the umts architecture

Understanding UMTS©Informa TelecomsSec2

The UMTS Architecture


1 UMTS ARCHITECTURE – THE REQUIREMENTS1.1 Aims of the UMTS Architecture 11.2 Key new features in UMTS vs. GSM/GPRS 31.3 The Two Modes of W-CDMA Access 51.4 Elements & Domains in a UMTS network 7

2 THE USER DOMAIN (USIM + ME DOMAINS) 9

3 THE ACCESS NETWORK3.1 The Access Domain and Interfaces 113.2 Requirements of the UTRAN 133.3 Further UTRAN features 173.4 UTRAN Architecture – General 173.5 The Node B 193.6 The RNC 213.7 RNC Terminology 233.8 Functions of the RNC 253.8.1 Controlling RNC Functions 253.8.2 Serving RNC Functions 253.8.3 Drift RNC Functions 253.9 Further UTRAN features 273.10 Handovers 293.10.1 Softer Handover 293.10.2 Soft Handover 313.11 Functions Of The UTRAN Protocols 33

4 THE UTRAN TRANSPORT NETWORK4.1 Requirements Of The Transport Network 354.2 The Options 354.3 ATM Operation 374.4 The ATM Cell 394.5 ATM and Quality Of Service 41

5 THE CORE NETWORK5.1 The Core Network Domain 435.2 Specific Entities in the UMTS Release ’99 Core

Network Architecture 455.3 The Circuit Switched Domain & GSM Core

Network Elements 475.4 The GSM Location Registers 495.5 The Packet-Switched Domain & GPRS Core

Network Elements 51

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Understanding UMTS


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Understanding UMTS

5.6 Operation in the Core Network CS Domain 535.6.1 User and Control Information in the CS Domain 535.6.2 Signalling in the CS Domain – SS7 Overview 555.7 Operation in the Core Network PS Domain 575.7.1 User and Control Information in the PS Domain 575.8 OTHER NETWORK ENTITIES 595.8.1 Some other entities for specific services 59

6 IN/CAMEL IN UMTS6.1 Intelligent Networks 616.2 CAMEL 63

7 CHARGING 65

8 CORE NETWORK TRANSPORT 67

9 MOBILE IP9.1 Mobile IP: Basics 699.2 Mobile IPv4 vs. IPv6 71

10 RELEASE 410.1 Release 4 – Control & Data Separation

in the CS domain 7310.2 The IP Multimedia Subsystem 7510.3 New Domain Concept in Release 4 and Beyond 77

11 NETWORK EVOLUTION11.1 3GPP Release ’00/Release 4 79

ANNEX 1 DOMAINS AND STRATAa.1 UMTS Domains 81


Understanding UMTS©Informa Telecoms


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1. UMTS ARCHITECTURE – THE REQUIREMENTS

1.1 Aims of the UMTS Architecture

The fundamental difference between GSM/GPRS and UMTS is in the need for thelatter to support high bit rate bearer services, plus the notion of negotiated QoS andtraffic characteristics. In particular, UMTS needs to support bursty and asymmetrictraffic in an efficient way, and to allow support of single and multimedia N-ISDNapplications and single & multimedia IP applications.

However, no one knows what precise future service requirements will be. Therefore itis essential that the UMTS system is designed to be as flexible as possible. For thisreason, a modular approach has been followed, with network nodes defined thatimplement some specific functionality, and open interfaces defined between suchnodes.

A modular approach also increases the chances of being able to implement futureseamless roaming between the various IMT2000 family standards.

In order to ensure that UMTS is implemented as quickly as possible, it also becameobvious that its design needed to take account of the cost for operators.

The optimisation of the signalling load as well as reduction in the overall transmissioncapacity are critical cost factors for operators, and so the aim is for an architecturewhich will minimise signalling traffic and optimise transmission infrastructure. Thearchitecture also needs to protect existing investments which operators have, andre-use as many elements of these as possible. In particular, the first release of UMTS(UMTS Release ’99) builds directly upon an evolved GSM (GSM Phase 2+) network,including the addition of GPRS.

Of course, different phases of release of UMTS will need to be compatible with eachother.

Fig. 1 – Aims of the UMTS Architecture

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• Flexibility

• IMT2000 interworking

• Minimise signalling

• Optimise transmission

• Protect existing investments

• Enable evolution

Modular approach, building on evolved GSM



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1.2 Key new features in UMTS vs. GSM/GPRS

The UMTS Core network in Release ’99 is based on the GSM/GPRS network. Mostof the individual elements are re-used, although they require extensions (e.g. MSC,VLR etc.), but the UTRAN elements are completely new. UMTS core networks mustbe able to interoperate both this new UTRAN and the existing GSM BSS accessnetwork.

