8.6 GPRS Roaming

This section describes how roaming of GPRS users between different networks is supported, and in particular how practical aspects of roaming impact the network model and what type of technical solutions are adopted.

8.6.1 GPRS Roaming Architecture

GPRS roaming enables subscribers to access their GPRS services while connected to a visited network. Roaming requires the ability to connect GPRS operators so that subscribers can move from one network to another and still access the GPRS service they have subscribed to.

According to the specification, the GPRS network architecture does not specify the location of the GGSN (i.e., the GGSN can be located either in the visited network or in the home network). Figure 8-13 describes the two possible models: in model 1, the GGSN for a visited MS is in the visited network, and in model 2, the GGSN is in the home network. In model 1, routing is efficient since IP packets can be routed directly from the GGSN to the Internet. However, in such case the home network has no control over the service being provided in terms of the following:

Figure 8-13. GPRS roaming model.

• Billing: Among other functions, the GGSN collects the charging information coming from the SGSN and generates additional charging information (e.g., based on packet volume). However, currently there isn't a fast solution to carry such charging information to the home network; therefore, service such as prepaid cannot be supported.

• QoS: It is not possible to guarantee end-to-end QoS since the visited network may not have the ability to support the QoS requirements of the user beyond the GGSN.

• Access to services: DNS queries, HTML searches, and other services will be provided by the visited network or by the public Internet.

In model 2, routing of IP packets may not be optimal. For example, if the MS is accessing a Web site local to the country of the visited network, packets will be routed from the Web server to the GGSN in the home network, then tunneled backward to the MS in the visited network through GTP. However, in such a scenario the home network has full control on the service provisioning, prepaid access, and end-to-end QoS.

For these reasons, GPRS operators have come to the realization that in order to provide services to roaming users and at the same time maintain control of service provisioning, the GGSN needs to be located in the subscriber home network. The GSM Association has therefore recommended GPRS operators to provide roaming services by locating the GGSN in the home network.

8.6.2 GRX: GPRS Roaming eXchange

The GSM Association (GSMA) has came to the realization that GPRS roaming based on bilateral relationships between individual GPRS operators is incredibly complex and expensive to maintain, in particular if the number of roaming partners is high. In fact, each operator will have to have N ( N - 1) dedicated links to other operators (given that N is the global numbers of operators for which roaming should be supported). The GSMA has therefore recommended the use of a GPRS Roaming eXchange (GRX) for the Inter-PLMN GPRS roaming scenario.

The GRX is built on a private or public IP backbone and transports GPRS roaming traffic via the GTP between the visited and the home PLMN (Figure 8-14). A GRX service provider has a network consisting of a set of routers and the links connecting to the GPRS networks. Moreover, the GRX network will have links connecting to other GRX nodes to support GRX peering between networks.

Figure 8-14. GRX roaming architecture.

The GRX service provider acts as a hub, therefore allowing a GPRS operator to interconnect with each roaming partner without the need for any dedicated connections. This allows faster implementation of new roaming relations, faster time to market for new operators, and better scalability since an operator can start with low-capacity connections to the GRX and upgrade them depending on the bandwidth and quality requirements of the traffic. Other benefits of GRX are as follows :

• Support of QoS: This aspect that will be very important for the GPRS services and, in particular, for the transition to 3G systems.

• Security: The interconnection between the home operator and the visited operator uses the private GRX networks, hence does not require the overhead of maintaining expensive IPSEC tunnels over the public Internet.

• DNS support: Through GRX it is possible to support a worldwide ".gprs" DNS root, where the various GRX operators will collaborate in managing the root and each operator's DNS servers will be connected to such roots to provide translation of DNS names specific to one operator.

In conclusion, GRX is introduced for GPRS roaming to facilitate the network operators for the interconnection between networks to support roaming and will play a very important role for the transition to third-generation systems.