Wednesday, 21 January 2015

Voice over WiFi (VoWiFi) technical details

VoWiFi is certainly a hot topic, thanks to the support of VoWiFi on iPhone 6. A presentation from LTE World Summit 2014 by Taqua on this topic has already crossed 13K views. In this post I intend to look at the different approaches for VoWiFi and throw in some technical details. I am by no means an expert so please feel free to add your input in the comments.

Anybody reading this post is not aware of S2a, S2b, Samog, TWAG, ePDG, etc. and what they are, please refer to our whitepaper on cellular and wi-fi integration here (section 3).

There are two approaches to VoWiFi, native client already in your device or an App that could be either downloaded from the app store or pre-installed. The UK operator '3' has an app known as ThreeInTouch. While on WiFi, this app can make and receive calls and texts. The only problem is that it does not handover an ongoing call from WiFi to cellular and and vice versa. Here are a few slides (slides 36-38) from them from a conference last year:

The other operators have a native client that can use Wi-Fi as the access network for voice calls as well as the data when the device is connected on the WLAN.

A simple architecture can be seen from the picture above. As can be seen, the device can connect to the network via a non-3GPP trusted wireless access network via the TWAG or via a non-3GPP untrusted wireless access network via ePDG. In the latter case, an IPSec tunnel would have to be established between the device and the ePDG. The SIM credentials would be used for authentication purposes so that an intruder cannot access ePDG and the core.

Now, I dont want to talk about VoLTE bearers establishment, etc. which I have already done here earlier. In order to establish S2a (trusted) and S2b (untrusted) connection, the AAA server selects an APN among those which are subscribed to in the HLR/HSS. The PDN-GW (generally referred to as PGW) dynamically assigns an IP address out of a pool of addresses which is associated with this APN. This UE IP address is used by the VoWiFi SIP UA (User Agent) as the contact information when registering to the SIP soft switch (which would typically be the operators IMS network).

If for any reason the SIP UA in the device is not able to use the SIM for authentication (needs ISIM?) then a username/password based authentication credentials can be used (SIP digest authentication).

Typically, there would be a seperate UA for VoLTE and VoWiFi. They would both be generally registering to the same IMS APN using different credentials and contact addresses. The IMS network can deal with multiple registrations from the same subscriber but from different IP addresses (see 3GPP TS 23.237 - 'IMS Service Continuity' for details).

Because of multiple UA's, a new element needs to be introduced in order to 'fork' the downstream media streams (RTP/RTCP packets) to different IP addresses over time.

3GPP has defined the Access Transfer Gateway (ATGW) which is controlled by the Access Transfer Control Function (ATCF); the ATCF interfaces to the IMS and Service Centralization and Continuity Application Server (SCC AS). All these are not shown in the picture above but is available in 3GPP TS 23.237. The IMS networks in use today as well as the one being deployed for VoLTE does not have ATGW/ATCF. As a result vendors have to come up with clever non-standardised solutions to solve the problem.

When there is a handover between 3GPP and non-3GPP networks, the UE IP address needs to be preserved. Solutions like MIP and IPSec have been used in the past but they are not flexible. The Release-12 solution of eSAMOG (see 3GPP TS 23.402) can be used but the solution requires changes in the UE. For the time being we will see proprietary solutions only but hopefully in future there would be standardised solutions available.

3GPP TS 23.234 describes more in detail the interworking of 3GPP based system and WLAN. Interested readers can refer to that for further insight.


Yeshwant Bhat said...

Currently I am involved in the solutions provided by one of the operators. It is still early stages.
The idea is that the preferred option for VoIP is LTE. Wi-Fi is the option when there is no LTE coverage.
There is also a provision for handover between Wi-Fi and LTE and Viceversa.

In this regard it seems that the UE can be IMS registered only from either LTE or from Wi-Fi. It may not be a limitation of the IMS per se but the limitation of the PGW in terms of not being able to have multiple PDN connections towards the same APN.

So essentially the UE is IMS registered either from LTE or from Wi-Fi.
It can initiate a call in Wi-Fi and then the call can be handed over to LTE. The UE maintains the same IP address across the LTE and Wi-Fi which is assigned by the EPC.
During the Handover the UE may have to re-register with the IMS network.

I would like to hear from others as to what the standards say, how far we are in terms of standardization and so on ...

MG Subba Krishna said...

In most of the discussions we see any WIFI interworking is often considered, in conjunction with ePDG or PDG or TWG.

But, if we have a fiber connection or a DSL connection for internet at home, and a cell phone enters the residential premises, the UE automatically hands into the WIFI network of the home.

The 4G VoLTE call if its on going will get disconnected in the premises of the house. Anyway to get over this??

Zahid Ghadialy said...

Hi MG Subba Krishna,

I tried it out in different scenarios at my home with VoLTE, VoWiFi, etc. and I did not get any failures at all.

Scenario 1: Start VoWiFi call at home and walk away, call automatically hands over to VoLTE

Scenario 2: Opposite of Scenario 1, start VoLTE call and enter home. The call continues on VoLTE and WiFi is connected for data

Disable VoWiFi

Scenario 3: Start a VoLTE call. WiFi is being used for data and VoLTE for voice. Walk away where there is no WiFi, call continues normally

Scenario 4: Start VoLTE call on macro, walk into home. Voice call continues normally while the phone connects to WiFi for data.

I did this test over UK operator EE with iPhone 6S.

Where did you notice your problem? Which network, which device?

MG Subba Krishna said...

I use Reliance JIO in india. My phone doesnt have VoLTE enabled. So, the 4G voice call was not getting connected. So its my bad. Apologize you for that.

Anonymous said...

when the UE moves to WiFI Realm [ ePDG ], will the LTE Radio bearers or the LTE Radio be in place? or the RRC will be completely disconnected?

wificallingblogger said...

Hello everyone,

For one of critical demo, i am trying to setup wifi calling in the Lab (all nodes ePDG, HSS/AAA, PGW etc) . I am using the Samsung phone (production build).

1. EAP-AKA has been selected
2. HSS also has EAP-AKA
3. ePDG has EAP only

With the above setup, ePDG is sending Authentication and Authorization request to HSS/AAA but HSS/AAA responding with identity mismatch.

Trying to disable the authentication in the device so it can bypass the authentication.

I can update any config required in ePDG or HSS/AAA so authentication can be bypassed. I need your expert inputs. Appreciate quick response as demo is tomorrow and trying all the possibilities.

Lakmal Nishanta said...

Hi Every one,

Do you know how to obtain the URL for ePDG. It should be of the format ""

laxman said...


Im trying to setup VoWiFi in my lab. The ue got authenticated and IPSEC tunnel has been established and the ue traffic is passing thru PGW. The UE is also getting ip address from p-cscf. My question is that, UE is not initiating sip request to P- CSCF. Should we configure any parameter in the SIM or UE so that SIP request will be initiated?

mohit lala said...

hi laxman is your problem resolved? how we can take traces in UE for this kind of problem?