SDAA112 September   2025 AM625

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Use Case
  5. 2Platform and Products
    1. 2.1 Hardware
    2. 2.2 Software
  6. 3Methods
  7. 4Overview of Steps to Bring Up the Interface and Establish A Connection
  8. 5Troubleshooting
    1. 5.1 Power and Initialization First
    2. 5.2 Verify Network Before PPP
    3. 5.3 Test Manual Dial First
    4. 5.4 Timing When Dialing Up
    5. 5.5 Serial Communication
    6. 5.6 Chat Script
    7. 5.7 PPP Options
    8. 5.8 Signal Strength and Antenna Setup
    9. 5.9 Modem Reset
  9. 6Summary
  10. 7References

3 Methods

PPP is often used for remote network access and internet connections. In this application, PPP provides a lightweight, serial-based mechanism for transporting IP packets.

The Linux kernel includes PPP support natively. The PPP Daemon (pppd) manages PPP connections. pppd works with the PPP driver of the kernel to set up, maintain, and configure PPP connections, including negotiating IP addresses and handling authentication.

Before PPP can operate, the modem must be configured over a serial port with AT commands. Once PPP is dialed up, the serial port becomes the data path between the processor and the network connection. Commonly used AT commands include:

AT Command Description
AT+CPIN? Checks whether the SIM card is unlocked and ready for use
AT+CEREG? Reports the registration status of the device on the cellular network
AT+COPS? Queries the currently selected network operator
AT+CSQ Returns the received signal strength indication (RSSI) and bit error rate (BER)
AT+CGDCONT? Displays the defined PDP context(s), including APN configuration
ATD*99# Initiates a PPP data call to establish a network connection
AT+QNWINFO Reports the active Radio Access Technology (RAT) and frequency band (Quectel Specific)

Once the modem is confirmed to be registered with a network and a data profile is active, PPP can be launched. A common workflow involves verifying the state of the modem through AT commands, confirming signal strength and registration, and then initiating PPP through a peer configuration file that invokes both chat and pppd. The chat utility issues the dial-up AT commands, while pppd latches onto the session once CONNECT is returned. The chat script automates the dial up process, avoiding timing issues do present if dialing were performed manually. To learn more about AT commands and the use for Quectel Modems and beyond, see Quectel_BG95&BG77&BG600L_Series_AT_Commands_Manual_V2.0.