Is wifi used when cellular data is turned on?

[macstatic]

If I have both cellular data and wifi turned on, will the phone choose wifi, or continue using cellular (since it was already on before I turned on wifi)?

Whenever I connect to wifi I try to remember to also turn off cellular data, but perhaps I don't need to do this?
(I try to save on cellular data usage whenever I can, especially with large videos, downloading new apps etc.)
I'm on iOS 15.7.

 

[russell_314]

If I have both cellular data and wifi turned on, will the phone choose wifi, or continue using cellular (since it was already on before I turned on wifi)?

Whenever I connect to wifi I try to remember to also turn off cellular data, but perhaps I don't need to do this?
(I try to save on cellular data usage whenever I can, especially with large videos, downloading new apps etc.)
I'm on iOS 15.7.

No that’s silly, you don’t need to turn off cellular data. If you want to test it, just disconnect your Wi-Fi router from the Internet. You shouldn’t get any connection and it will ask you do you want to use cellular at some point.

Just go under cellular settings, cellular, then scroll all the way down and make sure “Wi-Fi assist” is off

 

[macstatic]

It worked differently here.....
If I have wifi AND cellular data both turned on the phone connects to wifi (if the wifi router is turned on).
If however I turn off my wifi router (thereby losing wifi access) the phone switches over to 4G (I can see the "wifi" icon in the upper left section of the screen change to "4G"). And I've checked to see that Settings-Mobile data-Wifi assist is turned off.

 

[0128672]

If I understand what you're describing, it's normal. Your home or work wi-fi network will be prioritized over cellular unless you turn off wi-fi. There's no need to turn off cellular to prioritize wi-fi. Although I don't know specifically for your country, I would be fairly certain it works the same way in most places.

When you're out of range of your wi-fi, and want to limit the cellular data used, you can turn off cellular data for specific apps by going to Settings > Cellular (or maybe Mobile Data in your country). If cellular is disabled for an app and you try to use it, you'll be prompted to turn on cellular for that.

View or change cellular data settings on iPhone

On iPhone, turn cellular data and roaming on or off, set which apps and services use cellular data, and set other cellular options.

support.apple.com

Use cellular data on your iPhone or iPad - Apple Support

Find out how much cellular data that you're using.

support.apple.com

 

[macstatic]

Ah!! Thanks for clearing that up.
Yes, I tried turning OFF cellular data for Safari, surfed (via wifi), then turned wifi off, and sure enough: a message pops up saying:

Mobile data is turned off for Safari
You can turn on mobile data for this app in settings
[Settings] [OK]

So if I'm understanding it right, this is the way wifi vs. cellular data works on an iPhone:

1. Apps can be allowed or NOT allowed to use a cellular data connection (Settings-Mobile data)
2. For apps that are NOT allowed to use a cellular data connection it obviously makes no difference if the phone's cellular data is enabled on or not. Those apps will regardless ALWAYS use wifi for going online (and when wifi is unavailable they won't be able to go online at all).
3. Apps that ARE allowed cellular data will:
   • Use cellular data whenever the phone's cellular data is ENABLED, and wifi is DISABLED
   • Use cellular data when the phone's cellular data is ENABLED, wifi is DISABLED, but no wifi signal has been made (i.e. the router is turned off or out of range or you don't have the correct password)
   • Use wifi when the phone's cellular data is ENABLED, wifi is ENABLED and a wifi connection has been ESTABLISHED
   • Use wifi when the phone's cellular data is DISABLED, wifi is ENABLED and a wifi connection has been ESTABLISHED

Does that sounds about right?
To simplify things I've learnt that the simple way to ensure the connection prior to streaming videos, download apps, update iOS or some other large data, is to always take a look at the upper left side of the screen and see if it says "4G" or if that's replaced with a wifi icon. That way you'll always know if the data connection is currently using cellular, wifi or nothing at all (no "4G" or wifi icon is displayed).

I'm still a little confused about what "Wifi assist" actually does and in which situation it makes a difference. I'll look closer at the links you posted to see of that's explained.

 

[0128672]

Ah!! Thanks for clearing that up.
Yes, I tried turning OFF cellular data for Safari, surfed (via wifi), then turned wifi off, and sure enough: a message pops up saying:

Mobile data is turned off for Safari
You can turn on mobile data for this app in settings
[Settings] [OK]

So if I'm understanding it right, this is the way wifi vs. cellular data works on an iPhone:

1. Apps can be allowed or NOT allowed to use a cellular data connection (Settings-Mobile data)
2. For apps that are NOT allowed to use a cellular data connection it obviously makes no difference if the phone's cellular data is enabled on or not. Those apps will regardless ALWAYS use wifi for going online (and when wifi is unavailable they won't be able to go online at all).
3. Apps that ARE allowed cellular data will:
   • Use cellular data whenever the phone's cellular data is ENABLED, and wifi is DISABLED
   • Use cellular data when the phone's cellular data is ENABLED, wifi is DISABLED, but no wifi signal has been made (i.e. the router is turned off or out of range or you don't have the correct password)
   • Use wifi when the phone's cellular data is ENABLED, wifi is ENABLED and a wifi connection has been ESTABLISHED
   • Use wifi when the phone's cellular data is DISABLED, wifi is ENABLED and a wifi connection has been ESTABLISHED

Does that sounds about right?
To simplify things I've learnt that the simple way to ensure the connection prior to streaming videos, download apps, update iOS or some other large data, is to always take a look at the upper left side of the screen and see if it says "4G" or if that's replaced with a wifi icon. That way you'll always know if the data connection is currently using cellular, wifi or nothing at all (no "4G" or wifi icon is displayed).

I'm still a little confused about what "Wifi assist" actually does and in which situation it makes a difference. I'll look closer at the links you posted to see of that's explained.

Yes, this sounds correct. In day-to-day usage there's really no reason to disable wi-fi or mobile data/cellular. Your phone will use wi-fi if it's in range; otherwise it will use cellular (also if in range).

Wi-fi assist means if you wi-fi connection is poor, it will automatically switch to cellular. This doesn't really matter if you have an unlimited data cell plan, but for people with data caps, they might want to disable wi-fi assist. Here's more info about that:

About Wi-Fi Assist - Apple Support

With iOS 9 and later, you can use Wi-Fi Assist to automatically switch to cellular when you have a poor Wi-Fi connection.

support.apple.com