Page 105 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 91

traffic of more than just one BSC. In such an architecture, the PCU is implemented
in a cabinet physically independent from the BSC. Several BSCs are then connected
to a single PCU.
The interface between the PCU and BSC has not been standardized. This means that
the PCU and BSC have to be from the same supplier. If a network operator has BSCs
from multiple suppliers, they are constrained to also buy the PCUs from the same net-
work suppliers.

2.5.2 The Serving GPRS Support Node (SGSN)
The SGSN can be seen as the packet‐switched counterpart to the MSC in the circuit‐
switched core network. As shown in Figure 2.16, it lies between the radio access network
and the core network. It is responsible for user plane management and the signaling
plane management.

User Plane Management
The user plane combines all protocols and procedures for the transmission of user data
frames between the subscriber and external networks like the Internet or a company
intranet. All frames that arrive for a subscriber at the SGSN are forwarded to the PCU,
which is responsible for the current cell of the subscriber. In the reverse direction the
PCU delivers data frames of a subscriber to the SGSN, which in turn will forward them
to the next network node, which is called the gateway GPRS support node (GGSN). The
GGSN is further described in Section 2.5.3.
IP is used as the transport protocol in the GPRS core network between the SGSN
and GGSN. This has the big advantage that different transmission technologies can be
used on lower layers (Figure 2.16). Typically, fiber Ethernet links are used today.
To connect the SGSN with the PCU, the Frame Relay protocol was initially used for
many years. The decision to not use IP on this interface is somewhat difficult to under-
stand from today’s perspective. At that time Frame Relay was selected because the data
frames between SGSN and PCU were usually transported using E‐1 links, which were
quite common in the GSM BSS. Frame Relay, with its similarities to ATM, was well
suited for transmitting packet data over 2 Mbit/s E‐1 channels and had already been
used for many years in wide area networks. The disadvantage of using Frame Relay,
however, was that besides the resulting complicated network architecture, the SGSN
had to extract the user data frames from the Frame Relay protocol and forward them via
IP to the GGSN and vice versa.

Radio access network Core network

FR or IP IP
E-1 Ethernet, ATM
PCU SGSN GGSN
Gb Gn

Figure 2.16 Interfaces and protocols of the SGSN on layers 2 and 3.