3 Network Architecture 3.1 Introduction The successful deployment and worldwide acceptance of the second-generation (2G) mobile telecommunication systems combined with the need for more advanced and ubiquitous mobile services have paved the way to the initiative of the so-called third-generation (3G) mobile telecommunication systems. In this chapter, we will discuss the network architecture of 3G systems, i.e. we will describe how the network is built, what functional elements exist, what overall functionality is provided, etc. Through this discussion, the main differences between the architecture of 2G and 3G systems will be illustrated and the advanced features of 3G systems will become apparent. Finally, we will discuss the evolution of 3G systems, which shows how the 3G systems are expected to evolve in the future and lead to the next generation of mobile telecommunications systems. It is important to keep in mind that the architecture of 3G systems was based (1) on a number of market requirements and (2) on the characteristics of the installed infrastructure base pertaining to 2G systems. Indeed, since many network vendors had already invested in a large number of network elements, it was desired to keep those elements in 3G systems wherever possible. Also, both end-users and network operators have formulated a list of requirements for 3G systems. From the end-user’s point of view, the key requirements included worldwide operation, advanced services (with emphasis on multimedia applica- tions), intelligent terminals and enhanced quality. On the other hand, from the network operator’s viewpoint, key requirements included whatever was related to increased revenue and flexible management and operation, such as, enhanced network capacity (e.g. serve more customers in a given area), increased resources utilisation, advanced network management, enhanced security, flexible and fast service deployment, etc. It is also important to point out that the network architecture was not designed having in mind the provision of telephone services but it was rather designed with multimedia services in mind. The support of multi- media services in a mobile environment was one of the primary targets and it is considered a key feature of 3G systems. Moreover, mobile access to the Internet services was another key feature. This feature was considered important because the recent evolution of Internet and its remarkable popularity called for the migration of Internet services to the mobile environment. This migration accounts for the convergence between the mobile telephony and the Internet. In this context, 3G systems are not merely mobile telephone systems similar to their 2G Broadband Wireless Mobile: 3G and Beyond. Edited by Willie W. Lu Copyright  2002 John Wiley & Sons, Ltd. ISBN: 0-471-48661-2 counterparts, but they look more like multimedia systems with enhanced capabilities and Internet access. 3.1.1 Requirements for 3G Systems Some of the most important design requirements for 3G systems include the following: † global roaming; † wide range of operating environments, including indoor, low mobility, full mobility, and fixed wireless; † wide performance range, from voice and low speed data to very high speed packet and circuit data services; † wide range of advanced services, including voice only, simultaneous voice and data, data only, and location services; † advanced multimedia capabilities supporting multiple concurrent voice, high speed packet data, and high speed circuit data services along with sophisticated Quality of Service (QoS) management capabilities; † modular structure to support existing and future Upper Layer Signalling protocols; † seamless interoperability and handoff with existing 2G systems; † smooth evolution from existing 2G systems; † highly optimised and efficient deployments in clear spectrum, including cellular, PCS, and IMT-2000 spectrum; † support for existing 2G services, including speech coders, data services, fax services, SMS, etc. The qualitative and quantitative differentiation between 3G and 2G systems is considered as an important factor to the success of 3G systems. For this differentiation, the following service requirements are considered very important: † significantly higher voice quality; † wide range of voice and non-voice services including packet data and multimedia services; † high power efficiency, especially in the mobile; † efficient spectrum utilisation (which is mandatory for the provision of high data rate services); † wide range of user density and coverage; etc. 3.1.2 International Standardisation Activities The international standardisation activities for 3G systems have been mainly concentrated in the following bodies/regions: † European Telecommunications Standards Institute (ETSI) – Special Mobile Group (SMG) – in Europe. † Research Institute of Telecommunications Transmission (RITT) in China. † Association of Radio Industry and Business (ARIB) and Telecommunication Technology Committee (TTC) in Japan. † Telecommunications Technologies Association (TTA) in Korea. † Telecommunications Industry Association (TIA) and T1P1 in North America. Broadband Wireless Mobile: 3G and Beyond138 The second-generation systems, mainly dominated by GSM, IS-136, IS-95 and PDC, have been taken into account by the related regional bodies in the design of 3G systems. This was mainly driven by the need for backward compatibility. As a result, two types of 3G core networks were standardised: one based on GSM