Using GPRS and TCP/IP stack with SEM GSM modules
LZT 123 7699 R1A
9
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.
