Android Question How 3G/GPRS operate when the app is in the background and the screen is off

Hi
My application connect via GPRS/3G and sends data through Socket, the 3g/gprs is always open
while my application is in the foreground and the screen is on operate normally
when my application gets into the background and screen is on, not connect to the host
when my application is in the foreground and the screen is off also not connect to the host

If device operate via WiFi my application operate normaly in all situations (Foreground, BackGround, even if Screen is off)

I want my application send data to the host all the time when my application is in background or monitor is off
How do that ?
Thanks