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General Packet Radio Service (GPRS) and EDGE 107

MS SGSN DNS

PDP context act. request
DNS query (internet.t-mobile.com.026.350.gprs)
(APN = e.g. internet.t-mobile.com)
DNS reply (10.2.0.64)

GGSN

IP address: 10.2.0.64
Create PDP context request
(APN, TID)
Create PDP cont. response
(IP address for the subscriber)
Activate PDP context accept
(IP address for the subscriber)

Figure 2.29 The PDP context activation procedure.

In a second step, the SGSN uses the APN to locate the IP address of the GGSN that
will be used as a gateway. To do this, the SGSN performs a domain name service (DNS)
lookup with the APN as the domain name to be queried. The DNS lookup is identical to
a DNS lookup that a web browser has to perform to get the IP address of a web server.
Therefore, a standard DNS server can be used for this purpose in the GPRS network. To
get an internationally unique qualified domain name, the SGSN adds the mobile coun-
try code (MCC) and mobile network code (MNC) to the APN, which is deduced from
the subscriber’s IMSI. As a top level domain, ‘.gprs’ is added to form the complete
domain name. An example of domain name for the DNS query is internet.t‐mobile.
com.026.350.gprs. Adding the MCC and MNC to the APN by the SGSN enables the
subscriber to roam in any country that has a GPRS roaming agreement with the sub-
scriber’s home network and use the service without having to modify any parameters.
The foreign SGSN will always receive the IP address of the home GGSN from the DNS
server, and all packets will be routed to and from the home GGSN and from there to the
external network. Of course, it is also possible to use a GGSN in the visited network. To
do that, however, the user would have to change the settings in their device, which is
very undesirable. Therefore, most operators prefer to always route the traffic back to the
home GGSN and thus offer a seamless service to the user.
After the DNS server has returned the GGSN’s IP address, the SGSN can then for-
ward the request to the correct GGSN. The APN and the user’s IMSI are included in the
message as mandatory parameters. To tunnel the user data packets through the GPRS
network later on, the SGSN assigns a so‐called tunnel identifier (TID) for this virtual
connection, which is also part of the message. The TID consists of the user’s IMSI and
a two‐digit network service access point identifier (NSAPI). This allows a mobile device