UMTS Network Architecture (Third Generation Networks)

The UMTS network is divided into two logical networks, i.e., core network (CN) and radio access network (RAN). The CN and RAN are connected via an open interface. The network architecture is continuously evolving to enable a smooth transition from 2G to 3G, leveraging existing and new technologies. Several specifications based on this evolution have been released by 3GPP.

■ 3GPP Release 99/Release 3: Adds 3G radios i.e. UTRAN in enhanced GSM/GPRS core. This provides broadband interface.

■ 3GPP Release 4: Adds a softswitch/media gateway in the circuit-switched domain.

■ 3GPP Release 5: GERAN, first IP multimedia service (IMS) with SIP, QoS, and IPv6.

■ 3GPP Release 6: All IP network, multicast/broadcast multimedia services, WCDMA/WLAN interworking.

3GPP Release 99

Figure 5-1 shows the UMTS architecture as specified in 3GPP Release 99. The system architecture is based on the enhanced GSM Phase 2+ core network with GPRS and a new radio network called UMTS terrestrial radio access network (UTRAN). UTRAN is connected with the core network by the Iu interface.

UTRAN consists of several radio network subsystems (RNSs). An RNS is supported by the core network. Each RNS consists of base stations, termed as Node B in UMTS, and a radio network controller (RNC). The RNC is a BSC equivalent and controls several Node Bs. As shown in Figure 5-1, the 3G terminals (UE) interface with UTRAN using the Uu interface, which is a WCDMA-based radio link.


Release 99 architecture.

Figure 5-1 Release 99 architecture.

The Node Bs are connected to the RNC by Iub interfaces. Unlike the Abis interface, the Iub interface is well defined. This ensures interoperability in a multivendor environment where Node Bs and RNCs are supplied by different vendors. Another point to note here is that, unlike GSM BSCs, Node Bs are connected to each other by the Iur interface. This is required for inter-RNC handover.

A UE may attach to the several RNCs. The RNC that controls Node B is known as controlling RNC (CRNC). It is responsible for managing radio resources for all the Node Bs under its control. The RNC that controls the connection between a UE and the core network is known as a serving RNC (SRNC). In many cases, the CRNC and the SRNC are same. UTRAN supports soft handover. The soft handover occurs between Node Bs supported by different RNCs. During soft handover, the UE starts communicating with the new RNC, i.e., a drift RNC (DRNC), before it takes over the role of SRNC.

As shown in Figure 5-1, the core network consists of network elements to support subscriber control and circuit and packet switching. The core network also supports interfaces to the external network. The RNCs are connected to a 3G MSC by the Iu-CS interface, which supports circuit-switched services. Iu-CS is equivalent to the A interface in GSM. The RNCs are also connected to a 3G SGSN by the Iu-PS interface, which supports packet-switched data services. Iu-PS is equivalent to the Gb interface in GPRS. All the new interfaces, i.e., Iub, Iur, Iu-CS, and Iu-PS, are based on ATM.

In UMTS, the user equipment (UE) or mobile station (MS) comprises mobile equipment (ME) and a UMTS subscriber identity module (USIM).

Release 4 architecture

Figure 5-2 illustrates the Release 4 architecture. As can be noticed, the core network is evolved further and introduces changes in the CS domain. The 3G MSC functions are divided into two parts, i.e., MSC server and media gateways. The MSC server contains call control and mobility management logic. The MSC server also contains a VLR to hold mobile subscriber service data. The media gateway contains the switching function and is controlled by the MSC server. MGW terminates the bearer channels from the circuit-switched network. The same applies to the GMSC server, which is split into GMSC server and media gateway.

Separating the call control and physical interfaces has distinct advantages. It offers scalability and lower cost. Moreover, the information transfer between MS server, media gateways and other components are IP based. Therefore, many components in the core network, including SGSN, GGSN, and MSC server, can be hooked up on the intra PLMN IP backbone, taking advantage of shared and cheaper IP transport.

Release 4 architecture.

Figure 5-2 Release 4 architecture.

The MSC server uses ITU-T H.248 to control the media gateway. The ITU-T BICC (bearer-independent call control) protocol is used between the MSC and the GMSC server The core network supports coexistence of both UTRAN and GSM/GPRS radio access network (GERAN).

Release 5 architecture

Figure 5-3 shows the Release 5 architecture. The salient point for this architecture is that it is all IP based. The voice is over IP, and hence there is no need of circuit switching within PLMN. At the gateway, appropriate conversion is required to interconnect to legacy systems. The SGSN and the GGSN are enhanced to support circuit-switched services such as voice. The new roaming signaling gateway (R-SGW) and transport signaling gateway (T-SGW) are needed to provide interworking with the external system over legacy SS7 and SS7-over-IP. The call state control function (CSCF) provides call control functions for multimedia sessions. The media gateway control function (MGCF) controls media gateways, which are IP multimedia subsystems. The media resource function (MRF) supports features such as multiparty conferencing and "meet me."

Release 5 architecture.

Figure 5-3 Release 5 architecture.

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