I. UMTS Architecture

UMTS ArchitectureModule Objectives: Introduction to UTRAN, UTRAN Identifiers (RNTI), UTRAN Functionalities, UTRAN Protocol Models and Protocol Stacks.

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1 UMTS Network Architecture, Definitions

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I. UMTS Architecture

1.1 UMTS Network DomainsWith the success of GSM and the increasing demand for pure data transmission services, a new mobile communication system was necessary to support services with higher needs for network resources. For the ETSI (European Telecommunication Standards Institute) UMTS shall fulfill these demands. UMTS (Universal Mobile Telecommunication System) can be considered to be the successor of GSM/GPRS. As GSM also UMTS follows the basic structure of all communication networks. They can be divided into three parts: User Equipment (UE): The user equipment consists of mobile equipment plus the hard- and software that is necessary to provide services. The mobile equipment itself mainly has the task to provide access and transport services to the network. Access Network (AN): The Access Network is part of the fixed network. Any access network AN has two tasks. First the AN is responsible to enable the UE (user equipment) to access the network (e.g. establishing radio links between AN and UE). The second task is to transparently transport information between UE and CN (core network). In UMTS the AN has a special name : UTRAN (UMTS Terrestrial Radio Access Network). Core Network (CN): The Core Network CN is the second big part of the fixed network. The CN is the network that is responsible for the basic telecommunication service. This can be switching of circuits or routing of packets. The applications itself can reside within the core network, but can also be in an external network (e.g. a server in the internet).

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User Data UTRANAccess / Transport Signalling

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figure 1 Top level design of mobile communication network UMTS and types of information.

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In this course we will deal with UMTS Release 3 (corresponds to UMTS Release 1999). In this case the CN (core network) is taken from GSM/GPRS. This means the core network of UMTS Release 3 contains a CSCNDomain (Circuit Switched Core Network Domain), which is in fact a standard GSM network, and a PSCNDomain (Packet Switched Core Network Domain), formed by the GSMGPRS network part (SGSN, GGSN). These two parts, PS- and CSCNDomain, are independent of each other. For the UTRAN this will mean that it has to serve two core networks.

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Video/Telephony

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CS-CN

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Internet Session

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figure 2 Core network domains and corresponding services provided by UMTS.

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To provide a consistent set of term definitions, the 3GPP/ETSI made the following definition, that can be found in TS 23.101. These definitions divide the different network blocks according to their function with respect to a provided service. We have the following so called domains: User Equipment Domain: The user equipment domain is the mobile part of the network. It represents the UE as physical entity. There are two sub-domains inside: USIM domain: The USIM (User Services Identity Module) is the entity that contains the user identity and the user specific settings. Mobile Equipment domain: This domain is the hard- and software in the mobile phone, necessary to get access to the network and to support the core network services. Infrastructure domain: The infrastructure domain covers the fixed network part of UMTS, that means all physical entities controlled by the network operators. There are two sub- domains: Access Network domain: The access network domain is the UTRAN that serves the user. Core Network domain: The core network domain represent that part of the fixed network, that is responsible for the basic services (switching, routing, SMS). The core network itself is a physical entity. According to their function for a running service, the following logical domains can be distinguished: o Serving CN domain: The serving CN domain represent that part of the CN, that is currently serving the user. o Home CN domain: The home CN domain is that part of the CN, the home operator of the subscriber controls. In the home CN domain the permanent subscriber data base for the user can be found (HLR). o Transit CN domain: The transit CN domain covers all CN parts that do not belong to the serving CN domain, but are used to transport user data.

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Home NetworkZuU Cu S Mobile Uu I Equipment M

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Transit Network

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figure 3 Network functional domains of UMTS.

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1.2 Communication between Network Functional Domains StrataThe communication between the different domains can be divided according to their task. In this course we will restrict ourselves to the point of concern, UTRAN. As already mentioned UTRAN has to tasks : access between UE and UTRAN, transparent transport of signaling messages (not related to access) between UE and CN.

Therefore we can distinguish three types of signaling between UE, UTRAN and CN: access stratum (AS) : The access stratum covers all signaling exchange used to control the access of an UE to the network. Access stratum messages occur between UE and UTRAN and between UTRAN and CN. The difference between the access stratum UE-UTRAN and UTRAN-CN is, that the UTRAN-CN access stratum shall be independent of the radio technology used in UTRAN. This enables the CN to use several different radio access technologies. transport stratum : The transport stratum protocols and messages have the task to transport higher layer PDUs (protocol data units) and user data. Because UTRAN has the task to transparently transport data between UE and CN, there will be transport stratum messages between UE and UTRAN and between UTRAN and CN. non- access stratum (NAS) : The non-access stratum covers all messages of higher layers and user data, that do not deal with access or transport tasks. This covers pure application control (application stratum), service request and control (serving stratum), handling of subscription data and subscriber specific services (home stratum).

The strata are exactly defined in TS 23.101. It has to be noted, that a single protocol can belong to different strata (e.g. RRC belongs to AS and transport stratum).

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UE

UTRAN

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Access Stratum

Access Stratum

Transport Stratum Transport Stratum Transport Stratum Non Access Stratum (NAS)

figure 4 Network strata relevant for UTRAN

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2 UTRAN Architecture and UTRAN Identifiers

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2.1 UTRAN Network Architecture and EntitiesThe radio technology WCDMA, that is used in UMTS, in contained in the UMTS Terrestrial Radio Access Network UTRAN. Now UTRAN is a cellular radio system like GSM. This means that every coverage area of UTRAN is divided into cells. Every cell is served by an antenna providing radio coverage of this cell. The UTRAN architecture is strictly hierarchical. Every cell is served by one and only one so called Node B. The task of the Node B is to control the antennae of every cell and to perform the lowest layer tasks. This means the Node B handles the WCDMA physics. One Node B can handle several cells. The higher access and transport tasks of UTRAN are performed by a RNC (Radio Network Controller). Every Node B is connected to one and only one RNC. Again one RNC can support several Node Bs. The RNC provides for all cells of all connected Node Bs the access and transport tasks between UE and UTRAN, and the RNC is responsible to control the connection to the CN for the UE. One RNC together with all its Node Bs and their cells form a RNS (Radio Network Subsystem). The UTRAN itself consists of one or more RNS. UTRAN knows the following interfaces: Uu : Interface between UE and Node B (cell). Iub : Interface between Node B and its controlling RNC. Iur : Interface between two RNC. This interface is optional, it is necessary to support soft handover. The Iur interface can be implemented via virtual channels. Iu : Interface between RNC and CN. In fact one RNC can have at most one Iu interface to a SGSN (PS-CN domain), one Iu interface to a MSC (CS-CN domain), there can be multiple Iu interfaces to a broadcast domain (e.g. for cell broadcasting).

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