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Along the path of evolutions, a new network element may be introduced or an existing one may be eliminated from the AN and CN domain of a particular mobile communications network and its architecture. For example, the LTE system has a simpler architecture having fewer nodes, channels, and less signaling messages than the UTRAN system. Network element such as the GPRS/UMTS SGSN also exists in an LTE/EPS network during the entire 3GPP mobile communications systems evolutions with enhanced roles being added to it. An engineer working on the GSM system will find it easy to understand the various aspects of the GPRS/Enhanced Data for Global Evolution (EDGE) system. Similarly, one will understand the LTE system better having working experience on the UMTS system. Figures 2.10–2.12 show the architectural differences among the different 3GPP systems releases, i.e. Release 99, Release 4, Release 5, and so on. Compare these architectures with the GSM network shown earlier in Figure 2.1 with respect to the network element, interfaces, and signaling systems. More about the 3GPP group and its technical specification is described later in Section 2.5.

 Release 99: The First UMTS Release Architecture

The 3GPP Release 99 system architecture that was shown earlier in Figure 2.3 is illustrated again in Figure 2.10 below in a slightly different way.

3GPP Release 99 evolved from the previous GSM/GPRS system in terms of new RAT only and introduces the new AN called the UTRAN, comprising a NodeB and an RNC. This implies that only the air interface had evolved from the TDMA/FDMA in GSM/GPRS system to the WCDMA scheme in UMTS while adapting to the CN elements from the pre‐release 99 GSM/GPRS network, with necessary software enhancements. However, as shown in Figure 2.3, new interfaces are being added in the UMTS Release 99 architecture, and they are different from the GSM and GPRS networks.

Figure 2.10 System architecture 3GPP Release 99.

Figure 2.11 System architecture 3GPP Release 4.

 Release 4 Architecture: Voice Call Through IP Transport Network

In the 3GPP Release 4 system architecture, Figure 2.11, changes were made only in the CS domain. In the case of the 3GPP Release 99 architecture, the CS domain core network elements – MSC and the GMSC – perform both the user traffic transport and the control/signaling, i.e. call control and mobility, functions. However, in the Release 4 architecture, the MSC has been separated into two entities: CS‐Media Gateway (CS‐MGW) and MSC Server. CS‐MGW handles the transport of user traffic in the form 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.