Source: nytimes.com
Q. Can you tell me how Wi-Fi Assist works and does it use my iPhone data?
A. Apple’s Wi-Fi Assist feature is designed to keep your iPhone or cellular-model iPad connected to the internet even when the device’s connection to a wireless network is weak or unstable. To keep the connection, the Wi-Fi Assist function (available on the iPhone 5 and later) automatically switches to your device’s cellular connection. So, yes, the feature does consume part of the monthly data allowance you receive from your phone carrier.
Wi-Fi Assist does not use the backup connection from your cellular plan for everything. It does not kick in for apps that stream or download large files, and it does not enable itself automatically if you are traveling and data-roaming on other carriers’ networks. Wi-Fi Assist is supposed to work for apps actively running, not programs that may periodically download new content in the background.
When iOS 9 was released last fall, Wi-Fi Assist was turned on by default and caused much dismay from users with limited data plans whose monthly allowances were chewed up when the device automatically switched to the cellular signal for extended periods. If you are worried about conserving the data allowance doled out by your cellular carrier, you can turn off Wi-Fi Assist and other bandwidth-hogging apps in the iPhone’s settings.
To do that, open the iPhone’s Settings icon from the Home screen, select Cellular and scroll down to the bottom of the page until you see Wi-Fi Assist. Tap the button next to it to disable the feature. In iOS 9.3 and later, the Wi-Fi Assist setting shows how much cellular data it has used, which can be helpful if you want to use the feature to stay online but want to keep an eye on the data meter.
