How to Use GSM/3G/4G in Embedded Linux Systems

How to Use GSM/3G/4G in
Embedded Linux Systems

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Introduction
Embedded devices connected to the internet are growing every
day. In many cases, these devices will be installed somewhere
without a wire or wireless network connection. A good way to
keep the devices connected to the internet in these situations is by
seeking mobile network coverage. Therefore, this blog post will present
the necessary configurations to connect a device running Linux to the
internet through the PPP (Point-to-Point Protocol) link.and optimize cost.
In simple terms, modularity means that a system can be built by
assembling many small units.

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Hardware Specification
In this article we will be using the Colibri iMX6S with the base board called
“Colibri Evaluation Board”, both of which, are from Toradex. The Colibri
Evaluation Board is recommended specifically for project evaluation and
development. This product comes with a great number of interfaces like USB,
Ethernet, I2C, SPI, RS242, RS485, CAN and many others. We can also find
some multimedia interfaces like HDMI, LVDS, VGA and LCD already
supported on the Evaluation Board.
To follow this tutorial, you can use any USB or Serial modem to connect to the
internet.

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You just have to adapt the interface created at “/dev/” and check the “AT”
commands supported and implemented for your modem because each modem
has specific AT commands. In this case, we have used for this tutorial a Huawei
E173s 3G modem with a compatible SIM Card with data plan activated.
Kernel Preparing
To use the 3G USB modem through the PPP protocol, it is necessary to select
some features in the kernel configurations and then compile the Kernel. To
understand how to compile the kernel, I suggest following the link below.
http://developer.toradex.com/software-resources/arm-family/linux/board-
support-package/build-u-boot-and-linux-kernel-from-source-code

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Basically, the process to compile the Kernel can be summarized in the
following steps:
• Toolchain configuration
• Download the kernel from the correct repository and branch
• Configure the standard processor features
• Change the features for your application
• Compile

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When you are configuring the kernel features, go into menuconfig and be
sure to enable the following options:
CONFIG_PPP:
PPP (Point to Point Protocol) is a newer and better SLIP. It serves the same
purpose: sending Internet traffic over telephone (and other serial) lines. Ask
your access provider if they support it, because otherwise you can't use it;
most Internet access providers these days support PPP rather than SLIP.
Device Drivers --->
[*] Network device support --->
PPP (point-to-point protocol) support
PPP BSD-Compress compression
PPP Deflate compression

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[*] PPP filtering
PPP MPPE compression (encryption)
[*] PPP multilink support
PPP over Ethernet
PPP support for async serial ports
PPP support for sync tty ports
CONFIG_USB_ACM:
This driver supports USB modems and ISDN adapters which support the
Communication Device Class Abstract Control Model interface.
Please read <file:Documentation/usb/acm.txt> for details.
Device Drivers --->
[*] USB support --->
USB Modem (CDC ACM) support

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Necessary Software Packages
To configure and connect the 3G modem to the internet, Linux uses, in addition to
the kernel drivers, a software which manages the connection. This software is PPP.
Usually Toradex images already have the PPP software installed by default.
In case you do not have PPP installed, just add the following to your local.conf file
for build environments based on Yocto/OpenEmbedded:
Testing Communication with the Modem
After all the necessary drivers and software are installed, it is possible to check if
the module recognizes the 3G modem. The modem interface should appear in /dev.
Basically the driver simulates a USB connection as many serial connections.

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Execute the following command to search for the interfaces /dev/ttyUSB
If the files ttyUSB0, ttyUSB1 … e ttyUSB2 were found, the modem was succesfully
recognized by the system.
In case you want to conduct in-depth testing, it is possible to use a serial terminal
like “minicom” to start communicating with the modem and check if it is working
well. In my case the modem had a baudrate of 115200.

Configuring the PPP Files
After validating the communication with the modem, it is time to create the
configuration files responsible for connecting to the internet.
You should be able to find a folder inside /etc/ called ppp. Some files need to be
created inside this folder.
First we create the PPP options file:
$ vi /etc/ppp/options
_____________________________________
auth
crtscts
lock
hide-password
modem
mru 296

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mtu 296
lcp-echo-interval 30
lcp-echo-failure 4
noipx
persist
asyncmap 0xa0000
mru 1500
refuse-chap
ipcp-max-failure 30
logfile /home/root/ppp
In my case we use the PAP authentication file (may vary depending on Network
Provider).

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The next step is to create one more file inside /etc/ppp/peer In this file we have
configurations for our network provider and the path for the chat file:

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Now we create the file responsible for sending the AT commands to the modem so it
can connect to the internet. This file varies according to modem models/brands.

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Connecting to the Internet
Next we run ppp and analyze the log files. In the log files we can see when each AT
command has been executed.
After running ppp, we open the log file:
In my case, I received the following output:
tail -f ppp.log
Sent 3940 bytes, received 2843 bytes.
restoring old default route to eth0 [192.168.10.1]
restore default route ioctl(SIOCADDRT): Network is unreachable(101)

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Script /etc/ppp/ip-down started (pid 778)
sent [LCP TermReq id=0x2 "User request"]
rcvd [LCP TermAck id=0x2]
Connection terminated.
Script /etc/ppp/ip-down finished (pid 778), status = 0x0
ATZ
OK
AT+CGDCONT=1,"IP","zap.vivo.com.br"
OK
ATDT*99#
CONNECT
Script chat -v -f /etc/ppp/chat/vivo-3g.chat finished (pid 795), status = 0x0
Serial connection established.
using channel 2
Using interface ppp0
Connect: ppp0 /dev/ttyACM0

