Page 238 - From GMS to LTE

224 From GSM to LTE-Advanced Pro and 5G

Owing to a shortage of available IPv4 addresses, most network operators assign local
IP addresses and use Network Address Translation (NAT) to map many internal IP
addresses to a few public IP addresses on the Internet. This is similar to home network
Asynchronous Digital Subscriber Line (ADSL) routers, which also are assigned only a
single public IP address from the fixed‐line Internet service provider, and which then
assign local IP addresses to all PCs, notebooks and other devices connected to them. A
downside of this approach is that services running on mobile devices cannot be directly
reached from the outside world as the NAT scheme requires that the connection is
always established from the local IP address. Only then can a mapping be created
between the internal IP address and TCP or UDP port and the external IP address and
TCP or UDP port.
An advantage of NAT is that malicious connection attempts, for example, by viruses
probing the network for vulnerable hosts or data intended for the previous user of the
IP address are automatically discarded at the PDN‐GW. This not only protects mobile
devices to a certain degree but also helps to conserve power on the mobile device’s side
as malicious packets cannot keep the air interface connection in a power‐consuming
state when no other data is transferred. Details on this topic can be found in Sauter [9].
The PDN‐GW also plays an important part in international roaming scenarios. For
seamless access to the Internet for a user while traveling abroad, roaming interfaces
connect LTE, UMTS and GPRS core networks of different network operators in differ-
ent countries with each other so that a foreign network can query the user database in
the home network of a user for authentication purposes. When a bearer, for example,
for Internet access, is established, a GPRS Tunneling Protocol (GTP) tunnel is created
between the S‐GW in the visited network and a PDN‐GW in the user’s home network.
The process is nearly identical to that for the establishment of a user data tunnel on the
S5 interface as described before. To distinguish the scenario, however, the interface is
referred to as S8. Figure 4.4 shows this setup, which is also referred to as home routing.
Typically, the networks are connected via the IP Roaming Exchange (IPX) network, a
private IP‐based network separate from the Internet. The disadvantage of home routing
is that the user’s data is first transported back to the home network before it is sent to
the Internet. An alternative, referred to as ‘local breakout’, also exists in the standards.
Here, the connection to the Internet is established via a PDN‐GW in the visited net-
work. However, this is not widely used today.
Whether a standalone network node is used for the PDN‐GW or a combination of
several functions is embedded in a single node depends on the network operator and
the size of the network. In theory, MME, S‐GW and PDN‐GW could all be implemented
in a single device. In such a case, the S11 and S5 interfaces would be internal. In prac-
tice, the functionality is usually decoupled because of the different evolution of traffic
and signaling load. In a roaming scenario, the S‐GW and PDN‐GW are always separate
entities when default home routing is used.

4.2.6 The Home Subscriber Server (HSS)
LTE shares its subscriber database with GSM and UMTS. In these systems, the database
is referred to as the Home Location Register (HLR) and Mobile Application Part (MAP)
is used as the protocol between the Mobile Switching Center (MSC) and SGSN on the
one side and the HLR on the other. In LTE, an IP‐based protocol referred to as