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The UMTS System
The Universal Mobile Telecommunications System (UMTS) is a third generation (3G) mobile communications system that provides a range of broadband services to the world of wireless and mobile communications. This chapter presents an overview of the UMTS architecture specified by the Third Generation Partnership Project (3GPP), focusing on the network elements relevant to the study presented here.
The evolution towards an All-IP network, within the 3GPP, is occurring in several steps, known as releases [6, 7]. Earlier UMTS specifications, with a relatively strong retention of the current 2nd generation networks, were still switch centric. However, the introduction of a new IP platform, when fully specified, will provide the UMTS system with multiple wireless access options and full IP packet support.
The first version of the UMTS Specification, 3GPP Release 99, defines a system which adopts much of the functionalities of the GSM/GPRS core network and introduces a new wireless access technology, namely wideband code division multiple access (WCDMA). This access technology increases the 2G systems capacity to higher data rates enabling the support of advanced services. 3GPP Release 4, further optimizes the air interface, although the major changes are targeted at the core network circuit switched domain, still present in order to maintain full coverage for second-generation services. These changes result in the separation of user data flows and their control systems into different physical entities, the Mobile-services Switching Centre (MSC) and the Media Gateway (MGW) respectively. Finally Release 5/6 provides a migration from conventional Circuit-Switched speech services served by CS domain to enhanced IP-based services using a Packet-Switched domain. This is realised by the introduction of a new platform, the IP Multimedia Subsystem (IMS) that is added to the existing PS architecture.
In this chapter the basic aspects of the UMTS system are introduced with special emphasis on the Packet Switched Core Network and the IP Multimedia Sub-system. Two major architectural views are presented, the network element centric perspective and the interface-centric one. The first approach refers to the UMTS functional architecture, which defines the system in a network centric manner by allocating with them the major system-level management functions. The second, on the other hand, allude to the UMTS network from an interface-centric view, focusing on the system protocols.
2.2 ARCHITECTURE OVERVIEW
CN
The UMTS system consists of a number of logical network elements connected through open interfaces or access points. Functionally these elements are grouped into the Radio Access Network (RAN, UMTS Terrestrial RAN - UTRAN) and the Core Network (CN) [8, 9]. The UTRAN handles all radio-related functionality, Radio Resource and Mobility Management (RRM and MM). Whereas, the CN is responsible for switching and routing calls and data connections to external networks at the same time as managing session and mobility information, Communication and Mobility Management (CM and MM). The system is completed by the User Equipment (UE) or 3G terminal [8], which interfaces with the user and the radio interface. The high-level architecture is shown in Figure
3G MSC/VLR 3GJGMSC
3G SGSN 3G S Domain GGSN
Radio (Uu)
Figure 2.1 - UMTS functional Architecture
The collection of at least one logical network element of each type defines a UMTS Public Land Mobile Network (PLMN). The UMTS PLMN is a fully featured and operational network, either on its own or together with other sub-networks, that provides land mobile telecommunication services to the public. Each PLMN, typically established and operated by a single operator, is connected to other PLMNs as well as to other types of networks, such as Integrated Services Digital Network (ISDN), Public Switched Telephone Network (PSTN), Internet, and so on.
2.2.1 UMTS Radio Access Network
UTRAN is subdivided into individual Radio Network Systems (RNSs), where each one is controlled by a Radio Network Controller (RNC) [10]. Within a RNS, the RNC is connected to a set of NodeB elements, each of which can serve one or several cells. UTRAN is located between two new open interfaces, Uu and Iu. The Uu Interface is a WCDMA radio interface through which the UE accesses the fixed part of the system. Iu
connects the UTRAN to the CN.
The main task of UTRAN is to create and maintain Radio Access Bearers (RABs) for communication between User Equipment (UE) and the Core Network (CN). With RAB the CN elements are given an illusion of a fixed communication path to the UE. While in GPRS/EDGE networks the logical page link is defined between UE and SGSN, so-called Radio Access Bearers (RABs) are defined between UE and the UTRAN. Hence, the locations of several functions have been shifted from the SGSN which is part of the GPRS
CN to the Radio Network Controller (RNC) in UTRAN. A 3rd Generation Serving GPRS
Support Node (3G-SGSN) no longer comprises logical page link management functions. 2.2.1.1 NodeB
NodeB is the physical unit for radio transmission/reception in cells. Depending on sectoring (omni/sectored cells), one or more cells may be served by a NodeB. NodeB connects with the UE via the WCDMA Uu radio interface and with the RNC via the Iub asynchronous transfer mode (ATM) - based interface.
The main task of Node B is to perform the air interface Layer 1 processing (transfer information from transport to physical channels) although it also participates in some basic Radio Resource Management (RRM) operation. However, RNC is where the radio resources are managed.
