User's Manual Part 1

4 Product Overview — Introduction to GSM and GPRS
Revision 1.8 P/N 9002363
GPRS Network Architecture
To better understand GPRS, we take a quick tour beginning with the mobile PC and
traversing through the network. First, we have a notebook computer connected to a
GPRS-capable modem through a serial cable, Universal Serial Bus (USB), or PC Card.
The GPRS modem communicates with GSM base stations, but unlike circuit-switched
data calls that are connected to voice networks by the mobile switching center, GPRS
packets are sent from the base station (BTS) to what is called a Serving GPRS Support
Node (SGSN).
Enabling GPRS on a GSM network requires the addition of two core modules:
• Gateway GPRS Service Module (GGSN)
The GGSN acts as a gateway between the GPRS network and the Public Data
Networks such as IP. GGSN’s also connect to other GPRS networks to facilitate
GPRS roaming.
• Serving GPRS Support Node (SGSN)
The SGSN provides packet routing to and from the SGSN service node for all
users in that particular service area. It also keeps track of the mobiles within its
service area. The SGSN communicates with what is called the Gateway GPRS
Support Node (GGSN), a system that maintains connections with other networks
such as the Internet or private networks. A GPRS network can use multiple
serving nodes, but requires only one gateway node for connecting to an external
network such as the Internet.
When the mobile station sends packets of data, it is via the SGSN to the GGSN, which
converts them for transmission over the desired network, which could be the Internet
networks or private networks. IP packets from the internet addressed for the mobile
station are received by the GGSN, forwarded to the SGSN and then transmitted to the
mobile station.
To forward IP packets between each other, the SGSN and GGSN encapsulate these
packets using a specialized protocol called the GPRS tunnel protocol (GTP) which
operates over top of standard TCP/IP protocols. The details of the SGSN and GGSN are
both invisible and irrelevant to the user who simply experiences a straight forward IP
connection that just happens to be wireless. See Figure 3 GPRS Network Architecture
below for an illustrated display of the GPRS network.