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In the previous sections, we have described, in general, the AN and CN domains or areas of different mobile communications networks that are available today. Each network domain consists of various network elements as shown in Figure 2.6. This is a general and an introductory figure to provide the reader with an overall view of the various network domains and its elements of communication networks based on the GSM, GPRS, UMTS, and LTE systems. Note that in the LTE system, the network element HLR and Authentication Center (AuC) have been replaced by the HSS. For the expanded texts version of these abbreviated acronyms, refer to the abbreviation section of this book.

Figure 2.6 Network domains and their elements of mobile communication networks: GSM to 4G (LTE/EPS).

The vertical dotted lines shown in Figure 2.6 represent the logical interface between two network domains of a mobile communications network. More about the logical interfaces using which network elements exchange protocol information is described later in Section 3.1.2. For the description of the functions performed by each of the network elements shown in Figure 2.6, the reader is recommended to refer to the TS 23.002 [29]. Identification and other aspects of the 3GPP technical specification are described later in Section 2.5.

The network elements of the AN domain work using the respective and particular RAT of a mobile communications system.

The CN is further divided into the following domains.

 Circuit Switched (CS), which provides voice call services in case of the GSM and UMTS system.

 Packet Switched (PS), which provides data services in the case of the GPRS, UMTS, and LTE/EPS systems.

 IMS (IP multimedia subsystem), which is used to provide voice call services over an LTE/EPS network.

Note that some of the network elements are found in the CN domain only. For further details on the functions performed by the different network elements, refer to TS 23.002 [29]. In the next two sections, illustrations are presented to illustrate the end‐to‐end protocol information flow through the above network domains of a GSM network.