Page 380 - From GMS to LTE

366 From GSM to LTE-Advanced Pro and 5G

UMTS or circuit‐switched fallback (CSFB) in LTE. The reason for this is that most
network operators are still busy deploying VoLTE in their own networks and gaining
experience with the technology. Another reason might be that the initial VoLTE
roaming solution, referred to as ‘Local Breakout VoLTE Roaming’, requires a some-
what complicated interworking between the visited and the home network. As a
consequence, a second and much simpler roaming option has been designed that is
referred to as ‘S8‐Home Routing VoLTE Roaming’.
To better understand the two concepts, it is necessary to take a look at how interna-
tional circuit‐switched voice and data roaming is implemented in practice today.
When a subscriber roams abroad and attaches to a VPLMN for Internet access,
the visited MME (Mobility Management Entity, see Chapter 4) contacts the Home
Subscriber Server (HSS) in the user’s home network to get the subscriber’s profile
and authentication and ciphering parameters. Once the subscriber is authenticated,
the MME in the visited network contacts the PDN‐Gateway (P‐GW) in the home
network and requests the establishment of a packet data bearer, typically for Internet
access. The P‐GW in the home network then assigns an IP address for the user and
returns the information to the MME in the visited network, and a connection
between the Serving‐Gateway (S‐GW) in the visited network and the P‐GW in the
home network is established to transfer the user’s IP packets. This means that all
traffic to and from the Internet flows between the subscriber in the visited network
and the P‐GW in the home network and only from there to and from the Internet.
This concept is referred to as ‘home routing’ as data packets of a subscriber are
always routed to the home network and are not directly forwarded to the Internet
in the visited country. The advantages of this approach are that no configuration
changes are necessary on the user’s device when they roam abroad and that all net-
work‐operator‐provided services behind the P‐GW are available to the roaming
user as well. The disadvantage of the approach is that especially when the subscriber
uses a visited network far away from their home country, there is additional packet
delay. Also, mobile networks have to be interconnected by a high‐speed IP network
over which session management signaling and user data is exchanged. This network
is typically referred to as IPX (IP Exchange) and mobile networks typically use IP
connectivity to central roaming hubs that is completely separate from the public
Internet for security reasons [21].
In contrast, Mobile Switching Centers (MSCs) used for traditional circuit‐switched
mobile voice calls use an entirely different roaming setup. When roaming abroad, the
mobile device performs a location update to a circuit‐switched GSM or UMTS MSC in
that country. When an LTE network is used, a combined LTE attach is performed and
the visited MME registers the user to the MSC in the visited country that handles voice
calls at the user’s location. For details see Chapter 4. When the user then makes a
mobile‐originated call to a phone number in the country they are currently visiting, the
local MSC will handle the call itself and no interaction is required with a network ele-
ment in the home network. The visited network will collect billing information for the
call and send a billing record to the home network. If the device is in an LTE network, a
circuit‐switched fallback (CSFB) procedure is performed and the call setup then contin-
ues in the same way as for the GSM and UMTS example. In other words, circuit‐
switched telephony while roaming is treated locally and calls are not routed back to the
home network.