Page 382 - From GMS to LTE

368 From GSM to LTE-Advanced Pro and 5G

network function is required and both IMS cores have to support VoLTE roaming. As
this setup might be difficult to achieve in practice, 3GPP has studied a second VoLTE
roaming solution referred to as VoLTE S8‐Home Routing (S8HR) in TR 23.749 [23].
The solution takes its name from the S8 network interface that connects an MME in the
visited LTE network with the P‐GW in the home network of a subscriber, as shown in
Figure 5.15. NTT DoCoMo was the first network operator to deploy such a solution in
practice [24].
The basic idea of VoLTE S8‐Home Routing is that instead of using a P‐GW in the
local network, all VoLTE signaling and speech path traffic is tunneled to the home net-
work over the S8 interface in the same way as is done for Internet traffic. This way, no
additional functions in the visited network are necessary and, importantly, no inter-
working between the IMS in the visited network and the IMS in the home network is
required. In addition, it would even be possible this way to offer VoLTE S8HR in a vis-
ited network that is not yet VoLTE‐capable.
A downside of deploying a plain VoLTE S8HR solution without any additions is that
no dedicated bearer can be established for the speech path and hence, speech packets
would not be preferred over other traffic. Also, the missing dedicated bearer setup pro-
cedure would require changes to the IMS client in mobile devices. 3GPP thus envisages
that two features should be supported in S8HR deployments, the establishment of dedi-
cated bearers and SRVCC for handing over an ongoing VoLTE call to GSM or UMTS in
the visited network operator. In practice this works as follows.
When a mobile device establishes a VoLTE call in a visited network, the IMS in the
home network contacts the P‐GW and requests the establishment of a dedicated bearer
with QCI set to 1 as described above. The home network P‐GW then forwards this
request to the MME and S‐GW in the visited network and from there it is forwarded to
the radio network. This requires that the two network operators have previously agreed

Visited Network Home Network

HSS
voice packets +
LTE S6a IMS signalling
SGi Cx
MME S8 IPX S8 PDN-GW I-CSCF
S-GW Mw Mw
Gx Cx
S-CSCF
PCRF P-CSCF
Rx ISC
Sh
MMTEL
IMS part in
home network

Figure 5.15 VoLTE S8HR.