Guys, I have a basic question on the VoWiFi untrusted architecture.
Why the ePDG is designed to be behind the PGW?
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Make it as a secure gw and act like a firewall through trusted entity.
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That trust and security could have be achieved even if it is a GW which can directly talk with the P-CSCF too, right?
Could it be something to do with the dedicated bearer to be created and the UE to acknowledge?
To implement security with help of IPSec tunnel between the UE device and the Mobile Packet Core.
The ePDG plays vital role as this IPSEC tunnel coming from UE via Internet terminate on ePDG.
This ePDG further pass this traffic to PGW via s2b Interface.
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Yes I understand that.
Should go through the documentation.
Why can’t ePDG would have been designed to manage IPSec tunnel with UE and then directly deliver to P-CSCF?
Then users will be calling through VoWifi even when no LTE connection is not there.
VoWifi should only work when user has active LTE connection
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Hmm… @Tech_Playon. I understand that VoWiFi should work even if LTE coverage is not there (infact, no 3GPP coverage at all - no 2G, no 3G, no 4G network).
The WiFi can belong to any other backhaul, not relevant to the VoLTE operator at all.
See ePDG comes from wireline side and EPC comes from mobility side, s2A is the interfacs which makes Radius or HSS function.
The UE should be supporting VoWiFi capable with a USIM/ISIM.
Both are different purpose and app stacks.
Yes brother, I agree that.
But why can’t the ePDG be deployed closer to the IMS platform, why should it be behind the PGW, was my prime question.
It can’t support IMS protocols.
Like HLR has MAP and HSS has diameter.
Anyways, it should be a simple GW.
Receive from UE (over IPSec let’s say) and relay the SIP/RTP packets to the IMS platform, the P-CSCF.
Epdg can suport through encapsulation only.
My point is why ePDG should be behind PGW, even PGW supports through encapsulation only.
Does pgw support wifi protocols? radius?
It might not.
Let the ePDG support WiFi and AAA with HSS.
I am not denying that.
Why they complicated ePDG to have GTP to talk to PGW?
If they would have bypassed PGW, then it would be a thinner protocol stack for the ePDG na… like the below:
It’s standards not an engineering.
I agree, I might be missing the Core functionalities of an ePDG, that’s why not able to appreciate the standards.
I too will check and come back.
Thanks for the clarifications. 
I doubt UE will allow to call when it is not registered with network.
I have never seen VoWiFi working with no LTE registration.