In order to support more data intensive services, operators will need to upgradecapacity throughout their networks in order to cope with the expected increase intraffic, and use transport protocols which are more efficiently suited to data andpacket transport. Thus, in UMTS Release ’99, ATM is specified as the transportmechanism in the new interfaces in the radio access and between radio access andcore network.

A new, standard default speech codec (adaptive multi-rate) is also standardised forUMTS, and supports tandem free operation both to lower transmission costs and toimprove speech quality.

Among other key features provided by UMTS networks but not previouslystandardised within GSM:

• the enabling of set-up, renegotiation and clearing of connections (both circuit-switched calls & packet-switched sessions), with a range of performancecharacteristics. The connections can also vary during their lifetime. This providesfor flexible, multimedia services, in which media elements may be added orremoved dynamically during the call

• support for a range of traffic and performance for connectionless (multicast,broadcast, unicast) traffic, defined using bearer services. Once established, bearersdo not prevent the set-up of new bearers, again permitting services to be flexible

• support for the Virtual Home Environment (VHE), provided through toolkits

• the generation of additional charging records, for example based on number, callduration, traffic, QoS and so on, in order to provide operators with the capability tooffer new pricing models. These new charging methods are also set up in such away as to provide support for on-line billing

• interworking and roaming with PSTN, N-ISDN, GSM, X.25 and IP signalling, andwith their respective numbering schemes

• the measurement of traffic flows and so on, in order to optimise congestion controland other management & efficiency techniques

• support for IP mobility between different environments (e.g. fixed & mobile)

Fig. 2 – Some Key New Features of UMTS Networks

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• Upgrades to existing GSM/GPRS elements

• ATM transport

• New speech codec

• Flexibility in connection set-up, re-negotiation& clearing

• Flexibility in bearers

• Support for VHE

• Enhanced charging & billing support

• Interworking with other networks &numbering schemes

• Traffic flow measurements to enablemanagement efficiencies

• Enhanced IP mobility support



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1.3 The Two Modes of W-CDMA Access

Two modes are defined for the W-CDMA access scheme, according to the two differentways in which duplex operation is dealt with. Duplex refers to the combination of bothuplink (mobile to base station) and downlink (base station to mobile) transmission.

FDD (Frequency Division Duplex)This is the name for the UMTS mode which is designed to give wide area mobilecoverage in UMTS. It can support 384kb/s in a mobile environment and uses a 5Mhzfrequency band for uplink and a separate 5Mhz for downlink. This is termed pairedspectrum channel allocation and operators in the UK for example have two or threechannels of paired spectrum depending on their licence (10Mhz or 15MHz).

TDD (Time Division Duplex)TDD uses W-CDMA as the modulation scheme, as in FDD, but shares a single 5Mhzchannel for both uplink and downlink. To do this, the system allocates time slots forboth the uplink and downlink transmissions. The system is capable of very high datarates (up to 2Mb/s) but is not suitable for anything above pedestrian mobility, due tothe slower power control loop.

Note that adding TDD mode to FDD will require a new Node B, likely to be smaller,with lower power output and up to two antennas serving a range of 100m or so. TDDis therefore sometimes described as equivalent to a cordless level of mobility.Thetypical uses envisaged include offices, stations, supermarkets, airports and so on,where traffic levels may be high but users are moving slowly. The channel allocation iscalled unpaired spectrum, and is absent in Japan, although present in four of the fivelicences in the UK.

In future, a further multi-carrier UTRA mode is expected to define compatibilitybetween UMTS and cdma2000.

Given the different wide and local area advantages of the two access schemes, theconcept of cell hierarchies arises, and may also include GSM/EDGE for the widestarea coverage in the early stages of UMTS deployment.

t

f

t

f

GSM/EDGE? FDD TDD

Fig. 3 – Two Modes of W-CDMA Access

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1.4 Elements & Domains in a UMTS network

A UMTS network can be divided into the following physical domains:

1. User Domain

2. Infrastructure Domain, itself subdivided into:

a. Radio Access Network

b. Core Network

Each domain is further described in the following pages, and each may involve furthersubdivisions and elements.

An important feature in the standardisation of UMTS is that the internal functionalityof domains is NOT specified. Instead the interfaces between them are defined andopen. This means that in theory it is possible to have several network elements of thesame type, with the minimum requirement for a fully featured network being to haveone of each.

A UMTS system could be divided into sub-networks, operational either alone ortogether, but each with unique identities. A single such network is described as aPublic Land Mobile Network (PLMN), and may be connected to other PLMNs, orother networks such as ISDN, PSTN or the Internet.

For example, in practical application this might mean that a single physical UTRANinfrastructure could be shared by a number of core network domains. It also meansthat different domain elements can be more easily sourced from different equipmentmanufacturers, with interoperability ensured by the standardised interfaces, thusleading to more competition and greater operator choice in sourcing infrastructure.

