Not getting any push notification when iphone is in sleep mode

Hello friends,

In my previous threads i talk about my problem that i faced with my iphone and now got replaced with new iphone which is working fine except

wifi issue which i had already faced earlier (with my first iphone)

The Problem is "I am enable to get any push notification including email, facebook, ebuddy e.t.c" when my iphone goes to sleep mode (locked)

my wifi disconnects after 20 sec when the device is locked (sleep mode) even if cellular data/3G is disable. again when i press home button and slide to unlock its automatically connect. In this situation i am not able to get even a single push notification when my phone is locked why?

I also found a thread saying that this happened after update of iOS 4.3.1 and my iOS is uptodate means 5.0.1 and apple did this to save battery but my question is if wifi will not work in that moment how would i get the push notification? its totally waste then?

please help me to solve this problem its fraustating 😟