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Cellular Connectivity with Raspberry Pi 4B: Implementation and Challenges

8 min read
By Erick
pimodems

Introduction

Implementing cellular connectivity in embedded Linux systems presents unique challenges, particularly in the context of the Raspberry Pi 4B platform. While the Pi 4B offers significant processing capabilities and versatile I/O options, integrating cellular modems requires careful consideration of hardware interfaces, driver support, and system service interactions. This technical implementation explores the process of adding cellular connectivity to a Raspberry Pi 4B, focusing on the challenges of modem selection, system integration, and configuration of the ModemManager framework.

Project Requirements

The implementation addresses two specific networking requirements for a Raspberry Pi 4B-based IoT application:

  1. System-Wide Cellular Network Integration

    • Enable full TCP/IP networking capabilities through the cellular modem
    • Integrate with the Linux networking stack for system-wide internet access
    • Support automatic network interface configuration via NetworkManager
    • Ensure proper routing and DNS resolution for all system services

  2. SMS Messaging Functionality

    • Enable programmatic SMS message handling
    • Integrate with ModemManager's messaging interfaces
    • Support both message transmission and reception

These requirements extend beyond basic AT command support, necessitating proper integration with Linux networking subsystems and system management services.

Hardware Integration Options

The Raspberry Pi 4B offers two distinct approaches for cellular modem integration, each with specific advantages and limitations.

USB Integration

Advantages:

  • Standardized plug-and-play functionality
  • Mature driver ecosystem
  • Built-in power management
  • Hot-swap capability for maintenance

Limitations:

  • Higher component cost
  • USB port occupation
  • Potential Linux driver compatibility issues
  • Limited hardware-level control

GPIO Integration

Advantages:

  • Cost-effective implementation
  • Direct hardware control
  • Enhanced debugging capabilities
  • Flexible power management options

Limitations:

  • Complex power requirements
  • Manual voltage level management
  • Increased wiring complexity
  • Direct exposure to hardware risks

Implementation Considerations

The cellular connectivity integration presents several technical challenges:

  • PPP (Point-to-Point Protocol) configuration complexity
  • AT command interface management
  • Driver compatibility requirements
  • Power consumption optimization

Design Decision

After thorough analysis, this implementation utilizes the USB approach for the following reasons:

  1. Reduced development time for initial prototyping
  2. Better stability for production environments
  3. Simplified power management
  4. Established Linux kernel support

Modem Selection Analysis

ModemManager Official Support

ModemManager provides built-in support for various cellular modems through vendor-specific plugins. Notable supported manufacturers include:

  • Fibocom
  • Foxconn
  • Huawei
  • Intel
  • Quectel
  • Sierra Wireless
  • Telit
  • ZTE

Each plugin is specifically designed to handle the vendor's AT command set and initialization requirements, ensuring optimal compatibility and performance.

Selected Hardware: SIMCOM A7670E

Despite the availability of officially supported options, this implementation utilizes the SIMCOM A7670E USB modem. This decision introduces certain technical challenges as the device relies on ModemManager's generic SIMCOM plugin.

Technical Specifications:

  • LTE Cat-4 capabilities
  • USB 2.0 high-speed interface
  • Integrated TCP/IP stack
  • SMS functionality
  • Dual-SIM support
  • Cost: ~$20 USD

Initial Testing Environment

Initial testing was conducted on an Xubuntu development system to evaluate basic functionality and system integration. The testing utilized a Safaricom SIM card for network connectivity assessment.

System Integration Results

The system detected and initialized the modem through standard USB protocols:

kernel: usb 1-2.1: New USB device found, idVendor=1e0e, idProduct=9011
kernel: usb 1-2.1: Product: A76XX Series LTE Module
kernel: usb 1-2.1: Manufacturer: SIMCom Wireless Solution

ModemManager's response confirmed the use of the generic plugin:

ModemManager[1238]: <msg> [device] creating modem with plugin 'simtech' and '4' ports

This implementation acknowledges the technical implications of using an unofficially supported modem and documents the necessary adaptations for successful integration.

Network Configuration and Initial Challenges

After establishing the physical connection, the next step was to configure the mobile broadband communication. This process involves several key components:

  1. Mobile Broadband Configuration

    • Creating a new connection profile in NetworkManager
    • Configuring the APN settings specific to the carrier
    • Setting up authentication parameters if required
    • Establishing connection parameters for the mobile network

  2. System Response Analysis The system logs revealed several important events during this process:

NetworkManager[1131]: <info> [1738998084.2953] dhcp6 (wlp2s0): activation: beginning transaction
NetworkManager[1131]: <info> [1738998084.3165] dhcp6 (wlp2s0): state changed new lease
  1. Service Interactions The configuration attempt triggered multiple system services:
systemd[1]: Starting NetworkManager-dispatcher.service
dbus-daemon[1027]: Successfully activated service 'org.freedesktop.nm_dispatcher'
  1. DNS Configuration The system attempted to configure DNS settings:
tailscaled[1802]: dns: OScfg: {Nameservers:[100.100.100.100]}
systemd-resolved[859]: Flushed all caches.