2.2.1.2 Radio Network Controller 2.2.2 UMTS Core Network
The Radio Network Controller (RNC) is the switching and controlling element of the
UTRAN located between the Iub and Iu interface. It also has a third interface called Iur for The UMTS Core Network (CN) can be seen as the basic platform for all communication
inter-RNS connections. services provided to the UMTS subscribers. The basic communication services include
The RNC interfaces the CN for both Packet-Switched and Circuit-Switched service switching of circuit-switched calls and routing of packet data. In order to handle both types
domains and also terminates the Radio Resource Control (RRC) protocol that defines the of user traffic as well as the related signalling, the CN is functionally further divided into
messages and procedures between the mobile and UTRAN. The whole functionality of a two domains, circuit switched domain (CS domain) and packet switched domain (PS
RNC can be grouped into two parts, UTRAN Radio Resource Management (RRM) and domain).
control functions. The RRM is a collection of algorithms used to guarantee the stability of 2.2.2.1 Circuit Switched CN
the radio path and the QoS of radio connection by efficient sharing and managing of the The CS domain refers to the set of all CN entities offering a "CS type connection ". In such
radio resources. The UTRAN control functions include all of the functions related to set- connection dedicated network resources are allocated at connection establishment and
up, maintenance and release of Radio Bearers including the support functions for the RRM released at connection release.
algorithms. The entities specific to the CS domain are the Mobile-services Switching Centre (MSC),
Logical Role of RNC the Media Gateway and the Gateway MSC (GMSC). Release 4 explains in detail its
The RNC controlling one Node B (i.e. terminating the Iub interface towards the Node B) is functionalities [9]; more detail is not provided as the CS domain is not the basis of the
indicated as the Controlling RNC (CRNC) of the Node B. The CRNC is responsible for the investigation presented in this thesis.
load and congestion control of its own cells, and also executes the admission control and 2.2.2.2 Packet-Switched CN
code allocation for new radio links to be established in those cells. On the other hand, the PS domain offers "PS type connection", which transports the user
In case one mobile-UTRAN connection uses resources from more than one RNS, the RNCs information using autonomous concatenation of bits called packets. The information is split
involved have two separate logical or functional roles: into separated but related packets before being transmitted and is reassembled at the
¢ Serving RNC. The SRNC is the RNC that terminates both the Iu page link for the receiving end.
transport of user data and the corresponding RAN Application Part (RANAP) The CN PS domain in UMTS has two basic network elements evolved from 2.5G General
signalling to/from the core network. The SRNC also terminates the Radio Resource Packet Radio System (GPRS), the Serving GPRS Support Node (SGSN) and the Gateway
Control Signalling, that is the signalling protocol between the UE and UTRAN. The GPRS Support Node [11, 12]. The GGSN is the node that is accessed by the packet data
SRNC may also be the CRNC of some Node B used by the mobile for connection network due to the evaluation of the PDP address. It contains routing information for PS-
with UTRAN. One UE connected to UTRAN has only one SRNC. attached users. The routing information is used to tunnel N-PDUs to the UE's current point
¢ Drift RNC. The DRNC is any RNC, other than SRNC, that controls cells used by of attachment, i.e. the SGSN. The GGSN may request location information from the HSS.
the mobile. The DRNC is involved in the active connection through an inter-RNC As mentioned earlier, the PS CN has been extended with the IP multimedia CN sub-system
soft-handover. Each UE may have zero, one or more DRNCs. (IM CN) functionality. This new platform enables PLMN operators to offer their subscribers multimedia services based on Internet applications and protocols.
9 10
Figure 2.2 - IM Subsystem entities
2.2.3.1.4 Breakout Gateway Control Function
The Breakout Gateway Control Function (BGCF) selects the network in which PSTN/CS Domain breakout is to occur forwarding the session signalling to another BGCF if it is a different one. Once in the network in which the inter-working with PSTN/CS domain is to occur, it selects a MGCF which will be responsible for such inter-working. Therefore this logical entity acts as a signalling entity for call/session control.
2.2.3.1.5 Multimedia Resource Function
The Multimedia Resource Function (MRF) is split into Multimedia Resource Function Controller (MRFC) and Multimedia Resource Function Processor (MRFP). MRFP controls the bearer on the Mb reference point and provides media stream resources to be controlled
by the MRFC.
2.2.3.2 Media Gateway
The Media GateWay (MGW) terminates bearer channels from a circuit switched network and media streams from a packet network.
2.2.3.3 Media Gateway Control Function
The Media Gateway Control Function (MGCF) entity controls the MGW and performs translation at the call control signalling level between ISUP signalling, used in PSTN, and SIP signalling, used in the UMTS multimedia domain.
2.2.3.4 Home Subscriber Server
The Home Subscriber Server (HSS) is the master database for 3G/UMTS Rel5/6 IP users. It contains the subscription-related information to support the network entities handling the IP session. This entity also integrates the Home Location Register (HLR) functionality for both packet and circuit domain, which is thereon considered as a HSS subset. As depicted in Figure 2.3 HSS provides heterogeneous information from core network diverse domains.
2.3 UMTS PROTOCOL STRUCTURE
The UMTS protocols are used to control the execution of network functions in a coordinated manner between the different UMTS domains. The general UMTS protocol model is structured, as depicted in Figure 2.4, into horizontal layers and vertical planes, which are logically independent.
