of IP packets only between the RNC and the CS‐MGW. The MSC/GMSC Server handles the signaling (call control and mobility management) part only as shown, the dotted line between the RNC and MSC, in Figure 2.11.
The signaling activities last for a fraction of time only. The separation of the transport and signaling or control functions between MSC Server and the CS‐MGW is achieved through a new Release 4 feature called the Bearer‐Independent Core Network (BICN). Because of this, in the CN, IP has been introduced as the new transport network to transport voice call traffic in the form of IP packets only instead of 64 kbits/timeslot format as it was the case during the UMTS Release 99 and pre‐Release 99 core networks, i.e. GSM. In these 3GPP releases, a voice call is carried through 64 kbits/timeslot format over the A‐interface between the BSC and MSC.
3GPP Release 5 Architecture: All IP Network
The 3GPP Release 5 system architecture is shown in Figure 2.12. In comparison to the previous architectures, this evolution is based on the three major changes that were introduced in the Release 5 architecture:
An all IP network through the usage of the IP as the only transport network right from the NodeB to the CN elements.
The introduction of the IMS for multimedia services, e.g. voice call and Home Subscriber Server (HSS) in place of HLR (Release 4).
High‐Speed Downlink Packet Access (HSDPA) feature to increase the data transmission speed from the UTRAN to the UE in the DL direction.
Figure 2.12 System architecture 3GPP Release 5.
In this architecture, there is no CS domain, but any CS voice call is routed through the IMS Media Gateway (MGCF) node in Release 5 to the existing Release 4 network.
3GPP Release 8: First Version of LTE System
The network architecture of the Release 8 or the first version of the LTE system was shown earlier in Figure 2.4. From a comparative study of network architectures of the GSM, GPRS, UMTS, and LTE systems, the following characteristics of the Release 8, i.e. first version of the LTE network, can be summarized.
A simpler and fully PS network based on IP transport, from E‐UTRAN to Evolved Packet Core (EPC). The CS domain is no longer available in the LTE and EPC networks.
Both the AN and the CN domains of the LTE system has evolved, from the previous UMTS system, due to which it is also known as the System Architecture Evolution (SAE).
LTE/EPC network has a flat architecture with fewer nodes or network elements, resulting in reduced latency and faster exchanges of information between UEs and E‐UTRAN. This is because unlike the GSM and UMTS, there is no separate radio controller in the LTE system. Radio controller functionalities are integrated into the eNodeB, and it alone performs the similar functions performed by a GSM BSC and BTS or UMTS RNC and NodeB.
New EPC network elements – MME, S‐GW, and PDN gateway – have been added.
Another version of the LTE system architecture is shown in Figure 2.13. Unlike Figure 2.4, the following figure shows the interconnection of the Evolved Packet Core Network elements also, namely, the S‐GW, the PDN gateway, and the HSS.
The S‐GW handles and performs the user data transfer‐related function, e.g. packet forwarding and routing, of the EPC network. A PDN gateway, similar to the GGSN of a GPRS network, allocates an IP address to a UE and connects the EPC network to the external IP network. For an overview of the functions by these network elements, refer to TS 23.002 [29]. The EPC network in the 3GPP Release 8 architecture support PS services only. To provide a CS voice call service for a UE registered in an LTE/EPC network, alternate features are used such as the Circuit Switched Fall Back (CSFB) and IMS. More about the IMS and CSFB features are described in later Sections 6.2.1.1 and 6.2.3.
Figure 2.13 LTE system architecture with EPC nodes.
2.4 Mobile Communications Network System Engineering
In Section 2.2, the network domains of a typical mobile communications network have been introduced. Apart from these, there are several other aspects of a mobile communications network that enable network operators to run their network smoothly. Similar to any other systems engineering, a mobile communications network is also an interdisciplinary system that provides various management functions in realizing a successful network while enabling and offering various communications services to subscribers. At a high level, the essential and general systems engineering aspect of each of the mobile communications systems and networks based on the GSM, UMTS, LTE, and 5G systems can be divided into different management areas, as shown in Figure 2.14. These system engineering aspects span across the AN, the core network, and beyond.
2.4.1 Mobility Management
Mobility management aspects of a system engineering deals with the capabilities and functions performed by a mobile communications network for enabling the continuation of the current communication services being in use by a moving user. It also deals with keeping track of the current location of a mobile user so that the network could reach and alert the mobile device for an incoming call at any point in time. Other situations where mobility management functions are needed are as follows:
Figure 2.14 Mobile communications network systems engineering.
Whenever a mobile device is switched off and on again in the same area or different service areas.
The current state of the mobile device, i.e. idle and active and their transition.
5G system mobility management system engineering aspects are described later in Chapter 18.
2.4.2 Air Interface Management
In a mobile communications network, the air interface is used to transmit and receive data, i.e. signaling and voice traffic, over a wireless medium between a mobile device and its base trans‐receiver station. The air interface uses the radio frequency transmission and forms the basis for the physical layer between the mobile device and the base trans‐receiver station. Air interface management deals with the optimum allocation, re‐assignment, and releasing of the allocated radio frequency resources in terms of timeslots/channels among the mobile devices. The physical properties and structure