Page 383 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 369

to allow the establishment of dedicated bearers over the IPX roaming exchange network
and that the visited network supports the establishment of dedicated bearers.
Handing over an ongoing VoLTE call to GSM or UMTS in a visited network with the
Single Radio Voice Call Continuity (SRVCC) function works as follows. As described
above for SRVCC in the home network, the procedure is triggered by the eNodeB when
it notices that LTE radio conditions are deteriorating and when a GSM or UMTS cell is
available to continue the call. The eNodeB is aware that the subscriber is engaged in a
voice call as a dedicated bearer with QCI 1 is established. As a consequence it sends a
speech call handover request to the MME. The MME in the visited network then con-
tacts the local Mobile Switching Center (MSC), which has to be enhanced, like in the
home network example above, with the Sv interface so that it can receive the handover
request. Part of the handover request is the Session Transfer Number for SRVCC (STN‐
SR), which the MSC has received from the home network’s HSS during the LTE attach
procedure. Like the Mobile Station Roaming Number (MSRN) described in Chapter 1
for GSM, the STN‐SR is a number that temporarily identifies a subscriber. In this con-
text it is used by an MSC like a ‘telephone’ number to establish a speech path to another
circuit‐switched entity, i.e. an MSC, or, in this case, the IMS and a circuit‐switched
media gateway in the home network. In the home network, the IMS and media gateway
recognize from the ‘telephone number’, i.e. the STN‐SR in the SS7 IAM (Initial Address
Message, see Chapter 1) call establishment message, that the incoming call is for an
SRVCC procedure for the subscriber identified by this number. The IMS in the home
network then redirects the speech path to this media gateway. For further details see
3GPP TS 23.216 chapter 6.2.2 [25].
A shortcoming of this SRVCC approach is that the traditional SS7 ISUP/BICC signal-
ing between the visited MSC and the IMS/media gateway in the home network has to
be used. This only allows pre‐3GPP Release 10 SRVCC without the Access Transfer
Gateways in place as described above. As a consequence the home network IMS must
support two kinds of SRVCC procedure. Furthermore, advanced SRVCC procedures
such as a handover during the alerting phase, handover of conference calls, etc. are not
possible.

5.5 Voice over WiFi (VoWifi)

As discussed at the beginning of this chapter, the IMS service for LTE (VoLTE) has been
designed in such a way as to be as independent as possible of the LTE core and espe-
cially of the LTE radio network. Except for the interface to request application of qual-
ity‐of‐service measures, VoLTE is indeed independent of the network. To extend the
reach of voice services beyond cellular access networks, 3GPP has specified the means
to also use the VoLTE voice service over ‘untrusted non‐3GPP networks’, i.e. the Internet.
As smartphones and other devices typically have access to the Internet over a Wi‐Fi
interface, this extension is referred to as ‘Voice over Wi‐Fi’ or VoWifi for short. At the
publication date of this edition quite a number of network operators around the world
have launched the service. It is important to note at this point that while variants of
Voice over Wi‐Fi are offered by network operators and Internet‐based third‐party ser-
vices, this section discusses the VoWifi variant that is fully integrated into a network
operator’s IMS core and VoLTE service, as described in GSMA IR.51 [26]. On mobile