Sony Ericsson GPRS and TCP-IP with the SEM GSM modules Installation and operating instructions

Application Note
Using GPRS and TCP-IP with
the SEM GSM modules

First edition (August 2003)
Sony Ericsson Mobile Communications. publishes this manual without making any warranty as to the
content contained herein. Further Sony Ericsson Mobile Communications. reserves the right to make
modifications, additions and deletions to this manual due to typographical errors, inaccurate information,
or improvements to programs and/or equipment at any time and without notice. Such changes will,
nevertheless be incorporated into new editions of this manual.
All rights reserved.
© Sony Ericsson Mobile Communications., 2003

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Contents
1INTRODUCTION....................................................................................................................4
2OVERVIEW OF THE GSM/GPRS SYSTEM ......................................................................... 5
2.1 GSM MODULE AND APPLICATION ........................................................................................ 5
2.2 GSM/GPRS NETWORK ..................................................................................................... 5
2.3 APN................................................................................................................................. 5
2.4 DHCP SERVER ................................................................................................................. 5
2.5 INTERNET ......................................................................................................................... 5
2.6 WEB SERVER .................................................................................................................... 5
3MODES OF OPERATION...................................................................................................... 7
3.1 GSM MODULES WITHOUT TCP/IP STACKS.......................................................................... 7
3.1.1 Running GPRS through Windows ........................................................................... 7
3.2 GSM UNITS WITH TCP/UDP/IP STACKS............................................................................. 8
4PRACTICALITIES OF USING GPRS.................................................................................... 9
4.1 CONNECTION TIMING ......................................................................................................... 9
4.2 DYNAMIC IP ADDRESSES ................................................................................................... 9
4.3 STATIC IP ADDRESSING ..................................................................................................... 9
4.4 TCP AND UDP PROTOCOLS............................................................................................. 10
5REFERENCES..................................................................................................................... 11

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1 Introduction
The GSM/GPRS system allows direct access into the internet enabling
M2M applications to interface to web servers rather than going through the
PSTN which gives faster and cheaper communications with greater
flexibility.
This document is intended to explain how the overall system works and
how the SEM GSM modems with their embedded TCP/IP stack enable
transparent communications.

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2 Overview of the GSM/GPRS system
In Figure 1 a simplified diagram of the overall system is shown, the major
components of these are explained below.
2.1 GSM module and application
This is a GSM module with or without TCP/IP stack on board. If the unit
does not have a TCP/IP stack on board it must be contained on the
application be it microprocessor, PC, etc.
2.2 GSM/GPRS network
This includes the base station through to the APN with all of the
associated IP backbone, etc. The network takes care of the mobility
issues and getting data to the mobile station
2.3 APN
An APN is similar to a URL on the WWW, it is what the unit makes a
GPRS ‘call’ to. The service provider is able to attach anything to an APN
to create a VPN (Virtual Private Network) i.e. if a security company wanted
its own APN it might have ‘security’ no one else would then have access
to this, requirements for APN’s to be assigned vary between the service
providers.
Most service providers have an internet portal which they connect a DHCP
server to giving access to the internet i.e. Vodafone UK use the APN
‘internet’ for their portal.
2.4 DHCP server
This is what you negotiate your IP address with and where all of the data
goes through, it effectively acts as your service provider i.e. AOL.
2.5 Internet
The internet is the internet, no explanation should be required.
2.6 Web server
The users web server which has a static IP address or a URL that can be
resolved by a server. This should be running the users application which
can interpret and manipulate the data as required.

