Page 373 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 359

[…]
</cp:ruleset>
</communication-diversion>
In this example the no‐reply timer has been set to 25 seconds and the number to
forward the call to has been configured to ‘+443397788990’. The rule contains a ‘rule‐
deactivated’ element which means that the call forwarding settings are currently
not used.
To change settings on the TAS server a HTTP ‘PUT’ request is sent as shown in the
following example:

PUT /simservs.ngn.etsi.org/users/sip:+443393144238@ims.telekom.de/
simservs.xml/~~/simservs/communication-diversion?xmlns(cp=urn:ietf
:params:xml:ns:common-policy) HTTP/1.1
The body of the HTTP ‘PUT’ request then contains the same XML information as
shown above for the HTTP ‘GET’ request. To activate the call forward no‐reply feature
the ‘<rule‐deactivated/>’ parameter is omitted.

5.3.14 Single Radio Voice Call Continuity
Despite the wide deployment of LTE in recent years, GSM and to some extent also
UMTS networks still have better geographical coverage. As a consequence a mecha-
nism is required to transfer a VoLTE call to UMTS or GSM.
Handing over a call from UMTS to GSM is relatively simple because a circuit‐switched
connection in UMTS is transferred to a circuit‐switched connection in GSM. As VoLTE
is based on IMS and IP, however, it is necessary to hand over an ongoing IP‐based voice
call to a circuit‐switched UMTS or GSM channel. As a device cannot be active in LTE
and UMTS/GSM at the same time (Single Radio), a solution referred to as Single Radio
Voice Call Continuity (SRVCC) has been designed and improved in the standards over
the years. SRVCC is specified in 3GPP TS 23.237 [19]. The following description is
based on the SRVCC solution described in 3GPP Release 10. In practice, network
operators can also implement earlier versions, which, however, work in a largely simi-
lar way.
Figure 5.12 shows which IMS components are involved in a VoLTE handover to
UMTS or GSM. As before, the P‐, I‐ and S‐CSCF servers are responsible for establishing
and maintaining the voice session. In addition to the MMTEL Application Server, a
Service Centralization and Continuity Application Server (SCC‐AS) is involved in the
call establishment phase to collect all necessary information about a session in order to
be prepared to hand over a voice call to the circuit‐switched network as quickly as pos-
sible should this become necessary. The circuit‐switched network is represented in
Figure 5.8 by the MSC‐Server (MSC‐S) and the Media Gateway (MGW), which were
introduced in Chapter 1.
To enable transfer of a voice call as quickly as possible, voice data packets are no
longer exchanged directly between the two mobile devices but are instead led over an
Access Transfer Gateway (ATGW). The gateway is controlled by the Access Transfer
Control Function (ATCF), which is part of the P‐CSCF.
Figure 5.13 shows how a voice call is redirected in the network during an SRVCC
handover. In general the connection is established as described before. The difference