Page 107 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 93

To charge the subscriber for usage of the GPRS network, the SGSN and the GGSN
(which is described in more detail in the next paragraph) collect billing information
in so‐called call detail records (CDRs). These are forwarded to the billing server,
which collects all CDRs and generates an invoice for each subscriber once a month.
The CDRs of the SGSN are especially important for subscribers that roam in a for-
eign network. As will be described in Section 2.8.2, the SGSN is the only network
node in the foreign network that can generate a CDR for a GPRS session of a roam-
ing subscriber for the foreign operator. For roaming subscribers the CDRs of the
SGSN are then used by the foreign operator to charge the home operator for the data
traffic the subscriber has generated. For GPRS data traffic generated in the home
network, the GGSN usually generates the CDR, and the billing information of
the SGSN is not used.

2.5.3 The Gateway GPRS Support Node (GGSN)
Although the SGSN routes user data packets between the radio access network and the
core network, the GGSN connects the GPRS network to the external data network.
The external data network will in most cases be the Internet. For business applications, the
GGSN can also be the gateway to a company intranet [3].
The GGSN is also involved in setting up a PDP context. In fact, it is the GGSN that is
responsible for assigning a dynamic or static IP address to the user. The user keeps this
IP address while the PDP context is established.
As shown in Figure 2.18, the GGSN is the anchor point for a PDP context and hides
the mobility of the user from the Internet. When a subscriber moves to a new location,
a new SGSN might become responsible and data packets are sent to the new SGSN
(IRAU). In this scenario, the GGSN has to update its routing table accordingly. This is
invisible to the Internet as the GGSN always remains the same. It can thus be seen as the
anchor point of the connection, which ensures that despite user mobility, the assigned
IP address does not have to be changed.

Web server

SGSN
Subscriber
roams to a IP router
new location GGSN

SGSN

GPRS tunnel is redirected Routing in the Internet
to the new SGSN remains unchanged
Figure 2.18 Subscriber changes location within the GPRS network.