services like voice (Circuit Switched – CS domain in case of GSM and UMTS) and data (Packet Switched – PS domain in case of GPRS and LTE), multimedia contents to subscribers.
Note: The abbreviated version of all the terms found in mobile communications systems and networks are being used in this book. For the complete list of glossaries of the terms, their expanded texts, and definitions, refer to the 3GPP TR 21.905 [24]. A partial list of abbreviations is provided under the section “List of Abbreviations”. More about the 3GPP, technical specifications (TS), and technical reports (TR) are described in later sections.
2.1.1 GSM (2G) Network Architecture
Consider the legacy GSM CS network shown in Figure 2.1. It consists of the different network subsystems and elements: MS, BSS (Base Station Subsystem), and Network switching subsystems (NSS). A GSM network provides the CS voice call services to subscribers.
As illustrated in Figure 2.1, a GSM CS communications network is broadly divided into various subsystems, which are described below.
BSS
It consists of the network elements: BTS and BSC. A BTS is a hardware component that is installed to provide communications services in a GSM cell. A BTS transmits and receives information with mobile devices through radio frequency communications. A BSC is responsible for the allocation of radio frequency resources in one or more cells and controls one or multiple BTSs. The GSM BSS, consisting of BTS and BSC, is the interface between a mobile device and the rest of the GSM network or public switched telephone network (PSTN). A BTS is connected to a BSC through a logical interface called A‐bis interface. An MS communicates with the BSC through the physical air interface, known as Um.
Network switching subsystem (NSS)
It consists of the network elements: MSC, Home Location Register (HLR), Visitor Location Register (VLR), and so on. The MSC performs all the necessary functions to provide CS voice call services, both for the mobile originated (MO) and mobile terminated. The gateway MSC performs the routing functions on behalf of an MS that is being served by another MSC. A VLR contains information about all the MS currently being served by a particular MSC. An MSC contacts the VLR to find and retrieve the current location of an MS. The HLR is a central database that stores the permanent information of subscribers.
NSS is also known as the Core Network (CN) and facilitates seamless communication services to freely moving users within its coverage area or between the networks of different operators or between a mobile and fixed‐line network.
The vertical dotted lines in Figure 2.1 indicates the separation of one network element from another one or an entire network from another network through a particular interface with a set of protocol layers on it. Each such interface has its name, for example, air interface (Um), A‐bis interface, A‐interface, and so on, as shown in Figure 2.1. More about the mobile communications network interfaces, both physical and logical, are described in Chapter 3.
Figure 2.1 Network architecture and elements of a GSM network.
Figure 2.2 Network architecture and elements of a GPRS network.
2.1.2 General Packet Radio Service (GPRS‐2.5G) Network Architecture
The architecture of a GPRS network and its elements for packet data services only is shown in Figure 2.2. As shown in this figure, a GPRS network uses the GSM BSS and NSS. However, the BSS provides the GPRS IP packet data services through a separate hardware unit as well as a software component which is known as the Packet Control Unit (PCU).
A PCU performs all the packet services‐related functions independently but in association with the BSC. Also, two new network elements on the NSS end are SGSN and GSSN. They communicate with each other over an IP transport network. GGSN has the interface to the external PDN over an IP transport network.
Functionality wise, an SGSN in a PS domain performs similar functions to an MSC in the CS domain core network. SGSN keeps track of the current locations of MSs and performs the PS domain control, i.e. mobility management and session management, and user data transfer functions. An SGSN can be interconnected with an MSC to deliver CS domain‐related paging services to an MS in case it is engaged in a PS session. The GGSN serves as the gateway for a GPRS network to an external IP network. Apart from this, a GGSN allocates IP addresses to a registered MS. Both the SGSN and the GGSN use the IP transport network.
2.1.3 Universal Mobile Telecommunications System (3G) Network Architecture
Similarly, consider Figure 2.3 for the different network elements found in a Universal Mobile Telecommunications System (UMTS) or 3G network. A UMTS network contains network elements such as the UE (known as the MS in GSM network), NodeB (similar to a GSM BTS), Radio Network Controller‐RNC (similar to a GSM BSC), SGSN, GGSN, Gateway Mobile Switching Center (GMSC), and MSC. These network elements together provide both the CS and PS data services to subscribers as illustrated in Figure 2.3.
Similar to a GSM network, the RNC together with the NodeB is called the Radio Network Subsystem (RNS) in a UMTS network. RNS transmits and receives information with UEs through radio frequency communications. The flow of CS voice call and PS data call is being shown with a bold solid line in the diagram Figure 2.3. The SGSN, GGSN, GMSC, and MSC network elements are reused from the GSM network. Figure 2.3 is the first version of the UMTS network architecture, also referred to as the Release 99. The UMTS CN elements are adapted from the Pre‐Release 99 GSM and GPRS networks. Subsequent releases of the UMTS network are known as the Release 4, Release 5, and so on, which are described later in Section 2.3.2.
Figure 2.3 Network architecture and elements of a UMTS network.
2.1.4 LTE (4G) Network
