GPRS Keep Alive is your answer. Developed as a keep alive solution for your PDA and connection.
GPRS Keep Alive is a small program that will run in background and ping up to five configurable hosts on a predefined time interval thus effectively preventing disconnection of some cellular modems.
One small program, one click solution! Easily minimized and kept out of the way. Can be configured to detect connections and auto start!
To use the program, first enter up to five hosts (eg. http://www.yahoo.com, http://www.google.com etc), check the checkbox in front of each host to include it in the list of hosts that will be processed (pinged) and turn the pinging on by either pressing the "Turn GPRS Keep Alive ON" button or by configuring automatic startup of the program.