MAP signalling and another based on IS-41 signalling. The first is being standardised by the 3rd Generation Partnership Project (3GPP), whereas the second by the 3rd Generation Partnership Project 2 (3GPP2). In general, the 3GPP initiative harmonises and standardises the similar 3G proposals proposed by ETSI, ARIB, TCC, TIA, and T1P1. The radio access of the 3GPP system is based on WCDMA and the core network is an evolution of GSM core network, based on MAP. On the other hand, 3GPP2 harmonises and standardised the 3G proposals proposed by TIA and TTA. The radio access of the 3GPP2 system is based on the so-called cdma2000 and the core network is an evolution of IS-41 core network. In addition, major international operators have initiated a harmonisation process between 3GPP and 3GPP2 in the context of the ITU-R process, which aims to result in a globally harmonised technology. In the rest of this chapter, we will focus on the 3G system standardised by 3GPP, referred to as Universal Mobile Telecommunications System (UMTS). However, we have to keep in mind that the 3G system standardised by 3GPP2 features quite similar capabilities and it is designed based on similar requirements. UMTS has been standardised in several releases, starting from Release 1999 (R99) and moving forward to Release 4 (Rel-4), Release 5 (Rel- 5), Release 6 (Rel-6) etc. The features of each individual release are discussed later on. UMTS is continuously evolving and up-to-date information about the individual releases can be found in the official web site of 3GPP, www.3gpp.org. Typically, one new release is frozen each year – R99 was frozen in March 2000, Rel-4 was frozen in March 2001, Rel-5 was frozen in March 2002 and Rel-6 is currently being standardised. Before we get into the details of UMTS, let us briefly refer to the worldwide 3G proposals that were developed in the context of IMT-2000 program [1]. The standardisation process within ETSI started at the end of 1996. ETSI Special Mobile Group decided in January 1998 that the radio access scheme would be based on wideband CDMA (WCDMA) in paired bands (FDD), and on time-division CDMA (TD-CDMA) in unpaired bands (TDD). This radio access, called UMTS Terrestrial Radio Access (UTRA), fits into 2 £ 5 MHz spectrum allocation. ETSI has submitted the UTRA proposal to ITU-R in the context of IMT-2000 concept. Other IMT-2000 proposals from other standardisation bodies have also been submitted. China presented the ITU-R a TD-SCDMA proposal based on a synchronous TD-CDMA scheme for TDD and wireless local loop (WLL) applications. The Japanese standardisation body ARIB decided to propose a WCDMA system, aligned with the European WCDMA FDD proposal. TTA in Korea prepared two proposals: one similar to the ARIB WCDMA scheme and the other similar to the TIA cdma2000 approach. In the United States TIA prepared several proposals: UWC-136 (an evolution of IS-136), cdma2000 (an evolution of IS-95), and a WCDMA system called WIMS. T1P1 supported the WCDMA-NA system, which corresponds to UTRA FDD. WCDMA-NA and WIMS WCDMA have been merged into wideband packet CDMA (WP-CDMA) and all these technologies have been submitted to ITU-R. Network Architecture 139 3.1.3 General Aspects of 3G Systems It is important to note that, a 3G telecommunications system does not standardise the services themselves but rather it provides the means by which: † users can connect to services from anywhere (whether roaming or not); † billing and accounting functions are performed; † the network is managed; † security is provided; † radio resources are managed, etc. Services are offered to the users through a large variety of service providers and network operators. The user experience should be the same independent of the place and time. In other words, the user should perceive a virtual home environment (VHE) [2], wherein the same interface and service environment is maintained regardless of the location, i.e. regardless of the serving network and regardless of the access means. In a VHE the 3G network should be able to adapt the service provision to the particular capabilities of the terminal and access environment used in a particular time. Universal accessibility to services is established by enabling several access means to a common core network, (including fixed, mobile and satellite access) and multi-mode, multi-band terminals. As mentioned already, the key driver for 3G systems is the increasing demand for multi- media services. Demand is also increasing for access to multiple types of media, often used in various combinations. Thus 3G systems need to provide both narrow and wideband services (e.g. voice, data, graphics, pictures and video), in combination, on demand and on the move. This flexibility needs to be economically delivered, with costs understood by the user. It is also important to point out that, 3G systems aim to satisfy consumer (not only business) demands for personal mobile communications. Therefore, prices subscribers have to pay for the equipment and service usage should be kept to a minimum. This makes it necessary to provide common standards to build a widely accepted framework where: † low cost mass production for the manufacturers is made possible † open interfaces for the network operators, service and content providers are clearly defined. This framework should be global in order to allow the user easy service access all over the world and in both public and private networks. 3G systems will therefore offer ubiquitous services. 