Page 374 - From GMS to LTE

360 From GSM to LTE-Advanced Pro and 5G

2G/3G A / Iu MSC-S
GSM radio nw. MGW
UMTS CS core network
Sv
IMS core network
ATGW speech packets

SGi SCC -AS
MME Iq ISC
S-GW
PDN-GW SGi I-CSCF
LTE
Cx
Gx Mw HSS
PCRF Rx ATCF S-CSCF
P-CSCF Mw

Figure 5.12 IMS and MSC components required for SRVCC.

UE-A ATGW

speech path
speech path UE-B
Before SR-VCC
After SR-VCC
MSC-S / MGW
speech path

speech path
speech path UE-B

Figure 5.13 A speech connection before and after an SRVCC handover.

for SRVCC, however, is that the speech data streams of both sides terminate at an
Access Transfer Gateway for SRVCC Release 10. The eNodeB base station is aware
whether the mobile device supports SRVCC as this is signaled during the LTE attach
procedure. It can thus initiate the handover procedure to a 3G or 2G network when it
becomes necessary by requesting a handover of an ongoing voice call from the MME.
The MME in turn sends a ‘PS to CS Transfer Request’ to the MSC‐Server, which then
reserves the required circuit‐switched channels in the UMTS or GSM target cell and on
all required transport links. In addition the MME instructs the Access Transfer Control