Mobile Communications Systems Development. Rajib Taid
A wants to start a CS voice conversation (bold solid line) with User B. Prior to that, a dedicated and exclusive physical path/circuit (dotted line) for the entire duration of the call is required to be established between them through the interconnected switching or network elements. Similarly, User C wants to visit a web site, say www.abc.com, which is a typical PS data transfer scenario. In this case, the user’s mobile device sends a burst of data to the concerned network element NE1. Following this, depending on the concerned timer value and response from the webserver, the mobile device may release the ongoing signaling path that was established with the network element NE1. Network element NE1, in turn, shall forward the received packets to its peer network element NE2, en route to the web site server, say www.abc.com.
Table 2.1 Network elements comparisons.
| Mobile communication systems | ||||
| Network elements | GSM/GPRS | UMTS | LTE | 5G |
| GERAN | UTRAN | E‐UTRAN | NG‐RAN | |
| SGSN | SGSN | S‐GW | UPF | |
| GGSN | GGSN | PDN gateway | SMF (partially) | |
| HLR | HLR | HSS | UDM and AUSF | |
Figure 2.5 Illustration of circuit switched and packet switched data transfer.
2.2 Mobile Communication Network Domains
As illustrated in Figures 2.1–2.4, organizations and interconnections of different network elements constitute the Network Architecture for a particular mobile communications network, such as the GSM, GPRS, UMTS, and LTE networks. Interconnections are realized through different logical and physical interfaces as described later in Chapter 6. Related network elements of a mobile communications network are grouped and 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