Page 387 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 373

the IMS bearer from one radio access technology to another is completely transparent, as
the mobile device keeps its IP address and IP packets in the network operator’s core net-
work are simply routed in a different direction. While not required, the IMS and VoLTE
TAS server is nevertheless informed by the mobile device when the radio access changes
by performance of an IMS re‐registration with a P‐Access‐Network Identifier parameter
that contains information as to the radio network technology now in use. It is important
to realize, however, that this IMS registration has no impact on an ongoing VoLTE call
and the call would not be interrupted even if the re‐registration was not performed.

5.5.3 Wi‐Fi‐Preferred vs. Cellular‐Preferred
In practice, there are two modes of operation from a mobile device’s point of view in
which Voice over Wi‐Fi can be used. In the cellular‐preferred mode, the 2G, 3G or LTE
network is preferred for voice calls even if the device is connected to Wi‐Fi. The Wi‐Fi
network is still used for Internet connectivity, however. Connectivity to the ePDG over
Wi‐Fi is only established if no cellular network is available, e.g. in basements or rural
areas. One advantage of this approach is that over a cellular network the quality of an
ongoing call can be ensured while network coverage is present. Once the voice call is on
Wi‐Fi, quality can no longer be assured as voice packets in the local Wi‐Fi network are
usually not preferred over other packets. This is especially problematic when the Wi‐Fi
network is connected to a DSL or cable line with very limited uplink capabilities. Voice
quality and delay deteriorate noticeably as soon as other network users start transfer-
ring larger amounts of data in the uplink direction. The downlink speech path can also
be affected if, for example, a video‐streaming session takes up most of the available
downlink bandwidth. One major disadvantage of the cellular‐preferred approach is that
a handover of an ongoing voice call can only be performed between LTE and Wi‐Fi. If a
voice call is established over 2G or 3G and network coverage fails, a handover is not
possible as the voice call over 2G or 3G is a circuit‐switched connection, as described in
Chapter 1 (GSM) and Chapter 3 (UMTS), and therefore not based on IP. As a conse-
quence the voice call drops despite Wi‐Fi being available. Even if a voice call is estab-
lished on LTE it is possible that the network performs an SRVCC handover of the call to
2G or 3G before the mobile device initiates a handover of the call to Wi‐Fi.
Another VoWifi handover approach is to connect to the ePDG as soon as Internet
connectivity over Wi‐Fi becomes available. In this Wi‐Fi‐preferred mode, voice calls are
made over Wi‐Fi even if a cellular network is available. While this avoids potential call
drops when a device moves from cellular to Wi‐Fi as described above, the user is prone
to voice call quality issues when the Wi‐Fi network’s backhaul connectivity is over-
loaded. Which of the two VoWifi modes is used in a cellular network depends on the
operator’s preference and the mobile device implementation and configuration options
available to the user. While some network operators might prefer the Wi‐Fi‐preferred
option, other network operators might prefer the cellular‐preferred option. Again
depending on the relationship between network operator and device manufacturer, the
user may or may not get the option to select between the two modes.

5.5.4 SMS, MMS and Supplementary Services over Wi‐Fi
In addition to IMS voice services, the VoWifi solution also tunnels other network opera-
tor services through the IPSec connectivity to the ePDG. When a device is registered in