Internet Access over GPRS

Prerequisites

First make sure the board is properly attached to the pilot mainboard. You may use screws to make sure the board will be 100% accurately in place.

You need to install an GSM antenna in order to send and receive data from the module.

antenna

You need a SIM card that supports a 2G network. Most networks in Europe support this standard, but there are different networks that do not support this. Ask your mobile service provider if the SIM card is compatible with the 2G cellular network.

Basics

Plug in in the module and enable it. Run sudo pilot setup to configure the pilot motherboard and load the matching kernel driver. You should see the GSM module in the list. Once installed you can run pilot module to get an overview of what can be done with the installed modules.

First enable the module by running

echo 1 > /proc/pilot/module1/enable_gsm

/module1/ might be different depending on the slot you inserted your GPS module.

The module is now enabled and you should be able to communicate with it over the associated serial device.

A full documentation on all available AT commands is available in the AT commands datasheet for the SIM800 module.

You can get a copy here

Raw access of the modules output

The module operates with AT commands that you can access on the serial interface that is available on your linux host system. When plugged into slot 1 the device is called /dev/ttyP0.

To access the raw data you can use the screen command or another terminal emulator of your choice like minicom. The module sends its data at 9600 Baud. Connect to the module with the command screen /dev/ttyP0 9600

The module does display data only when you ask for it. To check wether it is operational type the command AT and press Enter. You will usually not see your own input. The module should respond with OK


You can leave screen by pressing

Ctrl + a than k to terminate the session

Setup PPP tunnel

To access the internet over GPRS you need to configure PPP and chat scripts like back in the old days when we still had dialup modems.

Install the system utilities

sudo apt-get update 
sudo apt-get install ppp 

Change directory to /etc/ppp/peers

Create a file called "gprsinternet" and paste these lines

connect "/usr/sbin/chat -v -f /etc/chatscripts/gprs -T internet"
/dev/ttzP0
115200
noipdefault
usepeerdns
defaultroute
persist
noauth
nocrtscts
local

Next create the file /etc/chatscripts/gprs and paste these lines

ABORT        BUSY
ABORT        VOICE
ABORT        "NO CARRIER"
ABORT        "NO DIALTONE"
ABORT        "NO DIAL TONE"
ABORT        "NO ANSWER"
ABORT        "DELAYED"
ABORT        "ERROR"

ABORT        "+CGATT: 0"

""        AT
TIMEOUT        12
OK        ATH
OK        ATE1

OK        AT+CGDCONT=1,"IP","\T","",0,0
OK        ATD*99#
TIMEOUT        22
CONNECT        ""

This is all you need to to to enable a PPP link ofer GPRS. To connect to the Internet start the connection with the command

pon gprsinternet

You can see if everything works by running tail -f /var/log/syslog.

Also ifconfig should display some information on the newly created PPP interface that looks like this:

ppp0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1500
    inet 10.170.75.132  netmask 255.255.255.255  destination 10.64.64.64
    ppp  txqueuelen 3  (Point-to-Point Protocol)
    RX packets 567  bytes 107736 (105.2 KiB)
    RX errors 0  dropped 0  overruns 0  frame 0
    TX packets 986  bytes 445004 (434.5 KiB)
    TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

To disconnect run

poff gprsinternet

Permanent Internet connection on bootup

Once your setup works, you might want to connect to the internet as soon as the device is turned on. To enable the GPRS datalink by default add the following lines in the file /etc/network/interfaces

auto gprsinternet
iface gprsinternet inet ppp
    provider gprsinternet

Example MQTT

One of the more commonly used examples of connecting embedded devices to the internet is using MQTT to transmit sensordata.

To give this a try install the mqtt client software apt-get install mosquitto-clients

Test your setup with the online MQTT broker client you can find here

Click the connect button and than SUBSCRIBE enter this resource hump/#

Now open your shell again and type

mosquitto_pub -h broker.mqttdashboard.com -m "hallo hallo" -t "hump/dump"

You should see the message "hallo hallo" on the resource "hump/dump" appear in real time. Please bear in mind that this information is not private and visible to everybody who connects to this open MQTT broker. This is very useful for debugging but should not be used for production environments.

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