Legacy Connectivity (3GPP Rel-4)

There are support for legacy network but first we need to understand Dial-up connections and GPRS. Both basically related to initial development of 3GPP standards (up to Rel-5). 

Specially for mobile user, which are used for providing information on small screen of mobile. So WAP was not fully compatible to public internet so WAP Gateways are used to get data and send request over public internet.

Dial up Connection:

Figure 1: GSM circuit switched access connection

While connecting with dial-up connection we use a E-164 number to dial to operator who connects to a WAP server or third party service provider. In initial deployment we have used RADIUS protocol so when a user dials up E-164 NAS number, user request of NAS connects to gateway, so at gateway there is authorization check is made and if allowed to access public internet, Gateway provide a private IP address to User terminal. By this way  a user can connect to public internet as Gateway publish a Public IP of user for other network and by getting data from that end , it map address of public identity to user private IP Address.

GPRS connectivity:

The GPRS nodes of each operator are interconnected on a private IP network.  The access connection (the technical name is PDP Context) can be thought of as a “flexible tunnel” through the GPRS networks that is established by the GPRS Tunneling Protocol (GTP). The arrangement is shown in Figure 2. The GTP tunnel extends from the SGSN to the GGSN. A mobile may support several PDP Contexts simultaneously.

Figure 2: The GPRS access connection
 

A GPRS user can access the following types of access point:

·        a Virtual Private Network connected to their home mobile operator (APN = <name of VPN>.mnc.mcc.gprs)

·        a third party ISP connected to their home mobile operator  (APN = <name of ISP>.mnc.mcc.gprs)

·        the Internet using either normal Internet protocols or WAP via the visited network’s connection to the Internet (APN = “Internet”).

APN name is an IP address of access point. APN is not URL of information source which user wants to interrogate, it is a access point by which a user can access many information source.

For the tunnel between the SGSN and the GGSN, i.e. the section across the GPRS backbone, there is a tunnel  identifier (TID) distinguishing each user’s tunnel. The tunnel ID relates to the GTP protocol running between the SGSN and the GGSN, and there are IP addresses for the source and destination SGSN/GGSN interfaces at each end of the tunnel. So this approach is known as circuit switching communication.