USER DOMAIN

RADIO ACCESS DOMAIN

CORE NETWORK

INFRASTRUCTURE DOMAIN

Uu Interface(Radio Interface)

Iu Interface

Other PLMN,Other Networks

Fig. 4 – UMTS Domains Overview

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2. THE USER DOMAIN (USIM + ME DOMAINS)

The user domain describes the equipment needed by the user to access UMTSservices.

Within this domain are further subdivisions into the Mobile Equipment (ME) domainand USIM domain. The combined ME and USIM is sometimes referred to as theMobile Station (MS).

The USIM domain contains the data and procedures allowing the ME to securelyidentify itself, and is linked to the ME by the defined Cu Interface.

The ME domain performs radio transmission and contains applications. It may itselfbe further subdivided, into the Mobile Termination MT (radio functions only) andTerminal Equipment TE (contains end-to-end application, and may be a separatedevice from the radio equipment, for example a laptop).

The functionality of the MT is entirely new in UMTS, in being able to interact with theaccess network over the all new UMTS radio interface, Uu. In almost all cases in theearly deployment of UMTS, the mobile terminal must also be multi-mode, able toreceive/transmit between both GSM-based and UMTS-based radio access schemes.

Note that it is also possible to define the following terms relevant to this domain, todescribe the various types of human user involved:

• the subscriber, who is associated with the home environment & responsible forpayment

• the user, who is authorised to use services by the subscriber (and may have theirown user profile)

• another party, for example the calling party in a call, the called party and so on.They may not be a 3G user

The User Domain is linked via the standardised Uu Interface (“air interface”) to theAccess Domain.

USER ACCESS CORE

MT

Radio

TE

Applications

UICC/USIM

Cu Interface

METE

External Applications

MS

Uu Interface

ME – Mobile EquipmentMS – Mobile StationMT – Mobile TerminationTE – Terminal EquipmentUSIM – UMTS Subscriber Identity Module

Fig. 5 – User Domain

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3. THE ACCESS NETWORK

3.1 The Access Domain and Interfaces

The Access Domain is in direct contact with the User equipment and the corenetwork. This split is intended to decouple access functionality from non-accessfunctionality.

The Access domain contains the physical entities to manage resources of the accessnetwork and provide the user with a way to access the core network domain. InUMTS, the Access Domain refers to the radio access mechanism, and is also knownas the UTRAN (UMTS Terrestrial Access Network).

For UMTS phase 1, only the new UTRAN is considered as part of UMTS Access.However the modular approach, and split between core and access network, meansthat there is no reason to preclude other types of access network developed later on.However all access methods will require use of the USIM.

In the early roll-out of UMTS, it remains relevant to also include the GSM/EDGE radionetwork as an alternative within the access domain, since interworking will berequired until UTRAN coverage is fully achieved.

The UTRAN is connected via another standardised, open interface, the Iu to the CoreNetwork Domain.

CoreNetwork

UTRAN(UMTS

Terrestrial Radio Access Network)

UserUu Iu

GERAN(GSM/EDGE

Radio Access Network)

Um

A

User

User

User

SAN(Satellite Access

Network)

Future Radio

Access

Fig. 6 – Access Domain & Interfaces




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3.2 Requirements of the UTRAN

In defining the UMTS Terrestrial Radio Access Network (UTRAN), a number ofrequirements and assumptions were identified. These are specified to ensuremaximum flexibility in the future evolution of the UMTS concept, and to ensure easyevolution to the UMTS concept from second generation networks. In addition, theyprovide flexibility in accessing the core network from not only the UTRAN, but fromevolved GPRS/EDGE GSM networks, Satellite access networks, fixed access(narrowband and broadband), and future access types such as the Broadband RadioAccess Network.

The UTRAN is considered a separate entity to the core network, with a definedinterface connecting them. This interface is designed to provide a logical separationof signalling and user data transport (this fits in with the evolved GSM networkspecified for use in UMTS at Release 99). The interfaces are designed to be fullyspecified, allowing as few options as possible and based on the logical model of theentities concerned. This ensures maximum compatibility between manufacturers.

All radio procedures and aspects are fully handled within the UTRAN, includingmobility of the radio connection (soft handover, relocation of serving entities etc.).This allows replacement of this radio access network with another access technology,fulfilling one of the basic requirements.

CORENETWORK

UTRAN

Fig. 7 – Requirements of the UTRAN

• Logical Separation of Signalling and Data Transport• CN and UTRAN functions separate from Transport Functions• Macro diversity fully handled in UTRAN• Mobility for RRC connection is fully controlled by UTRAN• Interfaces based on logical model of the entities (with as few functional options

as possible).




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3.3 Functions of the UTRAN

UTRAN functions have been specified to provide support for all radio activitiesneeded within the network infrastructure. They can be split into four main areas –System Access, Mobility, Radio Channel Ciphering, and Radio Resource Managementand Control.

