Connecting to the Internet over USB GSM Modem Print


This application note explains how to connect to the Internet from the STM32F7 Discovery running Linux.

The Linux PPP daemon and its shell scripts are used to initiate the connection, receive the networking parameters from the cell (IP, routing, DNS etc) and configure the system networking according to the received parameters. The pppd package integrated by Emcraft into the STM32F7 Discovery BSP was obtained from, release 20140504.

The functionality described below is available from the rootfs.uImage project provided by Emcraft for STM32F7 Discovery. The generic Linux kernel device driver driver for the USB Modems (CONFIG_USB_ACM) is used in this configuration. Emcraft tested the functionality documented below using the MTD-H5-2.0 HSPA+ USB Cellular Modem from MultiTech Systems.

Cellular Network Configuration

For starting and terminating the PPP connection, the standard scripts from the PPP Daemon package are provided in the rootfs projects. The scripts as follows:

  • pon <provider> - starts the PPP daemon with a specified configuraton in /etc/ppp/peers/. Without arguments, the /etc/ppp/peers/provider configuration is used.
  • poff - stops the PPP daemon and terminates the connection.

The following PPP configuration files are provided with the rootfs project:

  • /etc/ppp/peers/provider - contains the default PPP daemon configuration.
  • /etc/ppp/options-mobile - contains the general PPP daemon configuration options (see man pppd). Among others, there are two options enabled for debugging (dump and logfile /dev/console).
  • /etc/ppp/chatscripts/ - contains modem commands to establish connection with the network.
  • /etc/ppp/chatscripts/apn - contains the modem command to configure the Access Point Name specific to the provider.
  • /etc/ppp/chatscripts/pin - contains the modem command to configure the SIM-card PIN code, if required. Preconfigured for no PIN in the project.

For a US GSM network operator (AT&T, T-Mobile, Rogers, Bell, Telus, see in, the current configuration should work, after setting up an appropriate Access Point Name in /etc/ppp/chatscripts/apn and cell account information in /etc/ppp/peers/provider.

For a US CDMA network operator (Verizon, Sprint, Aeris), the modem model Multitech MTD-EV3 should be used. The project provides sample configurations /etc/ppp/peers/verizon and /etc/ppp/chatscripts/, which supposedly should work with Verizon.

To reconfigure the rootfs project for your SIM operator typically you will need:

  • Edit the etc/ppp/chatscripts/apn file in projects/rootfs, and change the APN settings appropriately.
  • Edit the etc/ppp/peers/provider file in projects/rootfs, and change the account information appropriately.
  • Build project/rootfs (Building Linux), and install it to the target (Installing Linux Images to Flash).

TCP/IP Connectivity Over PPP

Plug-in a USB GSM Modem device to the USB HS port (CN15 of STM32F7 Discovery). The system will print the appropriate messages to console about the hardware detected:

usb 1-1: new high-speed USB device number 3 using dwc2
cdc_acm 1-1:1.0: ttyACM0: USB ACM device
cdc_acm 1-1:1.2: ttyACM1: USB ACM device
cdc_acm 1-1:1.4: ttyACM2: USB ACM device
cdc_acm 1-1:1.6: ttyACM3: USB ACM device
cdc_acm 1-1:1.8: ttyACM4: USB ACM device
cdc_acm 1-1:1.10: ttyACM5: USB ACM device

Initiate connection with the cellular network:

/ # pon
pppd options in effect:
debug# (from /etc/ppp/options-mobile)
holdoff 10 # (from /etc/ppp/options-mobile)
persist # (from /etc/ppp/options-mobile)
logfile /dev/console # (from /etc/ppp/options-mobile)
maxfail 0 # (from /etc/ppp/options-mobile)
dump # (from /etc/ppp/options-mobile)
noauth # (from /etc/ppp/options-mobile)
user gdata # (from /etc/ppp/peers/provider)
password ?????? # (from /etc/ppp/peers/provider)
/dev/ttyACM0 # (from /etc/ppp/options-mobile)
115200 # (from /etc/ppp/options-mobile)
lock # (from /etc/ppp/options-mobile)
connect /usr/sbin/chat -v -t15 -f /etc/ppp/chatscripts/ # (from /etc/ppp/peers/provider)
crtscts # (from /etc/ppp/options-mobile)
modem # (from /etc/ppp/options-mobile)
passive # (from /etc/ppp/options-mobile)
hide-password # (from /etc/ppp/options-mobile)
novj # (from /etc/ppp/options-mobile)
noipdefault # (from /etc/ppp/options-mobile)
defaultroute # (from /etc/ppp/options-mobile)
usepeerdns # (from /etc/ppp/options-mobile)
random: nonblocking pool is initialized
Serial connection established.
using channel 1
Using interface ppp0
Connect: ppp0 <--> /dev/ttyACM0
rcvd [LCP ConfReq id=0x1 <asyncmap 0x0> <auth pap> <magic 0x96a24aa4><pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x4033167a> <pcomp> <accomp>]
sent [LCP ConfAck id=0x1 <asyncmap 0x0> <auth pap> <magic 0x96a24aa4> <pcomp> <accomp>] 
rcvd [LCP ConfAck id=0x1 <asyncmap 0x0> <magic 0x4033167a> <pcomp> <accomp>]
sent [PAP AuthReq id=0x1 user="gdata" password=<hidden>]
rcvd [PAP AuthAck id=0x1 ""]
PAP authentication succeeded
sent [CCP ConfReq id=0x1 <deflate 15> <deflate(old#) 15> <bsd v1 15>]
sent [IPCP ConfReq id=0x1 <addr> <ms-dns1> <ms-dns2>]
rcvd [LCP ProtRej id=0x2 80 fd 01 01 00 0f 1a 04 78 00 18 04 78 00 15]
Protocol-Reject for 'Compression Control Protocol' (0x80fd) received
sent [IPCP ConfReq id=0x1 <addr> <ms-dns1> <ms-dns2>]
rcvd [IPCP ConfReq id=0x1]
sent [IPCP ConfNak id=0x1 <addr>]
rcvd [IPCP ConfNak id=0x1 <addr> <ms-dns1> <ms-dns2>]
sent [IPCP ConfReq id=0x2 <addr> <ms-dns1> <ms-dns2>]
rcvd [IPCP ConfReq id=0x2 <addr>]
sent [IPCP ConfAck id=0x2 <addr>]
rcvd [IPCP ConfAck id=0x2 <addr> <ms-dns1> <ms-dns2>]
ioctl(SIOCDELRT): No such process(3)
local IP address
remote IP address
primary DNS address
secondary DNS address
Script /etc/ppp/ip-up started (pid 54)
Script /etc/ppp/ip-up finished (pid 54), status = 0x0

Verify that the connection is established (depending on the cellular network, the link may take up to 10 seconds to get established):

/ # ifconfig ppp0
ppp0 Link encap:Point-to-Point Protocol
inet addr: P-t-P: Mask:
RX packets:4 errors:0 dropped:0 overruns:0 frame:0
TX packets:6 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:58 (58.0 B) TX bytes:101 (101.0 B)

Check that the TCP/IP connection to the Internet over PPP has been configured:

/ # netstat -rn
Kernel IP routing table
Destination     Gateway     Genmask           Flags  MSS Window   irtt   Iface           U        0 0           0     ppp0   UH       0 0           0     ppp0     U        0 0           0     eth0
/ # cat /etc/resolv.conf

Ping a remote host to check that network is set up correctly:

/ # ping
PING ( 56 data bytes
64 bytes from seq=0 ttl=54 time=111.881 ms
64 bytes from seq=1 ttl=54 time=59.765 ms
64 bytes from seq=2 ttl=54 time=60.043 ms
64 bytes from seq=3 ttl=54 time=60.036 ms
--- ping statistics ---
4 packets transmitted, 4 packets received, 0% packet loss
round-trip min/avg/max = 59.765/72.931/111.881 ms

Download a file from a remote host:

/ # wget
Connecting to (
README 100% |*******************************| 1962 --:--:-- ETA

Close the TCP/IP connection:

/ # poff
Terminating on signal 15
Connect time 3.7 minutes.
Sent 2095 bytes, received 4681 bytes.
Script /etc/ppp/ip-down started (pid 73)
sent [LCP TermReq id=0x2 "User request"]
rcvd [LCP TermAck id=0x2]
Connection terminated.
Waiting for 1 child processes...
script /etc/ppp/ip-down, pid 73
Script /etc/ppp/ip-down finished (pid 73), status = 0x0