3G systems aim also to provide different kinds of mobility, typically, terminal mobility, personal mobility and service mobility. Terminal mobility is provided when a user is served while on the move, regardless of network boundaries. Personal mobility is provided when a user is not restricted to a special terminal when wanting to access his or her services. This kind of mobility is usually offered by means of a common smart card technology and the provision of the virtual home environment. Service mobility is provided when a user can access his or her personalised services independently of the terminal and serving network. Broadband Wireless Mobile: 3G and Beyond140 3.1.4 Chapter Outline In this chapter we will discuss the architecture of the 3G system standardised by 3GPP. As mentioned before, this 3G system is typically referred to as Universal Mobile Telecommu- nications System (UMTS). Section 3.2 discusses the generic network model of UMTS and defines the various high- level domains, such as the User Equipment domain, the Access Network domain, the Core Network domain, etc. It also discusses the functional strata, including the transport stratum, the serving stratum, etc. Section 3.3 focuses on the network architecture of several UMTS releases and their parti- cular features. In this context, it defines the various functional entities of a UMTS network, the various network interfaces and it discusses some fundamental differences with the GSM mobile networks. Section 3.4 thoroughly discusses the architecture of UMTS Terrestrial Radio Access Network (UTRAN), focusing on the internal UTRAN interfaces and the key functionality provided by UTRAN. Finally, section 3.5 discusses the network access security of UMTS, that is, the particular means provided to limit the access to network services and resources to authorised users, to encrypt the sensitive pieces of information, to verify the integrity of critical data, etc. Such functions are considered key in the deployment of UMTS. The high-level organisation of a UMTS network considered throughout this chapter is illustrated in Figure 3.1. Note that, the network is divided into the User Equipment domain, the Access Network domain, the Core Network domain and the Service domain. The core network is an evolved version of the GSM core and it is backwards compatible with GSM networks. Users access the services through the core network and by means of a particular Network Architecture 141 Figure 3.1 High-level domains in a UMTS network. access network. In UMTS, the core network is designed to be decoupled from the particular aspects of the access network and for this reason several different technologies can be used in the access network domain. Such network access technologies include the UMTS Terrestrial Radio Access (UTRAN), the standard GSM Base Station Subsystem (GSM BSS), the GSM/ EDGE Radio Access Network (GERAN), which is an evolved version of GSM BSS, the UMTS Satellite Radio Access Network (USRAN), various Broadband Radio Access Networks (BRAN), e.g. HIPERLAN/2, 802.11, etc. and also fixed access networks. At first stages of deployment, UTRAN and GSM BSS will dominate and will offer high interoper- ability between each other. In later stages, however, more access network options are expected to be interconnected to the UMTS core and progressively offer more access means to a common set of advanced and globally available services. 3.2 Generic Network Model The generic network architecture is a high-level representation that identifies the functional model and the physical model of the system. 3.2.1 Physical Model The physical model of the network provides a high-level physical configuration of the network. This configuration is represented by a number of physical domains connected to each other with a specific way. The UMTS physical model comprises two high-level domains: the User Equipment (UE) domain and the Infrastructure domain. This is illustrated in Figure 3.2. The reference point between these two domains is termed as Uu. 3.2.1.1 The user equipment domain The UE domain comprises of all equipment that is operated and owned by the user. The types Broadband Wireless Mobile: 3G and Beyond142 Figure 3.2 UMTS physical domains. of the equipment as well as their functional capabilities can vary between each other. However, all UE equipment must be compatible with one or more access technologies used to access the infrastructure domain. The UE domain is subdivided into two other domains: the User Services Identity Module (USIM) domain and the Mobile Equipment (ME) domain. The reference point between them is termed as Cu. The USIM includes a removable smart card that may be used in different user equipment types and it is used to provide terminal portability and terminal personalisation. The USIM corresponds to a subscription and provides the means for the infrastructure domain to securely identify the subscriber. The ME domain typically contains the equipment that execute the user applications and the radio interfacing procedures, such as physical and data link procedures. The ME domain may be physically realised in one equipment. For instance, a mobile terminal equipped with WAP microbrowser is an ME. However, it is customary to physically separate the equipment that executes the user applications and the equipment that governs the radio interface procedures. In such cases, the former equipment is referred to as Terminal Equipment (TE), while the latter is referred to as Mobile Terminal (MT) equipment. In general, a TE may be a laptop or a personal digital assistant (PDA), which runs the user applications. The TE is independent of any mobile radio issues, such as transmission, mobi- lity management, radio resources management, etc. All these mobile radio issues are handled by the MT, which terminates the mobile protocols and the associated procedures. 