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Easy GPRS User Guide
80000ST10028 Rev. 0 - 02/01/07
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The controlling application must relay on some ISP that may be the Network Operator of the
SIM to gain access to the internet through the GPRS connection.
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Because of the point before, the receiving application must have internet access either.
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Since the communication is based upon TCP/IP packets, then it is possible to talk
contemporarily with more than one peer.
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When required, the data security in internet shall be guaranteed by security protocols over
the TCP/IP that must be managed by the controlling application.
A modem can be in 4 different states:
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GPRS DETACHED, which corresponds to the "not reachable" condition for the GPRS
service;
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GPRS ATTACHED, which corresponds roughly to the "registered" condition for the GPRS
service;
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GPRS context activated, which corresponds to the “reachable on the network” condition with
IP address assigned (this is possible with AT command: AT#GPRS=1 and also some other AT
commands)
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CONNECTED, which roughly corresponds to the connected status;
A thing that must be noted on the GPRS connect, is the fact that, if the mobile IP address (the internet
address) is assigned by the ISP dynamically, then when the GPRS context of the device is not
activated it has no address and therefore it cannot be reached by internet requests. The same thing
occurs in the case the GPRS device has a static IP address assigned to it by the ISP, but it is
DETACHED.
In these cases there's no possibility for the internet peer to "call" the GPRS device through internet,
the only way to alert it is to call it in GSM mode (either a Data or a Voice call are suited) and the
GPRS module application must recognize the caller, eventually abort the GSM call and connect to the
internet in GPRS to receive the packets from the internet peer.
NOTE:
Mobile device can be reachable from internet network only if the IP assigned by the operator is
public IP; not all operators offer this service.
To explain further the differences between CSD and GPRS an example application made in both ways
will be shown.
1.1.1 CSD application example
Let’s suppose you have several remote meteorological measurement units spread around the territory,
and you want to access them wirelessly through a GSM module in CSD operation.
For each remote unit, there's a modem to connect with the server application, with its own SIM card
and unique phone number.
Now there are two possibilities:
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the server application calls on demand the remote units, provided it has stored their phone
numbers in a private database.