Page 377 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 363

located in a radio network that supports VoLTE and thus must forward the call to the
MSC at which the subscriber is currently registered. The MSC then pages the sub-
scriber in the traditional way as described in Chapter 1 (GSM) and Chapter 3 (UMTS)
and establishes a circuit‐switched channel. This functionality is referred to as
Terminating‐Access Domain Selection (T‐ADS) in 3GPP TS 23.221, chapter 7.2b [20].
After reselection to GSM or UMTS the mobile device performs a circuit‐switched
location area update procedure to inform the MSC that it is now located in its service
area. In addition the device also performs a routing area update to inform the packet‐
switched part of the GSM or UMTS network of its presence. Both registrations are
reported to the Home Subscriber Server (HSS) database. Performing a routing area
update means that the network also moves the LTE default bearers (i.e. the mobile
device’s IP addresses) from an LTE Mobility Management Entity (MME) to the GSM or
UMTS Serving GPRS Support Node (SGSN) that is responsible for the part of the radio
network the subscriber is currently located in. This way, IP connectivity is preserved
despite the change from one radio access network technology to another.
Some networks transfer all default bearers from LTE to GSM or UMTS in this pro-
cess, i.e. the bearer used for Internet access and the bearer used for VoLTE. This means
that the mobile device could still send and receive SIP messages and theoretically also
establish a VoLTE call. This does not work in practice, however, as GSM networks do
not have the required capacity for voice calls over packet‐switched GPRS. VoLTE calls
over UMTS networks would in theory be possible but network operators have typically
not deployed the necessary software in the network to support dedicated bearers for the
speech path. Therefore, T‐ADS is used for all incoming calls to check with the Home
Subscriber Server (HSS) database which radio access technology the subscriber cur-
rently uses. If the subscriber is registered in UMTS or GSM the call is forwarded to a
Mobile Switching Center and no SIP Invite message is sent to the mobile device. Mobile
devices are also aware that they shall only establish a VoLTE call while in LTE coverage
and thus also establish a circuit‐switched call via the Mobile Switching Center even
though they could send a SIP Invite message.
Some network operators only transfer the default bearer for Internet traffic when the
mobile has to reselect from an LTE network to GSM or UMTS. The VoLTE default
bearer on the other hand is terminated. The P‐CSCF can be informed of this, which in
turn triggers a VoLTE SIP deregistration procedure. The downside of this approach is
that when a mobile device returns from GSM or UMTS to LTE it has to reestablish a
VoLTE IMS default bearer and register with the IMS network again.
Interworking between the MSC and the VoLTE/IMS systems requires tight integra-
tion between the two systems. This is required not only for voice calls and reachability,
but also for supplementary services. Once a subscriber is VoLTE activated, supplemen-
tary service settings such as call forwarding configuration is no longer managed between
the MSC and the Home Location Register (HLR). Instead, supplementary service infor-
mation is now managed by the IMS Telephony Application Server (TAS). Therefore,
supplementary service change requests via the circuit‐switched MSC need to be for-
warded accordingly or mobile devices have to be instructed to always use XCAP as
described above if the network does not support supplementary service configuration
via the MSC for VoLTE subscribers. It should be noted at this point that not supporting
MSC‐controlled supplementary service control for VoLTE subscribers has the disad-
vantage that a user who moves a SIM card with a VoLTE subscription to a non‐VoLTE