3.2.1.2 The infrastructure domain The Infrastructure domain encompasses all the network equipment needed to support the end- to-end user connectivity. It is further split into the Access Network (AN) domain and the Core Network (CN) domain. The AN includes the physical entities (such as radio transceivers, base stations, etc.) that manage the resources of the AN and facilitate the user access to the CN. Ideally, the AN is independent of the CN and can implement any kind of access technology ranging from the legacy fixed local loop to wireless LAN technologies, satellite access technologies, and cellular broadband technologies. Any kind of AN can be interfaced to the CN as long as it complies with the specification of the Iu reference point. Having said that, it becomes evident that the functionality of CN is decoupled from the specificAN employed and the same CN can be reused with several different AN domains. For instance, access to the same CN could be realised through an AN based on DSL technology or through an AN based on GSM radio access technology. The CN domain includes the physical entities that facilitate end-to-end connectivity, transmission of user information and signalling and, in general, the provision of telecommu- nication services. To support mobile access, the CN implements mobility management procedures and location management procedures. The CN domain is further sub-divided into the Serving Network (SN) domain, the Home Network (HN) domain and the Transit Network (TN) domain. The reference points between these domains are illustrated in Figure 3.2. Strictly speaking, SN, HN and TN may be considered as different instances of the same domain. This means that, at one time, some physical entities may be considered as a SN domain and, at some other time, the same physical entities may be considered as a HN domain or a TN domain. As discussed below, the definition of a domain within the CN depends on several parameters, such as the mobile user’s location and on the mobile user’s service requests. Network Architecture 143 The SN domain is composed of all the physical entities that are directly connected to the access network. The user therefore makes use of the 3G services by directly communicating with the SN. All the service requests, including mobile terminated and mobile originated calls, are handled by the SN. In addition, the SN provides the mobility management func- tionality needed to accommodate the user mobility. Finally, the SN establishes and routes calls on behalf of the user and it interacts with the HN domain to take into account the home environment of the user, e.g. to learn what services the user is entitled to use, what particular subscription options have been activated, what home-based services are enabled, etc. Typi- cally, a SN serves a limited geographical area and, thus, while a user is on the go, he may enjoy 3G services through a sequence of SN domains. Each user is associated with a HN domain. This domain is effectively the network domain where the user has a 3G subscription and where some permanent user specific data is stored. Typically, a HN domain can provide the services of a SN domain for the users located in a given geographical area. However, this may not be considered as a general rule, i.e. it is possible for a HN domain to provide no SN services. When a HN domain can also provide SN domain services, then it acts as both SN domain and HN domain for the users that have a 3G subscription in this domain and are currently located in the area served by the domain. In such cases, the SN and HN domain merge into a single domain. However, when a user is located in the area served by a domain other than his HN domain, then his SN and HN domains are physically separate. In such case, we say that the user is roaming to another domain. One important characteristic of the HN domain is that its physical location is always the same no matter where the user is located. As shown in Figure 3.2, the TN domain is the components of CN that facilitates the communication between the UE and the remote party. If, for example, the mobile user makes a call to an ISDN user, then the ISDN network, which terminates the call, acts as a TN domain. Note that, for the ISDN user, this ISDN network is effectively a SN domain. The TN may be another SN, when the remote party is a mobile user served by another SN. In addition, when a call is established between two parties served by the same SN, then no TN exists. Therefore, the TN is defined on a per call/session basis and may or may not be needed for a call/session establishment. 