When GSM was designed, the network and the voice service running over it were inseparable. That’s because the GSM network was designed in the 1980s as a mobile voice network. Fast forward 20 years and the picture had changed significantly with LTE having been designed as a wireless network that only transports IP packets. Agreed, it took another 10 years before Voice over LTE (VoLTE) was ready to replace the CS-fallback (to GSM and UMTS) mechanism for voice telephony. Now Voice over Wifi that is deployed by first network operators shows that VoLTE really is network independent.
VoLTE is based on the IP Multimedia Subsystem (IMS) which is located behind P-GW and the Gi interface, i.e. it knows nothing about the radio infrastructure. The only thing it does in terms of network interaction is to request special treatment for voice data packets when a call is established but does so in a neutral way.
As the IMS is behind the P-GW and thus network independent, nothing speaks against also using the system for voice calls over other networks as well. Case in point is the Voice over Wifi (VoWifi) service now deployed by first network operators. The only extra piece of equipment that is required is the (evolved) Packet Data Gateway (ePDG) that acts as an IPSec Server towards the Internet and as a MME/S-GW towards a network operator’s core network. On a mobile device the IMS stack that is used for VoLTE needs only little modification to also work over an IPSec tunnel (over Wifi) to the ePDG. As the ePDG acts as an MME/S-GW towards the core network it is even possible to hand-over a running LTE voice call (VoLTE) to a Wifi call (VoWifi) and vice versa. To the network it looks like an inter-MME/S-GW handover. On the mobile side a few extra things have to be implemented as moving an existing IP address from an LTE bearer into an IPSec tunnel and vice versa requires some software extensions. But compared to getting the whole VoLTE software stack up and running that is rather a benign challenge.
If you are interested in the details of how VoWifi is implemented in practice, have a look at the following specifications:
- 3GPP TS 23.402 is the IWLAN spec that describes how untrusted non-3GPP access can be connected to the LTE core network.
- Handover between LTE and Wifi: In case the UE wants to change from LTE to Wi-Fi an IPSec tunnel is established and the IP address used by the LTE IMS bearer can be moved into the tunnel (Chapter 4.7.1). The same is done when moving from Wifi to LTE by setting the request type to ‘handover’ in the PDN-Connectivity request.
- The initial attach scenario on S2b can be found in Chapter 7.2.4 and Figure 7.2.4-1. For details on the IPSec tunnel establishment, 3GPP TS 33.402 (Chapter 8.2) is referenced.