How does iOS handle a connected Wi-Fi network that loses internet access?

Question

I'm trying to get my head around how iOS deals with a known Wi-Fi network that suddenly loses connectivity to the internet. If I'm connected to a network that's functioning normally and the network suddenly loses access to the internet (but the network SSID is still being broadcast), iOS appears to fall back to a cellular connection (if available); the Wi-Fi bars completely disappear from the status bar. When the Wi-Fi network regains internet access, however, the Wi-Fi icon then re-appears in the iOS status bar.

In this case, does iOS in fact stay connected to the Wi-Fi network in question but recognises the fact that no internet connectivity exists so falls back to the cellular network and periodically checks for internet access from the Wi-Fi network, or does this work in a different way entirely?

Answer 1

Yes, in this case iOS does stay connected to the WiFi network and periodically checks for internet connectivity.

Answer 2

Yes, iOS stays connected to the Wi-Fi network and the cellular network, but prioritises packets to go through the cellular connection until the Wi-Fi network starts carrying packages in a reliable way. This is a capability of the underlying lower-level OS, and macOS can also do this, for instance having a computer connected both to a wired and wireless connection (the network connection precedence order is defined manually by the user). The following Apple Support link describes how Wi-Fi Assist works.

Bear in mind that Wi-Fi Assist will not switch to the cellular connection if you are not actively trying to connect to the internet through a foreground application, if you're data roaming, or with large files or audio/video streaming. In all these cases the potential for a high cellular data usage — with little gain to the user — makes the OS decide not to engage with the cellular network.