System access functions involve broadcasting system information to allow the mobileto configure for access, admission control and radio channel congestion.

Mobility functions within the UTRAN are extensive in that they comprise handovers,Serving Radio Network Controller (SRNC) relocations, and additionally, UTRANRegistration Area (URA) and Cell updates for packet mode procedures. These areused so that the UE can fall back to a less active state whilst retaining its packet data“virtual connection”, known as a Packet Data Protocol Context (which describes thequality of service required as well as specifying the address). In this case, the mobileis tracked at URA or Cell level and paged accordingly when required to receive data.

Radio channel ciphering occurs in the UE and Serving RNC (at the RLC or MAClayer), unlike GSM, where only the air interface is ciphered.

UTRAN FUNCTIONS

Radio ChannelCiphering andDeciphering

Systems AccessControl (Admission,

Congestion,System information

broadcast)

Radio Resource Management and

Control

Mobility(Handover, SRNS

Relocation)

Fig. 8 – UTRAN Functions




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3.4 UTRAN Architecture – General

The UTRAN architecture comprises of one or more Radio Network Controllers (RNCs),each controlling a number of base sites, known as Node B. Each grouping of RNCand its associated Node Bs are collectively known as a Radio Network Sub-system(RNS). Hence an UTRAN is comprised of one or more RNS.

Standard interfaces connect each RNS to the Core Network (both Circuit Switchedand Packet Switched Domains), and to the User Equipment. These interfaces areknown as IuCS, IuPS, and Uu respectively.

The UTRAN internal interfaces are also standardised. The Iur connects RNC (andhence RNS), whilst the Iub connects the RNC and Node B.

Radio Network Controller

Iur

Iu (CS & PS)

Radio NetworkSub-system

(RNS)

RNC RNS

Iub

Node B

CORE NETWORK

Fig. 9 – UTRAN Architecture and Terminology




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3.5 The Node B

The term Node B refers to the base station equipment which communicates with thesubscriber’s handset via the radio link (and of course with the main network via atelecoms link).

It provides radio resources for a UMTS network, and uses UMTS channel allocation tocommunicate with the handset. It provides all the RF processing, enablingtransmission and reception information to and from the mobile terminal. Thisinformation is encoded using the W-CDMA scheme.

A single UMTS channel can be used on adjacent Node B sites and in different sectorsof the same Node B antenna system. A typical Node B may support a three sectorantenna and one or two UMTS carriers, although it is possible to configure up to sixsectors and up to three UMTS carriers. Each sector can be used as a different cell.

Node B tasks are as follows:

• conversion of data to and from the radio interface

• forward error correction

• rate adaptation

• W-CDMA spreading & despreading

• QPSK modulation (Quadrature Phase Shift Keying)

• measuring the quality & strength of connection

• determining the frame error rate

• handover between different sectors on the same Node B (“softer handover”)

• participation in power control, enabling the user terminal to adjust its power (“innerloop power control”)

Fig. 10 – Node B Functions


NODE BFUNCTIONS

• Radio Resource Provider• W-CDMA spreading

and despreading• QPSK Modulation• Signal quality & strength

measurement• Inner loop power control

• May support multiple cellsthrough sectored antenna

• Supports Softer Handover

• Converts data to/from W-CDMA transport• Forward error correction and frame error

rate determination• Rate adaptation



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3.6 The RNC

The RNC controls the operation of multiple Node Bs, managing resources such as allocating capacity for data calls, and providing critical signalling such asconnection set-up, plus switching and traffic routing functionality.

Compared to 2G systems, it is broadly equivalent to the BSC, but also includes somefunctionality of the MSC. In particular, it enables autonomous Radio ResourceManagement by the UTRAN by allowing RNCs to directly communicate (via the Iurinterface), eliminating this burden from the core network. So all handover processes,even where moving between cells controlled by different RNCs, are kept within theUTRAN. Compare this with the situation in GSM, where handover between differentBSC areas required involvement of the MSC, and hence the core network.

The RNC can manage many Node Bs, and allocates radio resources and maintainsthe equilibrium of a live and dynamic network. It must also interface with the corenetwork to provide access to the network operator services, applications, Internetand gateways to networks such as GSM and PSTN.

The Iub is the first example of a fully standardised base-station-to-controller interfacewithin commercial mobile networks, and is defined thus in order to increasecompetition between manufacturers in this very costly part of the network. Forexample it is now possible to source Node B and RNC equipment from differentvendors, and hence for specialist vendors for Node B only, for example, to enter themarket.

