Page 114 - From GMS to LTE

100 From GSM to LTE-Advanced Pro and 5G

between the GGSN and the destination makes its routing decision for a packet
based on the destination IP address and its routing table. In the fixed‐line Internet,
this approach is very efficient as the location of the destination address never
changes and thus the routing tables can be static. In the GPRS network, however,
subscribers can change their location at any time as shown in Figure 2.18 and thus
the routing of packets must be flexible. As there are potentially many IP routers
between the GGSN and SGSN, these would have to change their routing tables
whenever a subscriber changes location. To avoid this, the GPRS network does not
use the source and destination IP address of the user’s IP packet. Instead, the IP
addresses of the current SGSN and GGSN are used for the routing process. As a
consequence, user data packets need to be encapsulated into GTP packets to enable
them to be tunneled transparently through the GPRS network. If the location of a
subscriber changes, the only action that needs to be taken in the core network is to
inform the GGSN of the IP address of the new SGSN that is responsible for the
subscriber. The big advantage of this approach is that only the GGSN has to change
its routing entry for the subscriber. All IP routers between the GGSN and SGSN
can, therefore, use their static routing tables and no special adaptation of those
routers is necessary for GPRS. Figure 2.24 shows the most important parameters on
the different protocol layers on the Gn interface. The IP addresses on layer 3 are
those of the SGSN and GGSN, while the IP addresses of the user data packet that is
encapsulated into a GTP packet belong to the subscriber and the server with which
the subscriber communicates in the Internet. This means that such a packet con-
tains two layers on which IP is used.
When the GGSN receives a GTP packet from an SGSN, it removes all headers includ-
ing the GTP header. Later, the remaining original IP packet is routed via the Gi interface
to the Internet.

The Gi Interface
This interface connects the GPRS network to external packet networks, for example,
the Internet. From the perspective of the external networks, the GGSN is just an
ordinary IP router. As on the Gn interface, a number of different transmission technolo-
gies can be used. To increase the bandwidth or to add redundancy, several physical
interfaces can be used simultaneously.

SGSN GGSN

Subscriber Web server
Figure 2.24 GTP packet on the Gn interface.