Page 113 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 99

the telecom world for many years. Frame relay was also a predecessor of the ATM
protocol, which has gained a lot of popularity for packet‐based long‐distance transmis-
sion in the telecom world and which was heavily used in early UMTS networks, as will
be described in Chapter 3. Thus, its properties were very well known at the time of
standardization, especially for packet‐switched data transfer over 2 Mbit/s E‐1 connec-
tions. The disadvantage was that the user data had to be encapsulated into Frame Relay
packets, which made the overall protocol stack more complex, as in the wireless world
it is only used on the Gb interface. In newer versions of the 3GPP standards, a feature
was specified to also use IP as a lower layer protocol for the Gb interface. Today, the IP
protocol has mostly replaced Frame Relay, and fiber links are used on the physical layer
for transporting the IP packets of the Gb interface and data of other interfaces simulta-
neously over longer distances.

The Gn Interface
This is the interface between the SGSNs and GGSNs of a GPRS core network and is
described in detail in 3GPP TS 29.060 [5]. Usually, a GPRS network comprises more
than one SGSN because a network usually has more cells and subscribers than can be
handled by a single SGSN. Another reason for having several GGSNs in the network is
to assign them different tasks. One GGSN, for example, could handle the traffic of post-
paid subscribers, while another could be specially used for handling the traffic of pre-
paid subscribers. Yet another GGSN could be used to interconnect the GPRS network
with companies that want to offer direct intranet access to their employees without
sending the data over the Internet. Of course, all of these tasks can also be done by a
single GGSN if it has enough processing power to handle the number of subscribers for
all these different tasks. In practice, it is also quite common to use several GGSNs that
can handle the same kind of tasks for load balancing and redundancy reasons.
On layer 3, the Gn interface uses IP as the routing protocol as shown in Figure 2.23. If
the SGSN and GGSN are deployed close to each other, Ethernet over twisted pair or
optical cables can be used for the interconnection. If larger distances need to be overcome,
alternatives are IP over Carrier Ethernet optical links or IP over ATM, over various
transport technologies such as synchronous transfer mode (STM). To increase capacity
or for redundancy purposes, several physical links are usually used between two network nodes.
User data packets are not sent directly on the IP layer of the Gn interface but are
encapsulated into GPRS Tunneling Protocol (GTP) packets. This creates some addi-
tional overhead, which is needed for two reasons. Each router in the Internet

Figure 2.23 The Gn interface protocol stack. Gn
User data
GTP
UDP
IP
Ethernet, STM-1

SGSN GGSN