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Figures 2.1–2.4, in Section 2.1, provide the basic and introductory architecture of the mobile communications networks based on the GSM (2nd generation), UMTS (3rd generation), and the LTE (4th generation) systems. These architectures may be deployed individually by an operator to provide voice and data communication services to different categories of subscribers. For example, a subscriber may like to avail and subscribe to GSM service only; another subscriber may like to avail and subscribe to both the voice and data services through the UMTS network. Similarly, a subscriber may like to avail and subscribe voice call service through a UMTS network and mobile broadband services through an LTE/EPS network only. The different types of services that can be availed by a subscriber also depend on the capability of the mobile device. Within the same operator, a mobile user that is currently registered in an LTE/EPS network may move to a different part of the same city having UMTS coverage only. In this case, the User Equipment (UE) will access and register with the UMTS network and continue providing the communication services to the subscriber.

From the above examples, it appears that the mobile communications networks must have the capability and flexibility to provide communications services as per the mobilities and requirements of the subscribers are concerned. The UE/Mobile station (MS), access network, and circuit‐switched (CS) and packet‐switched (PS) domain core network elements of the GSM, GPRS, UMTS, and LTE networks provide such communications services through interworking among them.

Interworking of a mobile communications network can take place with the following types of networks:

 Traditional Public Switched Telephone Network (PSTN), as shown in Figures 2.1 and 2.3, connecting the GSM and UMTS Release 99 network with the PSTN.

 External IP data network, e.g. Internet, as shown in Figures 2.2, 2.3, and 2.13, connecting the GPRS and UMTS Release 99 network with the Internet.

 Packet domain communications networks.

Interworking among the networks is realized through a particular logical interface that connects their gateway or anchoring network element. For example, in Figures 2.2 and 2.3, GPRS and UMTS PS core network is connected with the Internet through the Gi interface. Similarly, in Figure 2.4, the LTE/Evolved Packet Core (EPC) network is connected with the Internet through the SGi interface.

In the next sections, we will discuss the interworking among the packet domain communications networks, i.e. LTE/EPS, GPRS, and UMTS networks. The interworking of these networks can be realized in the following ways:

 Interworking through enhanced network elements

 Interworking through legacy network elements