Page 388 - From GMS to LTE

374 From GSM to LTE-Advanced Pro and 5G

the VoLTE network, either over LTE or over Wi‐Fi, SMS messages are delivered over
SIP. This means that SMS messages are not delivered over 2G, 3G or LTE once the
device has registered for VoLTE voice services over Wi‐Fi. No enhancements on the
device or in the network are required for this as the type of radio access network is
completely transparent to the IMS.
Supplementary services such as changing the call forwarding settings use the IP‐
based XCAP protocol and are typically sent over the default bearer for Internet access
rather than the IMS default bearer. As the VoWifi solution only tunnels the IMS default
bearer over the default IPSec tunnel to the ePDG it is thus not possible to change the
supplementary service settings over this connection. It is therefore necessary that the
mobile device establish a temporary second IPSec connection to the ePDG with a dif-
ferent Access Point Name (APN), over which the XCAP traffic is exchanged. Once the
supplementary service interaction has finished the temporary IPSec tunnel is removed
again. In practice a temporary second IPSec tunnel is also used for sending and receiv-
ing MMS messages when a VoWifi connection to the ePDG exists.

5.5.5 VoWifi Roaming
In principle VoWifi is an IP‐based service over the Internet and hence from a technical
point of view is not limited to the country of the subscriber’s network operator. In prac-
tice, however, there are a few differences in the use and operation of the Voice over Wifi
service at home and for roaming abroad.
The only technical limitation for roaming abroad is that VoLTE roaming is typically
not deployed in practice yet. This means that it is not possible to perform a handover
of an ongoing VoWifi call to a visited LTE network abroad. Unless the mobile device
is certain that VoLTE roaming is available in the visited LTE network it must therefore
not try to send a PDN Connectivity Request with the request type set to ‘handover’.
As a consequence a VoWifi request will drop when the user leaves the Wi‐Fi cover-
age area.
A non‐technical limitation of the VoWifi service abroad is that some network opera-
tors prohibit IMS connectivity over the ePDG and the Internet if they detect that the
user is not in their home country. This can for example be determined by the network
operator by analyzing the source IP address from which the ePDG connection estab-
lishment is attempted or by checking in the Home Subscriber Server if the subscriber
was last seen in the 2G, 3G or LTE network at home or abroad. Whether it is wise to
block subscribers from using the service abroad is, in the light of competing non‐operator
voice services, more than questionable.
As VoWifi is a service operated by a telecommunication network operator, some
countries may require that all calls originated in their country must be interceptable by
local law enforcement. As this is not possible if the mobile device establishes an IPSec
tunnel to the ePDG in the subscriber’s home country, 3GPP has defined a number of
ways for the mobile device to determine if it is required to connect to an ePDG in the
visited country or if connectivity to the home ePDG can be used, as described in 3GPP
TS 23.402, chapter 4.5.4. If an ePDG has to be used in the visited country, the foreign
ePDG has to be connected to the home network. In practice it remains to be seen
whether devices will simply attempt to connect to the home network ePDG or if such
rules will actually be implemented and used in practice.