The key features of the RNC are:

• management of radio resources

• channelisation code allocation

• QoS monitoring

• handover of users between cells on the same site (softer handover)

• handover of users between cells on different sites (soft handover)

• handover of users between different UMTS carriers (hard handover)

• handover of users to GSM networks (hard handover)

• power control management of user and Node B equipment

• network alarm correlation

Fig. 11 – The RNC – General Functionality


RNC

CN

Node B

• Controls functions of multiple Node Bs• Radio resource management kept within the UTRAN• Interfaces with core network• Manages handover• Power Control Management



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3.7 RNC Terminology

The RNC operates in three main modes – Controlling, Serving, and Drift, dependingon whether an RRC connection is established, and how it is configured. Thedescriptions of each mode are with respect to a single User Equipment, since eachphysical RNC contains all the functionality needed for all three modes and is likely tobe acting in different modes with respect to different UE.

Controlling RNC

When mobiles are in idle mode, no RRC connection exists. Hence this mode simplydescribes the functionality of the RNC which controls the Node B on which themobile is camped (i.e. the selected Node B). Any RRC messages relevant to the UEare terminated at the UE and Node B.

Serving and Drift RNC

Once a mobile enters the RRC Connected mode, an RRC connection exists, andRRC messages relevant only to the UE are terminated at the UE and Serving RNC(SRNC).

In Soft Handover, the mobile is effectively served by two or more Node B. In the casewhere the Node B are connected to different RNC, the Serving RNC remains as theonly Serving RNC, whilst the new RNC (now called the Drift RNC, or DRNC) simplyprovides the radio resources necessary for the added radio link, and acts to carry theRadio Resource messages and user data between the SRNC and UE transparentlyover the Iur and Iub interfaces on the relevant channels.

As a result of Soft Handover, the original radio link may be deleted from the "activeset" of links, leaving the Serving RNC without any of its Node B in the active set. Inthis case, the DRNC could become the SRNC by a process called SRNC relocation.This procedure is considered optional.

If another RNC is involved in the active connection through soft handover, it isdeclared a Drift RNC. The Drift RNC is responsible only for the allocation of coderesources, with the original Serving RNC continuing to handle control functions suchas admission, radio resource control, congestion, handover and so on.

It is possible to reallocate the Serving RNC to the former Drift RNC, if this becomesnecessary.

CoreNetwork

ControllingRNC

MSC/VLRor SGSN

NodeB

"Idle" Mode

"Connected" Mode(After Soft Handover)

SRNC Relocation(Optional)

"Connected" Mode

CoreNetwork

ServingRNC

MSC/VLRor SGSN

NodeB

Soft Handover

CoreNetwork

ServingRNC

MSC/VLRor SGSN

NodeB

DriftRNC

NodeB

CoreNetwork

ServingRNC

MSC/VLRor SGSN

NodeB

DriftRNC

NodeB

CoreNetworkRNC MSC/VLR

or SGSNNode

B

ServingRNC

NodeB

Fig. 12 – RNC Terminology




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3.8 Functions of the RNC

3.8.1 Controlling RNC Functions

The CRNC controls one or more Node B. In practice, this is likely to be tens of NodeB. It is responsible for loading and congestion of cells, as well as allocating codesand controlling admission. System information broadcasts for mobiles in idle mode(or packet switched cell or URA paging modes) are originated from the controllingRNC.

3.8.2 Serving RNC Functions

The radio bearers and signalling radio bearers for mobiles in connected mode areterminated here (as well as in the User Equipment). All layer two (data link) processingof information to/from the radio interface is processed here for UE in connected mode(layer 1, the physical layer, is provided by the node B).

Outer loop power control is supported as well as the handover decisions.

Each User Equipment will have only one SRNC. The Serving RNC will often also bethe Controlling RNC for the Node B used by the mobile.

3.8.3 Drift RNC Functions

The DRNC is any RNC other than the SRNC which controls cells currently used bythe mobile. There may be zero, one or more DRNC at any one time for the specifiedmobile. The DRNC may itself be performing macro-diversity combining and splittingin support of Soft Handover.

No layer 2 processing of the data destined for, or received from, the radio interface isperformed in the DRNC.

Fig. 13 – Controlling, Serving and Drift RNC Functions


Controlling RNC (CRNC)

• Controls one or more Node Bs.• One Node B will have only one CRNC.• Controls load and congestion of own cells.• Executes admission control and code allocation for new radio links.

Serving RNC (SRNC)

• Terminates Radio Bearers and Signalling Radio bearers for the mobile (ie RRC is terminated here in RRC connected mode).

• Performs Layer 2 processing of data to/from radio interface.• Controls handover decisions.• Outer loop power control.• SRNC may also be CRNC for Node B(s) used by the mobile.• Each connected UE has only one SRNC.

Drift RNC (DRNC)

• Any RNC, other than SRNC which controls cells used by the mobile.• May perform macrodiversity combining and splitting.• No layer 2 processing, unless mobile is using common or shared

transport channel.• A mobile may have one or more DRNCs.



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3.9 Further UTRAN features

In addition to the elements just described, the main new feature of the UTRAN is theexistence of a new modulation scheme (W-CDMA) with two modes of access, FDDand TDD.

