logically divided into three domains, as follows:
Access Network (AN)
Core Network (CN)
Mobile Station or User Equipment (MS/UE)
AN and CN form the infrastructure domain of a typical mobile communications system and network.
2.2.1 AN Domain
An AN domain consists of equipment and systems that are responsible for radio frequency‐related transmission and reception in one or more cells to or from the UE or mobile device (MS). Across the different mobile communications networks, ANs are known as follows:
Radio Access Network (RAN) in the case of the GSM system.
Universal Terrestrial Radio Access Network (UTRAN) in the case of the UMTS system.
Evolved Universal Terrestrial Radio Access Network (E‐UTRAN) in the case of the LTE system.
Next‐Generation Radio Access Network (NG‐RAN) in the case of the 5G system.
Though the above ANs are responsible for radio frequency resource allocation and communication path control in general, they perform various functions differently depending on the radio access technology (RAT) being used. The GSM RAN and UMTS UTRAN perform both the CS and PS switched network functions; the LTE E‐UTRAN or 5G NG‐RAN performs PS network functions only.
2.2.2 Core Network (CN) Domain
A Core Network (CN) domain is the backbone infrastructure for a mobile communications system and network which consists of hardware and systems. A CN is also a gateway to the traditional PSTN system. CN primarily takes care of all the call control‐related functions. A CN also performs various functions such as the authentication, charging, and setup of end‐to‐end connections required for different telecommunication services. A CN is independent of the radio connection technology for a mobile device. In the case of the LTE system, both the core network and the RAN are fully based on IP PS due to which it is also called the Evolved Packet System (EPS). In the case of the 5G system also, both the core network and the RAN are fully based on the IP PS network.
From Figures 2.1–2.3, one can see that the GSM/GPRS and the UMTS differ in the AN only. However, UMTS reuses the core network elements (e.g. MSC, SGSN, GGSN, and so on) from the GSM core network. In the case of the LTE system, the entire architecture is, however, different from the GSM and UMTS system. Neither has it reused the AN nor has it reused the CN elements from the previous cellular systems. The CN of an LTE network works on top of an IP transport network, whereas the GSM and UMTS CN may use mixed transport networks.
2.2.3 Network Domains and Its Elements
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.
2.2.4 Example: End‐to‐End Mobile Network Information Flow
Figure 2.7 illustrates, in general, an end‐to‐end information flow starting from the MS/UE to the external network to offer communication services such as the voice, data, and multimedia contents in the case of a GSM, UMTS, LTE, or 5G network. The same figure can be used to illustrate the end‐to‐end information flow by replacing the RAN and CN elements of the respective communication systems, i.e. GSM, UMTS, LTE, and 5G systems.
In Figure 2.7, observe the extent of the dotted as well as the bold and solid line.
The dotted line terminates between the MS and the RAN. This is the radio frequency signaling information flow path between the MS and the RAN, which facilitates establishing and access dedicated network resources by the MS.
The dark solid and bold line terminates between the MS/UE and core network via the RAN. This is the actual user data/traffic information flow path between the MS and the CN, en route to an external network.
In a cellular system, radio signaling must be established prior to a user can start using various services like voice and data. So, a data/traffic path has been shown on top of the signaling path in Figure 2.7.