Node B
USER PLANE
CONNTROL PLANE
System Network Layer
2.3.1 Transport Network Layer
The transport network layer is the lowest of the UMTS protocol architecture thus providing facilities to transport and route both control and user traffic across all UMTS network interfaces.
The transport layer is subdivided into two protocols layers - the physical layer (L1) and the data page link layer (L2). The specified protocols can be grouped depending on the UMTS interfaces they refer to, UMTS radio interface or UMTS terrestrial interface. Figure 2.5 illustrates the UMTS transport protocols (coloured blue).
USER PLANE
CONNTROL PLANE Radio Network Layer
USER PLANE
CONTROL PLANE
Transport Network Layer
Figure 2.4 - UMTS protocol internetworking architecture
The horizontal decomposition separates (generic) transport aspects from (UMTS-specific) mobile networking aspects by dividing the UMTS protocol into three layers. The lower layer, named Transport network layer, is responsible for providing the general-purpose transport service for all UMTS network elements across the interfaces. Whereas, the Radio network and the System network layer, which are by definition UMTS system-specific protocols, divide UMTS system functionality among the network elements. Within all three layers it is then possible to distinguish between control aspects and user data transfer aspects, which creates the vertical structuring to the UMTS protocol model. While the control plane protocols ensure system-wide control of communication resources and services, the user plane protocols are used to transparently transmit user data. The next section will give a detailed overview of the UMTS protocols as defined by 3GPP. The different UMTS layers are analysed separately and if the data and user protocols domains differ, detailed information is given.
2.3.1.1 WCDMA Physical Layer
The physical layer of the UMTS radio interface is based on the WCDMA radio technology [17], whereas terrestrial interfaces are typically based on digital transmission technology, such as ATM, although they are open to operator definition.
In UTRAN the data generated at higher layers is carried over the air with transport channels, which are mapped to different physical channels in the physical layer. Transport channels are always unidirectional and either common (i.e., shared among several users) or dedicated (i.e., allocated to a specific user). The following transport channels are defined in
WCDMA [18]:
¢ Broadcast Channel (BCH) - Downlink common transport channel used to broadcast specific system and cell information.
¢ Forward Access Channel (FACH) - Downlink common transport channel used to carry control information and short user packets to a UE, the location cell of which is known to the system.
¢ Paging Channel (PCH) - Downlink common transport channel that carries control information to a UE, the location cell of which is not known by the system. When the network initiate communication with the terminal, i.e. paging procedures, uses the
PCH.
¢ Random Access Channel (RACH) - Uplink common transport channel intended to be used to carry control information such as requests to set up a connection and short user packets from the terminal.
¢ Dedicated Channel (DCH) - Downlink or uplink dedicated channel used to carry user data or control information intended for a given user.
Within UTRAN, the SRNC is responsible for the radio interface related activities for UE on the WCDMA transport channel level and the Node Bs actually only maintain the WCDMA physical channels as depicted in Figure 2.5.
2.3.1.2 Transport Network Protocols in Uu Interface
Over the Uu interface the L2 is defined with two specific UMTS page link protocols, the Medium Access Control protocol (MAC) [19] and the Radio Link Control protocol (RLC) [20]. MAC is responsible for mapping logical channels, characterized by the type of data transported, onto the appropriate transport channels. On the other hand, RLC provides segmentation/reassembly of variable-length higher layer Protocol Data Units (PDUs) into smaller RLC Payload Units (PUs) and it also provides error correction through packet retransmission. These transport layers are present in both user and control plane but offering different services to the upper-layers. On the control plane the RLC services, known as Signalling Radio Bearers, are used by the RRC network layer; whereas on the user plane are used by the service-specific protocol layers such Packet Data Convergence
Protocol (PDCP) on the PS domain.
2.3.1.2 Transport Network Protocols over terrestrial Interfaces Unlike in the radio interface the L2 transport network protocol for the UMTS terrestrial interfaces use two major existing protocols suites, the ATM protocol and the TCP/UDP/IP protocol family. Those non-specific UMTS protocols are introduced in the stack in combination with some adaptation protocols, such AAL(n) for ATM and a GPRS tunneling protocol (GTP) in the IP case. The IP-based transport is widely applied only within the CN PS domain backbone network and at the Iu interface for PS domain user plane traffic whereas the ATM transport protocol is dominating on the UTRAN side.
2.3.2 Radio Network Layer
The radio network protocols compose the next layer (L3) on top of the generic transport network protocols discussed above. The radio network extends from UE across the access network (UTRAN) and terminates at the edge nodes of the CN. Layer 3 protocols are defined to perform UTRAN-specific signalling and control.
The control plane protocols in the radio network layer execute all control needed for management of radio access bearers (RABs). UTRAN-specific control protocols exist in each of the four interfaces as depicted in the Figure
On top of the above-mentioned UMTS network layer, for the data plane, reside the application and transport layers, with IP defined protocols, necessary for the provision of the end-to-end communication. However, as they are not UMTS signalling specific, their operation analysis will be presented in the next chapter.
