Page 371 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 357

Figure 5.11 An SMS message being sent over SIP. Source: Gerald Combs / Wireshark.

5.3.13 Call Forwarding Settings and XCAP
Traditional mobile telephony offers a number of supplementary services such as
different kinds of call forwarding rules like:
Call Forward Immediately
●
Call Forward Busy (the user is currently on another call)
●
Call Forward No Reply (forwarding of the call if the user has not picked up after a
●
configurable time)
Call Forward Not Reachable (forwarding a call if the device is switched off or out of
●
coverage)
In practice, such supplementary services are implemented in the IMS Telephony
Application Server (TAS) and can be configured from the mobile device. Configuration
includes specifying a number to forward a call to for each individual type of call for-
warding, activating or deactivating individual forwarding functions and specifying the
timeout of the call forward no‐reply function. VoLTE IMS systems use the XML
Configuration Access Protocol (XCAP) for this purpose as described in GSMA IR.92
section 2.3.
XCAP messages are sent and received using HTTP requests over one of the LTE
default bearers or GSM/UMTS PDP contexts. In practice, network operators typically
do not use the IMS default bearer that is used for VoLTE SIP messaging. Instead the