In order to cope with broadband, multimedia traffic which could be circuit or packet,asymmetric or symmetric, a suitable upgrade to the transport layer transmissiontechnology was also required. ATM was selected for this in Release ’99, and isapplicable to the Iur, Iub and Iu interfaces.

Other basic features of the UTRAN are as follows:

• it is contained within only one UMTS network

• it supports set-up, renegotiation & clearing of connections with a range of trafficand performance characteristics

• it supports radio access bearers for broadcast and multicast applications

• it allows a mobile terminal to handle more than one radio access bearer servicesimultaneously

• it permits seamless handover of active radio access bearer services from a singleterminal between the cells of one UTRAN. This handover happens withimperceptible loss of speech and without degrading any QoS requirements for data

• it performs monitoring of cells in idle mode (cell reselection) and in active mode(handover)

• for UTRANs with different UTRA modes (TDD and FDD), cell selection and pagingprocedures will accommodate the fact that service areas may be covered by cellssupporting just one or both modes. It also supports handover between cellssupporting one or both modes

• it performs determination of the location of the mobile terminal

The market reality is that UTRANs will likely start as islands in a sea of GSM BSS, soUMTS is specified to support dual system UMTS/GSM terminals. Issues like cellreselection, paging procedures, handover and so on must therefore be supported inboth directions between GSM BSS and UTRAN (although the different bearercapabilities mean that some traffic flows may have to be released or renegotiatedduring handover).

Fig. 14 – Further New UTRAN Features


• W-CDMA

• ATM transport

• Flexible bearer support & connectionmanagement

• Handover functions

• Location determination

• Support for procedure and function interworkingwith GSM BSS



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3.10 Handovers

A handover primarily allows a moving mobile to remain connected with the networkas different coverage areas (cells) are transited. Alternatively, it allows the networkoperator to control congestion and cell loading by compelling a mobile to hand overbetween adjacent cells in the overlap region (or even between hierarchical overlaidcells).

Of increased importance for UMTS, though, is the possibility to hand over betweencells, frequencies, or even access network types for reasons of service requirements(data rates, capacity, and quality of service issues).

Different handover types exist. Hard handovers (as seen in GSM) are needed forhandover between different UMTS carrier frequencies and between systems. Softhandover provides handover between cells handled by different Node Bs, whilstsofter handover allows handover between cells handled by the same Node B.

Soft and softer handovers can be handled entirely within the UTRAN. Hard handoversmay be handled entirely within the UTRAN for handovers between carrier frequencies.The Core Network will be involved for inter-system hard handovers.

3.10.1 Softer Handover

In around 10% of connections at any time, the mobile will be served by more thanone cell or sector operating on the same frequency and provided by the same NodeB. With the same codes used, the received signals are simply input into the RakeReceiver as different components of the same signal. This process, together with theRake combining of any multi-path components enhances the signal.

Combining in this case is achieved entirely within the Node B and the UserEquipment. The process is known as micro-diversity. Only a single power control loopis active per connection, provided by the Node B.

Fig. 15 – Softer Handover (Micro Diversity)


SRNC

Node B

Combinedsignal received

via Rakeprocessing

CoreNetwork

• Communication via more than one air interface concurrently• Rake receivers at Node B and mobile station used to combine

signal (similar to multipath reception)• Occurs in about 10% of connections• Only one power control loop per connection is active.



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3.10.2 Soft Handover

In the case of the soft handover, combining is done in the RNC, with the differentarriving signals being continually assessed and the best signal chosen (every 10 –80ms) for inclusion in the combined signal. The process is known as macro-diversity.

Soft handover is generally thought to occur in about 20 – 40% of connections, andhence increases the overall requirement for transmission capacity in the UTRANtransport network. Additional Rake fingers are also required to cope with theincreased number of "wanted" paths.

One of the main reasons for employing the soft and softer handover techniques inCDMA is to mitigate the near-far effect, where a closer mobile contributesdisproportionately to the overall interference levels. Hence in all handover cases,power control is critical.

For softer handover, only one power control loop is active (i.e. only one Node Binvolved), but for soft handover, more than one power control loop is active (powercontrol is now being provided by more than one Node B). This does not present aproblem since the mobile simply responds to the Node B with the lower powerrequirement, minimising overall interference in the system.

Soft and softer handover can be used simultaneously.

Fig. 16 – Soft Handover (Macro Diversity)


SRNCCore

Network

Node B

Node BCombining/Splitting

Node B

DRNC

• Communication via more than one air interface concurrently.• Signal split/combined at RNC (best frame chosen)• Requires additional:

– Rake receiver channels in Node Bs – Transmission links Node B <-> RNC – Rake fingers in mobile stations

• Occurs in about 20 - 40% of connections• Power control active for each Node B (mobile responds to Node B

with lowest uplink power requirements).• Can be combined with softer handover



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UTRAN PROTOCOLS

3.11 Functions Of The UTRAN Protocols

The functions of each of the UTRAN control protocols are outlined opposite.

