Applications using cellular data while WiFi is activated?

Discussion in 'iOS 7' started by staticoranges, Aug 17, 2013.

  1. macrumors newbie

    Aug 26, 2010
    So last night I was downloading some of my music from iTunes via WiFi with iTunes Match. I downloaded about 650MB of music, and now I am seeing that although my phone showed that I was connected to my home's WiFi at the time, I was charged for that data from Verizon. It is also showing up on the application "cellular data usage" settings pane.

    Am I hallucinating, or was this a problem in one of the earlier betas of iOS 7?

    Any ideas as to how I should resolve this?
  2. macrumors 6502


    Oct 26, 2010
    Airplane mode and turn on wifi... that is temporary method
  3. macrumors member

    Jan 1, 2013
    The Netherlands
    You have a few options:

    1. Do not enable Cellular Data Use in iTunes & app Store settings
    2. Buy better WiFi, so you stay connected
  4. macrumors 6502a


    Aug 8, 2006
    Pretty sure its a bug. I think I had something similar happen around beta 1 or 2. I have only 1 bar 4g at my house but 60/10 wifi and I'd notice downloads going REALLLY slow on my iPhone. Stinkin thing WAS connected to wifi, but was being stupid and using my 1bar of 4g ATT instead. Haven't noticed it since beta 2 though... (I turned off cellular data for itunes to be safe)
  5. macrumors member

    May 7, 2009
    Actually there are cases where this happens, even with iOS 6.

    A good example is WhatsApp. I have very bad cellular reception in my house so I rely almost entirely on Wifi. Still, sometimes I see that WhatsApp is extremely slow when sending messages. This only happens when I'm in my house (or another place where GSM reception is flaky).

    I have done some Wireshark traces on my Wifi traffic, and I've noticed that when WhatsApp becomes slow in sending messages, the traffic of those specific messages never shows up in the Wireshark traces. They are being sent over cellular.

    This is usually triggered if you close WhatsApp (without killing it - it remains active in the background for 10 minutes) and put your iPhone to sleep. Wifi will be disabled, so WhatsApp will reconnect through cellular.

    When turning the iPhone back on, Wifi re-enables, but WhatsApp sticks to the cellular connection unless you kill it.

    I have reported this as a bug to the WhatsApp authors, but it doesn't seem so easy to fix.

    The point why I'm saying this is because applications may use cellular while Wifi is activated - and that's not specific to iOS 7.
  6. macrumors 6502a

    Nov 9, 2010
    The analogy is that you have a Mac with both Ethernet and WiFi enabled.
    Both interfaces have different IP addresses, one from you WiFi and one from Ethernet. Both from a different subnet.

    One can configure routing entries to instruct the OS to send packets with a certain destination over one specific interface.

    The same is happening on the iPhone when Cellular Data is enabled.
    You get an IP from your carrier and also a list of subnets that should be routed via that interface.

    This means as long as you don't turn of cellular data, some traffic will always use it, regardless of WiFi. DNS for example is probably always using cellular because otherwise the private hostnames used by carriers could not be resolved.

    Now for the complete technical BS: When you write an app that connects to the network, you can enumerate the available interfaces and use one of them as the source for your packets. Most developers don't care, because per default the interface with the matching route and highest priority is used.

    In theory you could force WiFi only traffic on a per App basis if you really want to do so. On the downside you always have to repeat these steps when the network state changes.
  7. macrumors 601

    Jan 16, 2008
    Bristol, UK
    Isn't this a new feature in iOS 7?

    If the iPhone is unable to connect using the active Wi-Fi connection, data will be routed over the cellular network.
  8. thread starter macrumors newbie

    Aug 26, 2010
    I understand that although I am connected to WiFi, my LTE service will sometimes be in use as well (although I wish there was an option to disable that, because my WiFi is unlimited and much faster than the service at my home and I'd rather not waste my phone's data)

    The main problem here is that iTunes Match is a service that uses a TON of data, which is why there is a specific setting to only let it be active when there is WiFi available. Since I have that setting enabled, it is extremely irritating that I ended up downloading so much over cellular data. I feel like this is a bug, and since I was connected to WiFi, the phone thought "WiFi is enabled, therefore I can start downloading music" but then resorted to LTE as a backup.

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