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Figure 1 GSM/GPRS

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3 Modes of operation
When using GPRS with the SEM modules some units have an embedded
TCP/IP stack others do not, theses are listed below:-
Units without the TCP/IP stack - GM29
- GM41
- GM47/GM48
Units with the TCP/IP stack - GR47/GR48
- GT47/GT48
3.1 GSM modules without TCP/IP stacks
These modules have limited functionality in that they will provide
communications across the network to the APN but the rest of the protocol
stack needs to be provided by the application.
The sequence of events
1. Create a data account using the at+cgdcont command i.e.
at+cgdcont=1,”IP”,”internet”. This hold the necessary information
required to make a GPRS ‘call’.
2. Make a GPRS call using the standard dial command but using the dial
string *99***1#
3. Once the unit has connected through to the APN correctly it will return
a CONNECT message and enters V.250 online data mode.
4. At this point the application needs to initiate a PPP/TCP/IP
conversation.
More details of the protocols and procedure used are available in GSM
7.60 or 3GPP 27.060.
3.1.1 Running GPRS through Windows
The fastest way to see how GPRS operates with a TCP/IP stack is to run
it against windows. To do this use the following steps.
1. Power the unit up and create a data account using the command
at+cgdcont=1,”IP”,”XXXXXX”
Where XXXXXX is the internet APN for the network you are using

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2. Install the modem onto the PC using the latest modem driver that can
be downloaded from the internet.
3. Set up a dial up networking session using the modem that you have
just installed. Make sure that all parameters in both the dial up
networking session and the modem properties are set to 9600bps.
4. Start to make the call using the dial string *99***1#, the unit should
now negotiate an IP address with the server and enable the PC to surf
the web.
Note : You may need to adjust some of your Internet explorer settings
to allow it to accept data from the dial up network connection.
If the connection fails the most common errors are :-
Error 738 – The most common cause of this error is that the SIM does not
have access rights to the internet DHCP server and it is unable to allocate
the unit an IP address.
Error 692 – This is generally attributed to either the modem not being on
or the PC talking at a different baud rate to that which the modem is
talking at.
3.2 GSM units with TCP/UDP/IP stacks
The GSM units that have TCP/UDP/IP stacks on them allow the
application to forget about how the communication takes places and
allows transparent communication between the unit and the IP address
which the users web server is sat on i.e. data will be passed to the MS it
will then packet it up and the user will see it arrive at the port of the IP
address that has been connected to.
The AT command operation of the TCP/IP stack is specified in Application
note LZT 123 7631 R1A.

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4 Practicalities of using GPRS
Below are a number of points that need to be considered when using
GPRS
4.1 Connection timing
When using GPRS with a session running most service providers will
monitor the traffic flow, if there is none for a predetermined period of time
then it will shut the connection down at either the DHCP server or the
APN, this is performed so that system resources are not taken up
unnecessarily.
To stop this happening you will need to send periodic keep alive bytes,
around 1 every five minutes would be sufficient.
4.2 Dynamic IP addresses
At present all of the IP address that are handed out by the networks are
dynamically allocated i.e. each time you start a new session the unit will
be assigned a new IP address. A session may be finished for a number of
reasons
•The unit is powered off and then back on again
•There has been no activity on the line for a predefined period of
time and the network has shut it down (see above).
•The unit has been out of coverage (or left coverage) for too long
and the network has again shut it down.
These are the most common causes but are by no means the only ones.
4.3 Static IP addressing
Giving a unit a static IP address is not yet in wide spread use although
they are being experimented with by some operators. A static IP address
will allow you to keep the same address regardless of whether you power
the unit off or not, it does present the operators with some problems but it
does allow the unit to support server functions as well as client functions
which are all that are currently implemented.

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4.4 TCP and UDP protocols
The two main protocols that the unit supports are TCP and UDP a brief
explanation about each is given below.
TCP – This provides a reliable connection to another point in the network
i.e. numbers frames and keeps track of acknowledgements.
UDP – This is a ‘fire and forget’ protocol normally used with time critical
information such as voice i.e. the unit will packet the data up and send it
across the network.
In the future the units will be able to support higher level protocols such as
XML, WBXML, etc.

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5 References
Most official documentation about TCP/IP is available as RFC’s from
http://www.ietf.org and they can provide essential technical background for
anyone requiring a deeper understanding of TCP/IP. A small sample of
the more pertinent RFC’s and their titles are outlined below but many
more are available.
1. RFC 791 Internet protocol
2. RFC 792 Transmission Control Protocol
3. RFC 394 File Transfer Protocol
4. RFC 1180 TCP/IP Protocol
5. LZT 123 7631 R1A Using AT commands to control TCP/IP stack
on GR47/GR48