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My Twitter feed is full of people telling me about mysterious data usage over cellular networks after installing iOS 6 or acquiring an iPhone 5. Adam Engst already penned an article explaining potential causes and solutions for fast battery draining in iOS 6 (see “Solving iOS 6 Battery Drain Problems,” 28 September 2012). This may have some bearing on the unexpected cell data consumption, too, especially given that he tracked his problems to corrupted Safari bookmarks syncing constantly through iCloud, which could happen while away from a Wi-Fi network.

But many of the reports I’ve received are from people whose iPhones are set to use Wi-Fi, and the phones show a Wi-Fi network connection item when woken from sleep. One Twitter buddy, Anthony Hecht, says AT&T told him that when his iPhone is in “idle mode” (standby), it always reverts to cellular, which is wrong. AT&T customer service also told him to turn cellular data off (Settings > General > Cellular Data) whenever it’s idle, which is crazy making. He has seen 9 GB in unexpected mobile use, largely while at home based on his online charge breakdown, in just a week.

Many people attributed this problem to usage by Apple’s Podcasts app, which has been documented to exhibit bad behavior when downloading and streaming over cellular (see “Does Apple’s Podcasts App Suck Cellular Data?,” 17 September 2012). It can download the same podcast file repeatedly. Even after Apple added a switch in Podcasts 1.1 to restrict data use to Wi-Fi, my colleagues can still track cellular downloads with the app, especially if a download or streaming was already in progress when walking away from a Wi-Fi connection.

But several people have also eliminated Podcasts and other podcasting apps as culprits. They can see from their online data usage and from iOS’s tracking of cellular data (or by using DataMan) that the device chews through hundreds of megabytes of cell data over short periods of time, and they don’t know why. Josh Centers is in the middle of a quest to figure this out, and I expect others are as well.

John Herbert seems to have found one particular bad use case when iTunes Match will download over a mobile broadband network even when all the switches to use cellular data with Music and iTunes Match are flipped to Off. His entry on the topic explains how these settings are currently ignored when you start to download items from the cloud or have music downloads in queue.

Verizon has released a “carrier settings update”, which is supposed to deal with technical issues of connecting an iPhone to a given mobile network, and it apparently has to do with an iPhone 5 using the cellular data network instead of Wi-Fi even when connected to a Wi-Fi network. This doesn’t explain AT&T users’ problems nor those of people with earlier iPhone models experiencing the same data consumption.

Over at the Economist’s Babbage blog, I suggested that it’s hard to pin down blame when one can’t currently measure per-app use and thus figure out what’s going on. That was possible with DataMan Pro, which Adam Engst started testing for review before Apple pulled it from the App Store, but for most people trying to figure out what’s happening is completely frustrating — and expensive! This isn’t “CellularDataGate,” but it’s clearly affecting more than just a handful of people, and could involve folks paying tens or even hundreds of dollars in excess data usage because of what might be a bug in iOS 6 or Apple-provided apps.