GPRS Connections

To move information from content and application servers to the handset and vice
versa, a path through the network must be defined. This is achieved using the PDP
context procedure, where the handset, SGSN, and GGSN all store data relating to
this virtual connection (Figure 3.26).
The GGSN defines the gateway to the content — maybe the Internet, MMS
server, or a WAP gateway. The GGSN is identified by a special identity known as
the Access Point Name — effectively a URI (Universal Resource Indicator, as used
for Web pages, etc.).
Because billing and other administrative functions may be based on this
GGSN, it does not make sense in most instances to change this gateway dynamically.
Hence, even when a user is roaming abroad, the GGSN is usually located in
the subscriber’s home network, and a path is defined through the serving networkand an inter-PLMN backbone network (incorporating GPRS Roaming Exchanges
(GRX)), to the home network. The border gateways define the edge of the administratively
separate networks with associated firewalls.
An alternative arrangement may be that the SGSN and GGSN are located in the
same network. This would certainly be the case where the subscriber is attached via
the user’s home network, but may also be true, for example, where connection settings
in the handset for Internet access use a generic APN (access point name), shared by
different operators, allowing access to the nearest GGSN (in the serving network).
Using different procedures, the connection can be maintained. If necessary, the
PDP context information can be moved from SGSN to SGSN without having to
reestablish either the connection or the PDP context.