RANAP includes those functions needed to manage location procedures which mayneed Core Network interaction, such as Hard Handover, and SRNS relocation. Radioaccess bearer management, security, paging, identity management, and transparenttransfer of Non-Access Stratum signalling are all supported by RANAP.

RNSAP provides functions which are split into four modules. Basic inter RNC mobilityis supported in order to provide soft handover between RNS and to transfer waitingdata during SRNS relocations.

In addition, support is provided for both dedicated channel traffic (transparentlytransferred between SRNC and UE in dedicated transport channels) and commonchannel traffic (transferred from the SRNC to the DRNC for inclusion in the commonchannels being supported by that DRNC – which is also acting as the CRNC for theNode B in question).

NBAP functions are classed as either common or dedicated, depending on whetherthey are concerned with common or dedicated channels. RNC in Controlling, Servingor Drift Modes are supported. The functions are generally concerned with the use orconfiguration of the radio channels, including paging, access requests, radio linkmeasurements, handovers and fault management.

NB

AP

RNSAP

RA

NA

P

CoreNetwork

RANAP FUNCTIONS INCLUDE:• Relocation SRNS & Hard Handover• Radio Access Bearer Management• Paging and ID Management• UE <-> CN Signalling Transfer (Transparently)• Security Mode Control• Location Reporting

RNSAP FUNCTIONS INCLUDE:• Basic Inter - RNC Mobility• Dedicated Channel Traffic Support• Common Channel Traffic Support• Global Resource Management (optional)(Implemented in Four Separate Modules Shown Above)

NBAP FUNCTIONS INCLUDE:Common -• Setup First Radio Link of UE• RACH, FACH & PCH Handling• Reporting Cell/Node B Measurements• Cell Configuration• Fault Management

Dedicated• RL Addition, Release & Reconfiguration for

one UE context• Dedicated and Shared Channel Handling• Softer Combining Support• Reporting of RL Specific Measurements• RL Fault Management

SRNC DRNC

NODE B

Fig. 17 – UTRAN Protocol Functions




35

4. THE UTRAN TRANSPORT NETWORK

4.1 Requirements Of The Transport Network

As a network of interconnected nodes, the UTRAN presents familiar problems to thedesigner of a transport network.

The UTRAN provides the User Equipment (UE) with access to the Core Network (CN)for both Circuit Switched and Packet Switched services as well as providing transportfor all signalling interactions, including those confined within the UTRAN, thosebetween the UTRAN and the Core Network, and those being transferred through theUTRAN from UE to CN or vice-versa.

The W-CDMA air interface has been designed to support services which vary widelyin terms of acceptable quality of service. Hence services with varying data rates,delay tolerance, delay variance, and acceptable error rates are all possible.

The UTRAN transport network has therefore been specified to support the varyingqualities of service required for all the data types.

4.2 The Options

In choosing the technology, reliability, cost, flexibility, scalability, delivery time scales,and not least suitability for the task at hand were all factors. Specifying a protocolespecially for UMTS was deemed not necessary since ATM (Asynchronous TransferMode) already existed and provided a relatively close match to the requirements.Fortunately, the transport protocol has been specified separately from the UTRANprotocols themselves, hence future flexibility in choice of technology is assured.

At the moment, ATM is specified rather than an Internet Protocol (IP) solution,however the continuing work on IP is bringing it closer to satisfying the requirements(and at an inevitably low cost).

Fig. 18 – UTRAN Transport Network Requirements


RNC

UTRANTransportNetwork

RNC

MSC

Rest ofNetwork

(CS & PS)

SGSN

RNC

NodeB

NodeB

Packet Switched& Circuit Switched

User Data & Signalling

Radio Bearers

Signalling Radio Bearers

Interface Control Information

• ATM Chosen



37

4.3 ATM Operation

Within a network of ATM switches, virtual channels and virtual paths through thenetwork from entry point to exit point can be provided by ensuring that the switcheshave the relevant identifiers and routing information available. This information can bepre-configured, or set up within the switches dynamically by specific signallingmessages as a requirement for a path or channel through the network arises.

Switching is achieved by the use of fixed length (53 Octet) cells with appropriateidentifiers. Each cell is identified at each ATM switch and simply directed on to thenext switch in accordance with the routing information held in the switch. Theidentifiers will change as the cell passes through the switch, however, each switch willhave been programmed with the correct identifiers and the overall path or channel willstill be valid.

Switching can occur on two levels – at the path level, which may simply switch byanalysing only the "virtual path identifier" (irrespective of the "virtual channelidentifier"); and at the channel level, where both identifiers are analysed and the cellrouted accordingly. This can allow flexibility in network provision by allowing simplerprocessing at virtual path switches, and more in-depth at virtual channel switches.ATM physical switches can of course have both levels of switching available.

