With my new iPhone 4, I thought "I'll just switch to a 200MB data plan" since I mostly use WiFi and have rarely had a month of greater than 150MB data usage.
What a mistake. With no apps running in the background, Push turned off and Notifications turned off, according to the myWireless AT&T app, my phone is using between 5 and 50 MB of data per night. The usage occurs between 11pm and 3am MT. Upon waking the phone up, it shows it is connected to WiFi, but I get a momentary flicker of the 3G icon. That's what makes me suspicious it is using 3G instead of WiFi when it goes to sleep. That behavior is contrary to documentation.
Articles and Links
There's no shortage of material to point at extolling variations of this problem. Some are old -- as far back as 2008 -- but the post frequency and number of new threads has increased, centered around the iPhone 4 launch and anyone transitioning to a 200MB data plan.
An Apple forum 3+ page post on iPhone 4 using Cellular Data when on standby and connected to wifi.
The 10 page Apple discussion entitled "Unknown data usage early morning".
An AT&T forum thread entitled "iphone 2am unauthorized data usage"
Another AT&T thread from 2009 stating similar complaints on iPhone OS 3.0
A claim that it is just a usage tally (which still doesn't explain the days I never leave the office WiFi).
"iPod touch can stay connected to Wi-Fi when asleep so you can receive incoming VoIP calls and notifications from compatible third-party apps." So, WiFi should stay connected.
"There’s no word on whether this applies to the iPhone as well (since cellular data is typically persistent anyway), but it would be a nice feature to have in all iOS devices — including iPad, especially when roaming with data turned off, or even just to prevent the delays that usually accompany reconnecting to Wi-Fi based remote control apps."
Older, 2008 post claiming "Wi-Fi is usually turned off a little while after in standby."
Conclusion
The only logical explanation is that everyone is just holding it the wrong way, thereby causing it to use more data on 3G than it should.
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