fi subsystem on a single 40 nm CMOS wafer. The functional partitions are as follows:

RF Front End:

On-chip power amplifier (+20 dBm@11b), low-noise amplifier, transceiver switch, and balun, requiring only an external 50 Ω antenna or a printed inverted F antenna.

Baseband Processor:

Hardware implementation of IEEE 802.11 b/g/n physical layer, supporting 20 MHz and 40 MHz channels, with a maximum PHY rate of 150 Mbit/s (HT40 MCS7).

Media Access Controller (MAC):

Hardware execution of DCF, EDCA, Block-ACK, A-MPDU/A-MSDU, and immediate retransmission, reducing CPU usage.

Security Engine:

Independent AES-128/256, GCMP, CCMP, TKIP accelerators, supporting WPA3-SAE, TLS1.3, WAPI, and WPS, with no CPU intervention required during the handshake process.

Network Interface:

Up to four virtual network interfaces (netif), sharing the same RF through time division multiplexing.

Resources:

Internal 64 kB TX/RX DMA circular buffer, CPU Cache can be dynamically configured; a system clock of 40 MHz crystal is sufficient. Peripherals only require antenna matching network, π-type filter, and power supply.

ESP32-S3 WiFi in STA Mode

02

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Wi-Fi Operating Modes

Mode

Function Description

Typical Applications

STA

Client, associates with external access point

Sensors, gateways

Soft-AP

Ad-hoc network, up to 10 STAs can associate

One-click network configuration, local monitoring, hotspot

STA+AP

Acts as both STA and Soft-AP

Relay, transparent gateway

Sniffer

Only receives 802.11 frames, does not send association frames

Protocol analysis

Promiscuous

Promiscuous listening, can capture all MAC frames

RSSI ranging, positioning

03

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TCP/IP Protocol Stack

The ESP32-S3’s protocol stack is an open-source TCP/IP embedded network protocol stack implemented using the lwIP library. The TCP/IP protocol stack does not completely correspond to the traditional OSI model. The TCP/IP protocol stack is a simplified model that emphasizes the actual protocol implementations and the real operation of the Internet. In contrast, the OSI model is more comprehensive and idealized, providing a framework to describe the interactions between different systems. The following diagram compares the layered architecture of the IOS protocol stack with the TCP/IP protocol stack.

ESP32-S3 WiFi in STA Mode

OSI Layer

TCP/IP Layer

ESP32-S3 Implementation

Key Resources

7 Application Layer

Application Layer

LwIP Socket + mbedTLS + User Protocol Library

Software

6 Presentation Layer

Application Layer

mbedTLS Encryption, Compression

Hardware AES

5 Session Layer

Application Layer

LwIP PCB, Sockets

Software

4 Transport Layer

Transport Layer

LwIP TCP/UDP

Software

3 Network Layer

Network Layer

LwIP IPv4/IPv6, ARP/DHCP/ICMP

Software

2 Data Link Layer

Network Interface Layer

Wi-Fi MAC / Ethernet MAC

Hardware

1 Physical Layer

Network Interface Layer

2.4 GHz PHY / RMII PHY

Hardware + Antenna

04

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STA Mode Configuration Process

Enable Process:：

  • Initialization: esp_netif_init() → esp_event_loop_create_default() → esp_wifi_init().

  • Set Mode: esp_wifi_set_mode(WIFI_MODE_STA).

  • Configure Parameters: Fill in SSID, password, channel, and encryption method in the wifi_config_t structure.

  • Start RF: esp_wifi_start().

  • Trigger Connection: esp_wifi_connect().

  • Event Feedback: The kernel delivers WIFI_EVENT_STA_CONNECTED and IP_EVENT_STA_GOT_IP in the default event loop.

Connection Mechanism:：

  • Probe Phase: STA sends Probe Request in the 2.4 GHz band, AP replies with Probe Response.

  • Authentication Phase: Uses Open System or SAE/WPA2-PSK four-way handshake.

  • Association Phase: Association Request/Response exchange, establishing AID.

  • DHCP Phase: Obtains IPv4 address via DHCP (or static IP).

Data Path:：

  • TX: Application layer data → LwIP TCP/IP → MAC layer → Baseband modulation → RF transmission.

  • RX: RF demodulation → Baseband → MAC filtering → LwIP → Socket.

  • Modem-sleep: DTIM cycle wake-up, average current 15–20 mA.

  • Light-sleep: CPU and RF short power-off, average current 2–5 mA.

  • Deep-sleep: Only RTC runs, current <10 µA, can be awakened by timer or GPIO.

05

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Program Design

1.Global Initialization

  • Before any Wi-Fi action, three things must be completed:

  • nvs_flash_init()—Open non-volatile storage to save Wi-Fi calibration data and last connection configuration;

  • esp_netif_init()—Create LwIP virtual network card framework, laying the foundation for subsequent IP, DHCP, DNS;

  • esp_event_loop_create_default()—Establish system event post office, where Wi-Fi driver will deliver WIFI_EVENT and IP_EVENT.

