Hey Oscar, great question—this is a super common scenario when moving from WiFi to cellular on ESP32, and luckily there are solid solutions that let you reuse most of your existing RTOS-based code without a full rewrite. Let’s break this down step by step:

1. Replacing WiFi with 3/4G: The Core Concept #

Instead of treating the cellular module as a "serial-to-Ethernet adapter" you need to wrap, the right approach is to use PPP (Point-to-Point Protocol) over Serial to integrate the module directly into ESP32’s existing TCP/IP stack (lwIP). Here’s how it works:

• Your 3/4G module acts as a cellular modem that establishes a PPP connection to the carrier’s network via AT commands.
• ESP32’s lwIP stack recognizes this PPP link as a standard network interface—just like its built-in WiFi. This means your existing web server and web client code can work almost unchanged, since they use the same socket APIs.

Step-by-Step Implementation: #

• First, configure the cellular module via AT commands (exact commands vary by module, check your docs):
  • Attach to the cellular network: AT+CGATT=1
  • Set your carrier’s APN: AT+CGDCONT=1,"IP","your-carrier-apn"
  • Initiate a PPP connection: AT+CIICR
  • Verify connection and get assigned IP: AT+CIFSR
• On ESP32, enable the PPPOS (PPP over Serial) driver:
  • If using ESP-IDF, the pppos component is built-in. Create a PPPOS task to handle serial communication with the module, and lwIP will automatically register this as a network interface.
  • If using Arduino, libraries like TinyGSM or ESP32-PPPOS wrap this logic into easy-to-use functions, supporting most popular modules (SIM7600, BG96, etc.).

2. Do You Need a "Serial-to-Ethernet" Library? #

Short answer: No, you don’t need a dedicated serial-to-Ethernet library. The PPPOS driver (either built into ESP-IDF or via Arduino libraries) does exactly this—it translates serial PPP frames into network packets that ESP32’s TCP/IP stack can understand. This is far more efficient than building a custom layer, since it leverages the standard PPP protocol supported by almost all cellular modems.

3. Running WiFi and Cellular Simultaneously, or Switching Between Them #

Both options are totally feasible with ESP32’s flexible lwIP stack:

Simultaneous Operation #

ESP32 supports multiple network interfaces at once (WiFi STA/AP + PPP). You can:

• Bind your web server to the WiFi AP interface so local devices can access it via WiFi, while your web client uses the PPP cellular interface to upload sensor data to the cloud.
• Configure lwIP’s routing table to prioritize traffic: send local LAN traffic to WiFi, and internet-bound traffic to the cellular interface. Most frameworks handle this automatically, but you can tweak routes manually if needed.

Switching Between Modes #

To toggle based on coverage:

• Add a WiFi signal strength check (use WiFi.RSSI() in Arduino or esp_wifi_sta_get_rssi() in ESP-IDF).
• If WiFi signal drops below a threshold, gracefully shut down the WiFi interface (close sockets, disable AP/STA), then initialize the PPP cellular connection.
• When WiFi is restored, reverse the process: disconnect the cellular modem, re-enable WiFi, and re-bind your server/client to the WiFi interface.
• Always clean up resources (close sockets, free serial ports) during switching to avoid crashes.

Practical Tips #

• Module Selection: Pick a module with good PPP support and TTL-level serial (avoids RS232-TTL converters). SIM7600 (4G LTE) is a great choice—it’s widely used, well-documented, and affordable.
• Serial Stability: Use ESP32’s hardware serial (UART0/UART1) instead of software serial for better reliability. Match the baud rate to your module’s default (usually 9600 or 115200).
• Error Handling: Cellular connections can drop—add reconnection logic (e.g., check if the PPP interface is up every 30 seconds, and re-run the AT command sequence if it’s down).
• RTOS Compatibility: Keep your existing core split (web server on core 0, client on core 1) and run the PPPOS task on a separate core or low-priority task to avoid blocking your existing workload.

内容的提问来源于stack exchange，提问作者Oscar