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 Architecture
Figure 2.4 shows the network architecture of the LTE system, refer to TS 36.300 [92], which consists of the different network elements: UE, Evolved (e)NodeB, and Evolved Packet Core (EPC) network to provide PS services only. LTE network, which provides higher data rates, was evolved from the previous UMTS system. The term called “Evolved”, denoted by “e” or “E” can be found in any descriptions related to the LTE system. The eNodeB performs the radio communication‐related functions and controls one or more cells, similar to a UMTS NodeB +RNC. Apart from this, eNodeB performs the radio resources allocation, UE scheduling, and forwarding of the user traffic/data functions to the S‐GW. An eNodeB is interconnected with other eNodeBs over the X2 interface, and together, they form the Evolved‐UMTS Terrestrial Radio Access Network (E‐UTRAN) of an LTE/EPS network.
The LTE EPC contains the network elements such as Mobility, Management Entity (MME), Serving Gateway (S‐GW), and Home Subscriber Server (HSS). An MME performs the signaling or controlling, e.g. mobility management, session management, and related functions between a UE and the EPC network. The HSS performs the similar functions of an HLR found in the GSM and UMTS system. The HLR/HSS is a database that stores the subscriber’s permanent information in a mobile communications network. Unlike the previous systems, i.e. GSM and UMTS, in the LTE system, the various management functions related to signaling and user data/call aspects are assigned separately to the MME and S‐GW. In an LTE network, the E‐UTRAN and the EPC are collectively known as the Evolved Packet System (EPS). An eNodeB is connected to the MME (for signaling) and S‐GW (for user traffic) over the S1 interface; refer to Figure 2.4.
Figure 2.4 Overall network architecture and elements of an LTE network.
Source: © 2015. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2015, 3GPP.
The LTE system is an all IP‐based network, i.e. all the network elements communicate with the existing IP transport network only. This differentiates the LTE communication network from its predecessors, GSM, GPRS, and UMTS networks, which uses other transport networks, such as ATM and Frame Relay, apart from the IP transport network.
From the comparisons of Figures 2.1–2.4, it appears that the number of network elements in an LTE/EPS network has reduced. This has led to a smaller number of protocols and interfaces between network elements compared to the GSM and UMTS system. For an overall description of the functions performed by each of the network elements of the respective mobile communications network described above, refer to the TS 23.002 [29].
2.1.5 GSM, UMTS, LTE, and 5G Network Elements: A Comparison
Based on the similar functions performed, one can compare the different network elements of a GSM, UMTS, and LTE network. A side‐by‐side comparison of different network elements of GSM, GPRS, UMTS, and LTE networks is shown in Table 2.1 below. The BTS and BSC of a GSM network are known as the NodeB and RNC in the UMTS system. Similarly, the functions performed by a BTS and BSC of a GSM network are performed by the eNodeB only that is found in an LTE network which was shown in Figure 2.4.
2.1.6 Circuit Switched (CS) vs Packet Switched (PS)
At the beginning of Section 2.1, the types, i.e. CS as well as PS, of services being provided by a typical mobile communications network was mentioned. In the case of a CS call, an end‐to‐end dedicated physical circuit establishment is required prior to the starting of voice call service on it. However, no such dedicated physical resources or path is required to be established for a PS call. A refresher illustration showing this basic difference between a CS and PS call is shown in Figure 2.5.
In Figure