2.Create STA Network Card Instance

Call <span>esp_netif_create_default_wifi_sta()</span> to return an <span>esp_netif_t *</span> handle, on which all subsequent IP layer operations (DHCP, DNS, static address) will depend. This function internally binds the network card to the Wi-Fi driver, requiring no additional setup.

3.Initialize Wi-Fi Driver

wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();esp_wifi_init(&amp;cfg);

Load the internal Wi-Fi firmware into RAM and start a driver task with priority 23 and stack size of 6 kB. The cfg contains default values such as country code and DMA buffer size, which can be directly generated using macros.

4.Set Working Mode

esp<span>_wifi_set_mode(WIFI_MODE_STA)</span> fixes the RF to the “station” role. Other optional values include <span>WIFI_MODE_AP</span>, <span>WIFI_MODE_APSTA</span>, <span>WIFI_MODE_NULL</span>, and once set, all subsequent configurations must follow the structure type corresponding to that role.

5.Fill in Connection Parameters

wifi_config_t wifi_config = {    .sta = {        .ssid     = "YOUR_SSID",        .password = "YOUR_PASSWORD",        .scan_method = WIFI_ALL_CHANNEL_SCAN,        .sort_method = WIFI_CONNECT_AP_BY_SIGNAL,        .threshold   = { .rssi = -127 },    },};esp_wifi_set_config(WIFI_IF_STA, &amp;wifi_config);

SSID and password can be up to 32 bytes; other fields filled with 0 indicate automatic selection of channel and security policy. To force connection to a specified MAC, additionally set .bssid_set = 1 and fill in .bssid[6].

6.Start RF and Trigger Connection

First <span>esp_wifi_start()</span> opens the RF, then <span>esp_wifi_connect()</span> initiates the 802.11 association and four-way handshake. These two steps can be executed consecutively; if you want to scan first and then connect, you can place <span>esp_wifi_connect()</span> after <span>esp_wifi_scan_start()</span><span><span>.</span></span>

7.Wait for Events and Obtain IP

Register in the event loop

esp_event_handler_register(WIFI_EVENT, ESP_EVENT_ANY_ID, event_handler, NULL);esp_event_handler_register(IP_EVENT, IP_EVENT_STA_GOT_IP, event_handler, NULL);

Wi-Fi Configuration Program:

static void wifi_scan_task(void *pvParameters){    /* 1. Initialize network interface layer */    ESP_ERROR_CHECK(esp_netif_init());    /* 2. Create default event loop (for Wi-Fi driver to deliver events) */    ESP_ERROR_CHECK(esp_event_loop_create_default());    /* 3. Create default STA network card */    esp_netif_create_default_wifi_sta();    /* 4. Initialize Wi-Fi driver */    wifi_init_config_t cfg = WIFI_INIT_CONFIG_DEFAULT();    ESP_ERROR_CHECK(esp_wifi_init(&amp;cfg));    /* 5. Set to STA mode */    ESP_ERROR_CHECK(esp_wifi_set_mode(WIFI_MODE_STA));    /* 6. Start RF */    ESP_ERROR_CHECK(esp_wifi_start());    /* 7. Scan parameters: NULL means default full channel scan, true means blocking until complete */    ESP_ERROR_CHECK(esp_wifi_scan_start(NULL, true));    /* 8. Get results */    uint16_t number = DEFAULT_SCAN_LIST_SIZE;    wifi_ap_record_t ap_info[DEFAULT_SCAN_LIST_SIZE];    uint16_t ap_count = 0;    memset(ap_info, 0, sizeof(ap_info));    ESP_ERROR_CHECK(esp_wifi_scan_get_ap_records(&amp;number, ap_info));    ESP_ERROR_CHECK(esp_wifi_scan_get_ap_num(&amp;ap_count));    ESP_LOGI(TAG, "Total APs scanned = %u", ap_count);    for (int i = 0; i &lt; number &amp;&amp; i &lt; ap_count; i++) {        ESP_LOGI(TAG, "SSID        %s", ap_info[i].ssid);        ESP_LOGI(TAG, "RSSI        %d", ap_info[i].rssi);        print_auth_mode(ap_info[i].authmode);        if (ap_info[i].authmode != WIFI_AUTH_WEP) {            print_cipher_type(ap_info[i].pairwise_cipher, ap_info[i].group_cipher);        }        ESP_LOGI(TAG, "Channel     %d\n", ap_info[i].primary);    }    /* Task ends, delete itself */    vTaskDelete(NULL);}

Main Program Flowchart:

ESP32-S3 WiFi in STA Mode

This article’s case uses the following hardware and software:

1. SOC Model: ESP32-S3-N16R8;

2. Software Development Environment: ESP-IDF 5.3.1, VSCode IDE (VSCodeUserSetup-x64-1.102.1 version);

3. ESP32 Project Version: IDF version (FreeRTOS);

4. Program Download: Type C USB (USB to Serial) interface;

5. The hardware and software case in this article is for personal learning only and not for commercial use.

References for this article:

“ESP32-S3 Technical Reference Manual Version 1.7”;

“ESP32-S3 Series Chip Technical Specification Version 2.0”.

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