Despite the proper configuration of APN settings and network parameters, the modem failed to establish an online connection. This outcome necessitated a deeper investigation into the modem's behavior and system configuration requirements.

Debugging ModemManager Interactions

To effectively diagnose communication issues between ModemManager and the modem, we can employ several logging methods:

Debugging with Modem Manager

The most comprehensive method is to enable debug logging for ModemManager:

# Stop the ModemManager service
sudo systemctl stop ModemManager

# Start ModemManager in debug mode
sudo ModemManager --debug

ModemManager Initialization Analysis

Running ModemManager in debug mode (sudo ModemManager --debug) provided detailed insights into the modem's initialization sequence. Let's analyze the key stages:

1. Device Identification

[modem0] loaded model: A7670E-LASE
[modem0] loaded revision: A110B03A7670M7
[modem0] loaded equipment identifier: 862771071659240

The modem properly identifies itself with model, revision, and IMEI information.

2. Capability Detection

[modem0] device allows (3GPP) mode combination: 2g
[modem0] device allows (3GPP) mode combination: 3g
[modem0] device allows (3GPP) mode combination: 4g
[modem0] device allows (3GPP) mode combination: 2g, 3g, 4g

The modem supports multiple network modes including 2G, 3G, and 4G combinations.

3. IP Protocol Support

+CGDCONT: (1-15),"IP",,,(0-2),(0-1),(0-1),(0-2)
+CGDCONT: (1-15),"IPV6",,,(0-2),(0-1),(0-1),(0-2)
+CGDCONT: (1-15),"IPV4V6",,,(0-2),(0-1),(0-1),(0-2)

Supports IPv4, IPv6, and dual-stack configurations across 15 possible contexts.

4. SIM Card Status

[modem0/sim0] loaded operator identifier: 63902
[modem0/sim0] loaded operator name: Safaricom
[modem0/sim0] loaded IMSI: 639021331274317

The SIM card is properly detected and read, identified as a Safaricom SIM.

5. Critical Issues Identified

  1. AT Command Timeouts:
[modem0] port ttyUSB1 timed out 2 consecutive times
[modem0] port ttyUSB1 timed out 3 consecutive times

Serial communication instability detected.

  1. Command Failures:
[ttyUSB1/at] <-- '<LF>+CME ERROR: 100<CR><LF>'

Multiple AT commands failing with generic errors.

  1. Network Registration Issues:
[modem0] couldn't load current allowed/preferred modes: Unknown error

Problems with network mode configuration.

6. Communication Interface Analysis

The modem creates multiple interfaces:

ttyUSB0: Primary AT command interface
ttyUSB1: Secondary control interface
ttyUSB2: Additional interface (possibly for data)
enxae0c29a39b6d: Network interface

7. Response Timing Analysis

AT Command Response Times:
- Simple queries (AT+CPIN?): ~10ms
- SIM operations (AT+CRSM): ~30-40ms
- Network operations: >100ms

AT Command Analysis and Driver Considerations

Failed AT Command Analysis

Looking at the debug logs, several critical AT command failures stand out:

  1. Network Mode Query Failure:
AT*CNTI=2
<-- ERROR
[modem0] generic query of supported 3GPP networks with *CNTI failed: 'Unknown error'

This suggests ModemManager is using an AT command not supported by the A7670E.

  1. Preferred Network Configuration Failure:
AT+CPOL=,2
<-- +CME ERROR: 21
[modem0/sim0] setting preferred network list format failed: 'Serial command timed out'

The modem either doesn't support this command or requires different syntax.

  1. Facility Lock Queries:
AT+CLCK="PN",2 
<-- +CME ERROR: 100
AT+CLCK="PU",2
<-- +CME ERROR: 100

Multiple facility lock queries failing with generic errors.

Driver Support Analysis

These failures indicate potential issues with ModemManager's built-in SIMCOM driver:

  1. Current ModemManager Support
  • Using generic SIMCOM plugin
  • Limited command set support
  • No specific A7670E optimizations
  1. Required Actions

a) Driver Installation:

  • Check for specific A7670E Linux driver package
  • Verify kernel module requirements
  • Consider USB_ModeSwitch configuration

b) ModemManager Plugin:

  • Check if newer ModemManager versions have better A7670E support
  • Consider creating custom ModemManager plugin for A7670E
  • Verify supported AT command set
  1. Vendor Documentation
  • Review A7670E AT command manual
  • Verify correct initialization sequence
  • Confirm supported command set

Recommended Next Steps

  1. Driver Verification:
# Check current USB driver
lsusb -t
# Check loaded kernel modules
lsmod | grep option
  1. ModemManager Version Check:
mmcli --version
apt search modemmanager
  1. Custom udev Rules:
# Create custom udev rule for A7670E
sudo nano /etc/udev/rules.d/99-a7670e.rules

# Add specific rules for the modem
SUBSYSTEM=="usb", ATTRS{idVendor}=="1e0e", ATTRS{idProduct}=="9011", RUN+="/sbin/modprobe option"

The next section will detail the implementation of proper driver support and ModemManager configuration for the A7670E modem.

Part 2: Driver Support for A7670E: Kernel Module Implementation

Part 3: Testing A7670E Modem Features with CellularPi

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