Wifi connection doesn't stay alive when asleep, with iOS4

Knowing that iOS4 support wifi stay alive in asleep is really happy thing. However I got som开发者_高级运维e different experience: I've got 2 iPods, iPod 2nd gen & iPod 3rd gen. Both updated to iOS 4 beta. iPod 2g can stay wifi on in sleep while iPod 3g always disconnect wifi in sleep. I ping both of them while they're in sleep mode, only iPod 2g keeps reply echo.

Could any often a hint to show the point I'm doing wrong?

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Test with adding this on your plist :

UIRequiresPersistentWiFi = YES

With this the wifi is alive during all the life of your app.