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rcvd [LCP ConfReq id=0x1 ]
sent [LCP ConfReq id=0x1 ]
sent [LCP ConfAck id=0x1 ]
rcvd [LCP ConfAck id=0x1 ]
sent [LCP EchoReq id=0x0 magic=0xa068db01]
sent [PAP AuthReq id=0x1 user="vivo" password=]
rcvd [LCP EchoRep id=0x0 magic=0x96baf40f]
rcvd [PAP AuthAck id=0x1 ""]
PAP authentication succeeded
sent [CCP ConfReq id=0x1 ]
sent [IPCP ConfReq id=0x1 ]
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
rcvd [IPCP ConfNak id=0x1 ]

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rcvd [IPCP ConfReq id=0x1]
sent [IPCP ConfNak id=0x1 ]
rcvd [IPCP ConfRej id=0x6 ]
rcvd [IPCP ConfReq id=0x2 ]
sent [IPCP ConfAck id=0x2 ]
rcvd [IPCP ConfAck id=0x8 ]

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local IP address 179.133.47.109
remote IP address 179.133.47.109
primary DNS address 187.100.246.254
secondary DNS address 187.100.246.251
Script /etc/ppp/ip-up started (pid 805)
Script /etc/ppp/ip-up finished (pid 805), status = 0x0
As seen in the log file, the chat script is executed step-by-step until we get the following:
You can validate the necessary command sequence to get connected using the
serial terminal. Once it is validated, you can create your own customized chat
script.
After checking for an IP address from the network provider, look for whether the
ppp0 network interface is active:

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Testing the Connection
To test if the connection has been successfully established, ping an external ip address:
To configure DNS, we can use the following command and then ping a URL, for
example:

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Image 1: A
running web
browser
using 3G
connection.

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We can avoid the file being rewritten when system is restarted with the following
command:
Now it is possible to connect using SSH from an external computer:
Configuring a Host Name for our IP Address
Another thing we can do is to associate our IP address to a host name. For this
purpose, we used the NoIP services (www.noip.com). We then created a free
account and added a Host. In our case, we chose toradex.noip.me.

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Image 2: NoIP Add Host page.

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After the Host name is created we can again connect to the module using SSH:
In our case, when the 3G modem connection is restarted, we always receive a new IP
adress. If we try to reconnect to the module, we no longer have access. A workaround
was to use a method called Dynamic DNS(Dynamic Domain Name System). NoIP also
offers Dynamic DNS services. Further information for using this method can be found
here. Basically we send an http request to NoIP with information like IP, hostname,
username and password.
To accomplish this request, a small Python program was made. It gets executed when
we start the 3G modem connection and follows the sequence as listed below:

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Image 3: Python Application Fluxogram.

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#!/usr/bin/python
import sys
import requests
import netifaces as ni
user = 'xxxxxxx'
pswd = 'xxxxxxx'
ni.ifaddresses('ppp0')
ip = ni.ifaddresses('ppp0')[2][0]['addr']
myhostname = 'toradex.noip.me'
payload = {'hostname' : myhostname , 'myip' : ip}
r = requests.get("http://dynupdate.no-ip.com/nic/update", params=payload,
auth=(user,pswd))

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print " "
if "good" in r.text:
print "Hello", user, "!"
print "Your IP was successfully updated to:", ip
print myhostname, "is up and running!"
if "nochg" in r.text:
print "Hello", user, "!"
print "Your IP", ip, "is still active, no change needed"
if "nohost" in r.text:
print "The given Host name", myhostname, "does not exist under specified account"
print "Please review your Host name and try again"
if "badauth" in r.text:
print "Login and/or Username incorrect"
print "Please correct your credentials and try again"
if "911" in r.text:

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print "Sorry for the incovenience but we are experiencing some problems right now"
print "Please try again later"
print "noip.com says:", r.text
print " "
After creating the Python program and making it executable with chmod +x, we can
move it to /etc/ppp/ip-up.d/, which is a folder where scripts are executed after ppp
connection or create a small script which calls our Python program.
Inside /etc/ppp/ we have the files that we created at the beginning of the article as
well as other files like ip-down, ip-up, ppp_on_boot. Feel free to explore each one of
these files.
ip-up is the script which calls another script or program found inside ip-up.d folder.
In this case, we created a small script inside ip-up.d

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Note: the script is not terminated with .sh
Note that all the print commands inside our Python program can be seen in the log file:
If everything is configured correctly, when we receive a new IP from our network
provider, the system automatically updates our host created at NoIP, so it will be
possible to connect to the module despite the IP address being changed.
Based on all the information provided in this article, it is possible to achieve many
other network activities. One of them is to share the 3G modem network with another
computer via Ethernet. For this we configured a basic LAN network between two
machines and ran the following command:

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Apart from internet sharing, it is also possible to conduct port forwarding and many
other network features.
Final Thoughts
As seen in this article, a Linux system has many network functionalities. We just
need to properly configure them and have access to ppp0 network. Another
important point, is that regardless of the used interface, the programming method
remains the same. The task of choosing the interface to use is made automatically by
Linux depending on the routes. For those who require network connection in their
projects and also need system flexibility, the use of Linux in embedded systems is
the way to go.
References
http://metodiew.com/huawei-e173s-and-ubuntu/
http://developer.toradex.com/product-selector/colibri-imx6
http://developer.toradex.com/product-selector/colibri-evaluation-board

Thank
you!

How to Use GSM/3G/4G in Embedded Linux Systems

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How to Use GSM/3G/4G in Embedded Linux Systems