3.2.2 Functional Model From a functional point of view, a 3G network can be decomposed into a number of func- tional planes, each one providing the level of functionality needed to realise a given set of services. These planes are characterised by a hierarchical relationship: a functional plane uses the services provided by the functional plane below it and provides services that are available to the functional plane above it. The decomposition of the overall functionality into several functional planes provides for a highly structural architecture and effectively maps the high- level functional requirements into smaller sets of clearer and more specific requirements. In turn, this facilitates the development and makes it easier to identify the functional entities required. The high-level functional architecture of a 3G network is illustrated in Figure 3.3. This figure shows three primary functional planes, which in the specifications of 3GPP are referred to as strata [1]: the application stratum, the serving stratum and the transport stratum. A specific part of the transport stratum is termed the ‘access stratum’. Figure 3.3 also shows the Broadband Wireless Mobile: 3G and Beyond144 physical network entities that interact within each functional stratum. Where the interaction between two entities is represented by a dotted line, it means that this interaction is not specific to UMTS and possibly different interaction mechanisms could be employed. The application stratum is the highest-level stratum and encompasses only two peer appli- cations, one running at the TE and another running at the ‘remote party’. The remote party could be a network server, another TE, a value-added server operated by an authority other than the 3G operator, etc. These two applications communicate transparently through the 3G network, which means that they don’t have to deal with any specific communication issues other than the communication between each other. The functional strata below the application stratum deal with all the transport and connection management issues and effectively provide a transparent communication path between the two applications. It should be noted that, the protocols used in the application stratum are not necessarily specified in 3GPP specifications. This leaves space for a vast range of applications that could be or could not be specifically developed for 3G systems. However, 3GPP specifications include also some application layer protocols, such as the USIM Application Toolkit (USAT), the Mobile Execution Environ- ment (MExE), etc. In general, a remote application should authenticate a user before allowing him to utilise the application services and it could also provide for application level data confidentiality. Such security mechanisms are of considerable importance in the application stratum. Application-level security mechanisms are needed because the lower functional strata may not guarantee end-to-end security provision. Lack of end-to-end security could be envisioned when, for instance, the remote party is accessible through the Internet. The serving stratum is mainly used to provide access to services. Through the serving stratum a user (or an application) may request to have access to specific services. The serving stratum aims at providing the user with the requested services, or with the services that are accepTable 3.to him. In general, the serving stratum is a control stratum that received service requests from the application stratum and then configures the transport stratum to provide accepTable 3.transport (or bearer) services. For example, protocols that functionally belong to the serving stratum include the call control protocols. In particular, the serving stratum includes protocols across the following interfaces (see Figure 3.3): Network Architecture 145 Figure 3.3 UMTS functional model. † TE – MT: These protocols support exchange of control information to enable the TE to request specific services. † MT – SN: These protocols allow the MT to request access to services provided by the serving network domain. † USIM – MT (not shown in Figure 3.3): These protocols support access to subscriber- specific information for support of functions in the UE domain. As illustrated in Figure 3.3, there could also be serving-stratum protocols across the SN– TN interface and across the TN–Remote party interface; however, these protocols may not be specific to 3G architecture. As opposed to the serving stratum, which effectively deals with signalling, the transport stratum aims at providing the correct transport mechanisms to transport the actual user data between various network interfaces. The transport mechanisms across each interface are tailored to deal with the specific characteristics of that interface. For instance, the transport mechanisms across the radio interface need to cope with the radio transmission issues, such as, efficient modulation, power control, interference cancellation, etc. Obviously, such issues do not exist across other interfaces, e.g. between the SN and the TN. As identified in Figure 3.3, the 3GPP specifications specify transport mechanisms applicable only between the MT and the AN, and between the AN and the SN. For providing the required transport function- ality, the transport stratum includes mechanisms such as: † mechanisms for error correction and recovery; † mechanisms to encrypt data; † mechanisms for adaptation of data to fit into the transmission format supported by the transport resources (e.g. adaptation of 13 kbps data to be transferred into a 64 kbps trans- port channel); † mechanisms for data transcoding to support interworking between entities using different data encoding formats. The transport stratum in a 3G system, which aims at supporting multimedia services, should be capable of providing a vast range transport channels, each one featuring a different set of communications characteristics. The part of the transport stratum that is specific to the AN technology is called the Access Stratum (see Figure 3.3). This stratum provides services related to the transmission of data over the radio interface and the management of the radio interface. In particular, the protocols across the MT – AN interface support the transfer of detailed radio-related information to coordinate the use of radio resources, and the protocols across the AN – SN interface provide access of the SN to the resources of the access network. The latter interface is independent of the specific structure of the access network. 