Page 376 - From GMS to LTE

362 From GSM to LTE-Advanced Pro and 5G

over the call. In practice it can be observed that in an ideal case it takes about 2 to 3
seconds from the sending of an LTE low‐signal‐strength measurement report, configu-
ration of GSM measurements, report of the result and reception of the handover com-
mand at the mobile device. During that time the voice call needs to continue on the LTE
network so the procedure must be started well before the device runs out of LTE cover-
age, i.e. with a margin of several dB before reception becomes critical.
A voice call can be in several states when an SRVCC handover is required. The sim-
plest case, which has been discussed above, is that the call is already established, i.e. the
terminating subscriber has accepted the call and a speech path is present. Another state
a voice call can be in is the alerting phase. Depending on how quickly the terminating
subscriber accepts the call, this phase can take many seconds, which increases the prob-
ability that the subscriber runs out of LTE coverage while the call is being alerted. The
initial 3GPP specifications only defined the SRVCC procedure for calls that have been
fully established, not for the alerting phase. If the subscriber is running out of LTE
coverage during the alerting phase the call would drop. Therefore, 3GPP specified an
extension referred to as Alerting‐SRVCC (aSRVCC) to close this gap. Both the network
and the mobile device have to support the extension and mobile devices signal support
to the IMS network with a ‘+g.3gpp.srvcc‐alerting’ tag in the SIP ‘Contact’ header.
Other states a call can be in when an SRVCC is required are, for example, established
conference calls, call‐hold, or the user being active in one call while a second call is
currently incoming (call waiting). To allow for these special call states to be handed over
into a circuit‐switched connection, 3GPP has additionally specified SRVCC for mid‐call
services. Like above, both the network and the device have to support the functionality
and the mobile device announces support with a ‘+g.3gpp.mid‐call’ tag in the SIP
‘Contact’ header.
One state not included in any of the SRVCC enhancements described above is sup-
port to perform a handover from IMS to a circuit‐switched channel before the alerting
phase, i.e. the short time between the initial SIP Invite message and the SIP ‘180 Ringing’
message. This gap has been closed in 3GPP TS 24.237 with the ‘Before Ringing SRVCC’
(bSRVCC) enhancement. And finally, 3GPP also included an optional procedure to
transfer an ongoing circuit‐switched GSM or UMTS call to VoLTE with ‘Reverse
SRVCC’ (rSRVCC).
In practice there are few if any network operators with legacy radio technologies who
do not support at least the basic form of SRVCC for established one‐to‐one voice calls.
Support for the more advanced features is less widespread and it will depend on how
fast LTE networks reach coverage levels similar to the legacy technologies whether the
more advanced features, which add significant complexity in the core network, will be
widely deployed in practice.

5.3.15 Radio Domain Selection, T‐ADS and VoLTE Interworking with GSM
and UMTS
In addition to handing over an ongoing VoLTE call into a GSM or UMTS circuit‐switched
channel handled by a Mobile Switching Center (MSC), backward compatibility is also
required when no LTE network is available at all. In this case, a VoLTE‐capable device is
registered in a GSM or UMTS network as it would be if it was not VoLTE‐capable at all.
For incoming calls, the IMS core needs to be aware that the subscriber is not currently