Chapter 2 describes the architectures and their domains of the GSM, GPRS, UMTS, and LTE networks. The standardization processes used by the 3GPP and evolutions of mobile communications networks based on the GSM, GPRS, UMTS, LTE, and 5G systems are also described. The System Engineering aspects of mobile communications networks are described briefly.
Chapter 3 describes the architecture and different types of protocols found within the mobile communications systems and networks, which are based on the legacy GSM, GPRS, UMTS, LTE system as well as the 5G system. Various interfaces for peer‐to‐peer protocol layers communications are also described.
Chapters 4 to 9 describes some of the essential functions that are performed by the mobile communications systems and networks, which are based on the legacy GSM, GPRS, UMTS, LTE, and the emerging 5G system to provide seamless communications services to mobile users within a particular network or across the networks.
2 Network Architectures, Standardizations Process
Introduction
This chapter provides the introductory and high‐level architectures of the legacy mobile communications systems which are based on the Global System for Mobile Communication (GSM), Universal Mobile Telecommunication System (UMTS), and Long‐Term Evolution (LTE) systems. However, being legacy and general, some of the contents of this chapter apply to the 5G system also. The architecture of the 5G system shall be covered in Part IV of this book. We begin with the basic network architecture of the legacy GSM system, followed by the architectures of UMTS and LTE systems as well. Following this, we present the different domains or areas of a mobile communications network along with their network elements that are interconnected together to provide communication services to users. The evolution of different mobile communications systems is presented. We also cover the system engineering aspects of a mobile communications network that span across its different domains or knowledge areas.
We then present the standardization processes used for mobile communications systems and networks that are available currently. Standardization is important for successful developments and integrations of multivendor network elements of a mobile communications system and network. Different features added, in terms of a release, during the evolution of a particular mobile communications system are also summarized.
2.1 Network Elements and Basic Networks Architectures
A mobile communications network based on the GSM (2G)/General Packet Radio Service (GPRS) (2.5G), UMTS (3G), and LTE (4G) technology comprises the interconnected network of different communicating entities/nodes, called as the network element. Based on a particular communications technology, a mobile communications network comprises several network elements. In the case of a GSM and GPRS network, the network elements are MS (Mobile Station), BSC (Base Station Controller), BTS (Base Transceiver Station), MSC (Mobile Switching Center), SGSN (Serving GPRS Support Node), and GGSN (Gateway GPRS Support Node). In the case of the LTE/Evolved Packet System (EPS) network, the network elements are User Equipment (UE) (mobile station), eNodeB, Mobility Management Entity (MME), Serving Gateway (S‐GW), and Packet Data Network (PDN). These interconnected network elements provide end‐to‐end communication 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