3.3 Network Architecture In this section, we present the network architecture of the 3G system specified in the 3GPP specifications [10]. The development of these specifications follow a phased approach, that is, specifications are being developed in phases, commonly referred to as releases. As we have seen in section 3.1.2, the first 3GPP release is known as Release 1999 (R99), the second as Release 4 (Rel-4), the third as Release 5 (Rel-5), etc. In every new release a list of new Broadband Wireless Mobile: 3G and Beyond146 [...]... releases GERAN uses in addition the Iu interface GERAN uses an EDGE (Enhanced Data rates for Global Evolution) radio interface that is based on the TDMA technology of legacy GSM but introduces a set of additional radio channels with different structure, different encoding and different modulation from the typical GSM channels These radio channels provide for increased capacity on the radio interface An overall... interface is not standardised 3.3.2 3GPP Release 4 3.3.2.1 Features of 3GPP Rel-4 The key features introduced in 3GPP Rel-4 are described below Note that all of them apply to later releases too Support of GSM/EDGE Radio Access Network (GERAN) Apart from the legacy GSM radio access network and the UMTS terrestrial radio access network (see section 3.4), 3GPP Rel-4 supports an evolved GSM radio access network,... transmission However, since most of the modern media 148 Broadband Wireless Mobile: 3G and Beyond encoders today perform variable bit-rate encoding, it becomes evident that PS domain is not only efficient for data traffic but also for multimedia traffic This fact accounts for the significant interest that has been developed for carrying multimedia services (including voice and video) over the PS domain Unavoidably,... 3GPP Rel-4 supports an additional type of radio access network as compared to 3GPP R99 – the GSM/EDGE Radio Access Network (GERAN) Also, another important difference from 3GPP R99 is that the MSC is split into two functional elements, one operating in the control plane and another operating in the user plane The first is referred to as MSC Server and the latter is referred to as Media Gateway function (MGW)... Wireless Mobile: 3G and Beyond call directly to the VGCS/VBS anchor MSC, based on the information contained in the dialled number The Gateway GPRS Support Node (GGSN) is a network element in the PS domain that serves as a gateway providing connectivity to external packet data networks (PDNs) over the Gi interface It could be considered as a typical IP router implementing additional functionality for supporting... services, etc At this point, it is instructive to point out the difference between a UMTS system and a 3GPP system A 3GPP system is typically composed of an evolved GSM core network that can be connected to several radio access networks (such as the UTRAN, the GSM radio access network, and other fixed or wireless access networks) through the standardised interfaces A, Gb and Iu A UMTS system is similar but... TS 23.002 [3] and TS 31.120 [48] respectively In the following sections we discuss some of the key aspects of the 3GPP R99 core network First, we briefly discuss the key network elements and network interfaces illustrated in Figure 3.4 Later we discuss the most important features of the 3GPP R99 network The UTRAN architecture is discussed in section 3.4.1 3.3.1.2 Core network elements The Authentication... combination of transport mechanisms Standardized transport mechanisms Although the transport mechanisms in Rel-4 and onwards may be freely chosen by the operator, these transport mechanisms need to be standardised to allow interworking across different operators Therefore, the transport mechanisms for bearer control, call control, and other signalling are based on standardised technologies, such as SS7 or... (for a particular type of service) these protocols are compatible, then the IWF may be bypassed The main difference between an MSC and a typical switch in a fixed network is that the MSC performs additional functions such as functions for radio resource allocation and for mobility management For providing service mobility, the MSC supports procedures for location registration and procedures for handover... applies configuration hiding, the S-CSCF forwards mobile terminating signalling to the appropriate I-CSCF † Route mobile terminating requests to the CS domain for subscribers who have requested to receive incoming sessions via the CS domain † The S-CSCF generates Charging Data Records (CDR), which are forwarding to a mediation gateway and are used for billing the utilised services The Media Gateway Control . set of additional radio channels with different structure, different encoding and different modulation from the typical GSM channels. These radio channels. and it discusses some fundamental differences with the GSM mobile networks. Section 3.4 thoroughly discusses the architecture of UMTS Terrestrial Radio Access Network