Universal Mobile Telecommunication System (UMTS)
Network architecture

The UMTS network architecture is composed of three main parts as suggested in Figure 16. Note that the main blocks in network architecture remain the same also for newer UMTS releases; only their functions are gradually changed or enhanced. The UMTS network architecture is logically divided into three parts; UE (User Equipment), the UTRAN, and the CN (Core network). The individual parts are separated by interfaces defined by 3GPP standardization body. The main purpose of interfaces is to enable direct communication between individual entities to facilitate their optimal cooperation and coordination.

The structure of UMTS network

The first part of UMTS network architecture (i.e., the UE) includes two blocks:

- Mobile Terminal (MT) - Physical device handling all communication on Uu interface, i.e., mobile phone, laptop or PDA. The MT is the same device as Mobile Station (MS) defined in 2G networks.
- UMTS Subscriber Identity Module (USIM) – The card including user specific data such as identification, authentication algorithm, and other subscription information. Again, the USIM is analogical to SIM formerly used in 2G networks.

The second part of UMTS network architecture is represented by the UTRAN, which is the most modified when compared to 2G networks. The UTRAN consists of two network elements:

- Base station (NodeB) – The NodeB is analogical to the BTS in 2G network. To that end, the NodeB is composed of the pole, antenna part and necessary hardware and software. The main purpose of the NodeB is to enable the UEs to connect to the 3G network as it is connected by means of radio channel (i.e., via Uu interface). Hence, the NodeB control uplink and downlink radio paths and ensures radio channel functions.
- Radio Network Controller (RNC) – The RNC entity is analogical to BSC node formerly used in 2G. Similarly, the RNC is responsible for controlling of several connected NodeBs. The RNC and former BSC have several similar functions, such as responsibility for radio resources management (allocation of radio resources to the NodeBs, admission control, congestion control, etc.). Still, the RNC has larger responsibility when concerned with mobility management of individual users than the BSC. In 2G networks, the MSC and SGSN play more important role regarding mobility issues.

The interesting thing regarding the NodeB is the name selected for it. One can ask why exactly NodeB? In 2G networks (GSM), the base station is logically named as BTS, which stands for Base Transceiver Station. When the 3G network has been initially designed, individual entities in the network were referred to as Nodes, i.e., mobile equipment has been formerly known as NodeA, base station has been labeled as NodeB, etc., in order to differentiate names with 2G networks. All entities have been one by one renamed appropriately, but the name for base station remained as NodeB since no more suitable name had been found.

The last part of UMTS network is composed of the CN. The CN is logically divided into Circuited Switched (CS) and Packet Switched (PS) domains. Since the structure of CN is nearly the same as in case of 2G networks, only the main differences are listed here (detail description of individual network elements in CN can be found in section 4):

- As already mentioned, part of mobility management has been moved from the CN to the UTRAN in UMTS.
- Security features are enhanced in UMTS as new ciphering algorithms are adopted. The ciphering execution is performed in RNC instead of the CN.
- The speech handling function is done by CN instead of BSS as in case of 2G.

## IP Multimedia Subsystem

The important part of UMTS CN is an IMS. The IMS is important for flexible, rapid development and deployment of new 3G services. To be more specific, the IMS provides a standard framework for deployment of next generation IP-based application using wide range of integrated media, video, text, and data.

The IMS has been firstly introduced in UMTS Release 4 where its architecture has been separated from access network to provide independent service control. The UMTS Release 6 extends the IMS toward an access independent networks infrastructure. As a consequence, IMS can be integrated also with other access networks based GPRS or EDGE technologies.

The IMS standards adopted a Session Initiation Protocol (SIP). The SIP establishes IP connection between the UEs (e.g., for voice or video sessions) or IP connectivity between the UE and application servers.