Page 384 - From GMS to LTE

370 From GSM to LTE-Advanced Pro and 5G

devices, VoWifi functionality is fully integrated into the device, i.e. there is no separate
application the user has to install. Except for a small icon in the status bar or a different
network name, the use of Wi‐Fi instead of LTE for a voice call is transparent for the user.

5.5.1 VoWifi Network Architecture
Figure 5.16 shows how the LTE and IMS network has to be extended to offer VoWifi
services. The only new component that is required is the evolved Packet Data Gateway
(ePDG), which is specified in 3GPP TS 23.402 [27] and placed between the Internet on
one side and the core network and IMS platform of a network operator on the other
side. On the Internet side, the ePDG behaves like a Virtual Private Network (VPN)
gateway. From the core network point of view, the ePDG acts like an LTE MME (Mobility
Management Entity) and S‐GW (Serving‐Gateway), which were discussed in Chapter 4.
A mobile device connects to the core network and the IMS of its home network
operator over Wi‐Fi by establishing a VPN tunnel to the ePDG. The standard IPSec (IP
Security) protocol and mechanisms that are also used by other IPSec‐compliant VPN
servers (e.g. to connect remote workers to the Intranet of their companies) are used.
Chapter 8.2 of 3GPP TS 33.402 [28] describes which IPSec options are used for VoWifi.
Like in LTE and VoLTE, the secret key stored on the SIM card and in the HSS (Home
Subscriber Server) in the network (see Chapter 4) is used during ePDG tunnel estab-
lishment to mutually authenticate the subscriber and the network and to generate the
encryption keys for the VPN tunnel.
Figure 5.17 shows the message flow of a typical VoWifi ePDG IPSec tunnel establish-
ment. In the first step the mobile device has to get the IP address of the ePDG of the
subscriber’s home network. This is done by assembling a Fully Qualified Domain Name
(FQDN) and sending it for IP address resolution to any DNS server on the Internet. The
construction of the FQDN for ePDGs is standardized and includes the Mobile Country
Code (MCC) and the Mobile Network Code (MNC) of the user’s home network opera-
tor. The MCC and MNC are part of the user’s IMSI (International Mobile Subscriber

Internet SWx
Wifi IPSec ePDG
tunnel
S2b
PDN-GW
VoLTE + S5
VoWifi
capable UE MME SGi
LTE S-GW
Cx
P-CSCF I/S-CSCF HSS
Mw
ISC
MMTEL
(VoLTE)

Figure 5.16 VoWifi network architecture and the ePDG.