VoWiFi: Why the ePDG is designed to be behind the PGW?

But @needoos.th, the whole point of VoWiFi has come for the use case where LTE coverage is not there or weak.

Yes, VoWiFi registration fails if the UE is not registered in LTE.

Weak coverage, but LTE registration is necessary.


I assume the MME latches with the HSS over S6a interface for LTE, while the AAA latches with the HSS over S6a Interface for WiFi.
So, LTE might not be required at all for VoWiFi to work.
Anyways, I am a newbie to VoWiFi.
Suggestions and Corrections appreciated.

I am quoting based on experience, never seen it working anywhere.
Lot of time I have been at places where there is full wifi coverage and no network.
No one was able to reach me😁.

AAA has an interface with PGW that possibly is to get IP assigned to UE for communication with IMS.

Other problem may be with emergency calls , you cannot locate users based on wifi connection.


Oh yeah, IP address allocation is one thing. This might be the reason ePDG is behind PGW. Convincing.

@Pritam.Nandy, both should be independent enough na… my device (Vivo) is supporting both independently.

See the WiFi calling symbol beside the WiFi symbol. This is when “VoLTE HD call” is off and “WiFi calling” is alone ON.

Do you mean if you disable LTE Network, your VoWiFi Registration is still through in your UE?
Just FYI, in my case it is not.

VoWiFi is allowed even when no cellular for most operators.
In case operator doesn’t allow VoWiFi without cellular coverage, it can provision that as well.

This is all operator and regulatory requirements.

Can you show the VoWiFi icon in your notification bar with LTE turned OFF?

It works even in limited service, you can check Jio in India, Airtel doesn’t.

Airtel is working with total Data disabled, VoLTE disabled and VoWiFi alone enabled.

Do you know any operator (across globe) which does not require this?

I have checked with Jio only.

Data disabled, VoLTE disabled… don’t mean no LTE registration.
When you don’t see UE registered (no signal) then is the case - not you manually disabling some feature in phone.
It is required for user location tracking for emergency calls.
Without LTE registration, VoWiFi call will be made from any corner of the country without emergency services able to locate user.

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Yes @RMatolia, should be. That is the sole purpose of VoWiFi.

Your data is still enabled, can you disable Data Completely and try again?

Data is disabled only.

Many, need to check though.

Turn ON APM Mode, since Jio is all IP Network even if you turn OFF data VoLTE works, although yours does not seem Jio (‘E’ icon visible).

In my case with Airtel (and same for Jio too), with LTE Registration disabled, as you can see even though WiFi calling feature is turned ON but there is no VoWiFi registration, this is what I meant.

Also it depends upon UE vendor implementation, half of it.
If you see VoWifi Registration request, then NW might be rejecting, in case not, UE implementation to not send such request.

Vo-WiFi actually a big relief for operators and customers as well.

As we know indoor coverage is a big challenge for operators and even people don’t allow them to put sites in residential areas due to EMF awareness.

Hence they have this solutions where users can do voice calls through their own fiber or wifi solutions.
So there will be less load on outdoor sites and customers perception won’t be poor.