ATMNETWORK

ATM Switch

A

A

B

B

• Virtual channels/paths through the network are set up by O&Maction or dynamically using signalling

• Channels and paths identified using VCIs (Virtual ChannelIdentifiers) and VPIs (Virtual Path Identifiers) in the ATM Cell Header

Fig. 19 – ATM Operation




39

4.4 The ATM Cell

On any link between switches, the cells for a single path or channel will be allocatedas required (asynchronously) within the overall synchronous cell stream.

The cell itself is made up of 48 octets of data (which may include higher layer controlinformation) with 5 octets of ATM header information. This information includes:

The Virtual Path Identifier (VPI)The Virtual Channel Identifier (VCI)Payload Type (PT)Cell Loss Priority (CLP)Header Error Correction (HEC)

The VPI and VCI are used in the switching process. The PT identifies the type ofpayload. The Cell Loss Priority allows cells to be prioritised in terms of which onescould be discarded first in congestion situations. HEC provides a mechanism forchecking for errors within the header (only).

The ATM Cell:

DATA

Contains User Data andAdaptation Information (Quality

of service requirements)

48 Octets 5 Octets

HEADER:(VPI/VCI/PT/CLP/HEC)

ATM Cell Streams:

Asynchronous allocation of cellsin synchronious stream

A B

Continuous stream of cells

Fig. 20 – The ATM Cell




41

4.5 ATM and Quality Of Service

It is not the ATM cell itself which provides the necessary control and protocols tosupport different Qualities of Service (QoS), but the specified adaptation processwhich occurs between the data to be transported and the ATM cell.

The adaptation process introduces extra overhead (control data) onto the data to beplaced within the 48 octets of data within the ATM cell. In terms of protocol, the ATMAdaptation Layer (AAL) lies directly between the data to be carried and the ATM layer.Four different AALs have been specified for use with ATM, and two have beenadopted within the UTRAN – AAL2 and AAL5.

The characteristics of each are shown opposite. Between them, they provide supportfor all necessary UTRAN QoS requirements.

• Variable bit rates• Packet type data• Segmentation & Reassembly• Constant delays not required• Suitable for signalling, packet user,

data transfer etc

• Variable bit rates• Circuit type data• Segmentation & Reassembly• Constant delays required• Suitable for multimedia, video etc

AAL-ATM Adaptation Layer

ATM NETWORK

AAL5 AAL5

AAL2AAL2

AAL5

AAL2

Fig. 21 – ATM and Quality of Service




43

5. THE CORE NETWORK

5.1 The Core Network Domain

The core network contains the physical entities providing support for the networkfeatures and telecoms services, for example the management of user location, controlof network services; and switching & transmission mechanisms for signalling & userinformation.

An important logical split is made within the core network for UMTS Release ’99,between a circuit-switched and packet-switched domain.

CS (circuit-switched) DomainThis refers to the set of all core network entities offering “CS type connection”, i.e. one for which dedicated network resources are allocated at connectionestablishment and held until connection release. PSTN and ISDN are examples ofother circuit-switched networks. In UMTS the CS domain provides data servicesupport of at least 64kb/s.

PS (packet-switched) DomainThis refers to all core network entities for “PS type connection”, i.e. one whichtransports user information using autonomous concatenation of bits called packets,where each packet can be routed independently of the previous one. The Internet isthe most well known example of a packet-switched network, although other publicdata networks (PDNs) do exist, including Mobitex, RAM, CDPD and so on. In UMTS,the PS domain provides support for data service capability of up to 2Mb/s.

The Iu interface is therefore subdivided into IuCS and IuPS in order to support connectionof each of these core network domains to the single access network (UTRAN).

PS Domain CS Domain

PS Networks CS Networks

UTRAN

IuCSIuPS

User Domain

CORENETWORK

DOMAIN

Fig. 22 – The Core Network Domains




45

5.2 Specific Entities in the UMTS Release ’99 Core Network Architecture

It is possible to define three categories of network elements in Release ’99:

1. GSM core network elements: MSC, VLR, HLR, AuC and EIR

2. GSM enhancements (GSM Phase 2+):

– GPRS to support packet-switching

– CAMEL (and other toolkits) as a basis for the VHE

3. New UMTS-specific enhancements, in particular the new UTRAN and USIM. Thisnew UTRAN can be connected to the GSM Phase 2+ core network.

We have already discussed the basic elements of the UTRAN, whereas the first twocategories above refer to elements within the core network.

Release ’00 takes the first steps towards integrating circuit switched domain andpacket switched domain transports.

Fig. 23 – The Concept of Strata


1. GSM Core Network Elements

• circuit switching

• databases

2. GSM Phase 2+ Enhancements

• packet switching through GPRS

• CAMEL and other toolkits

3. UMTS specific elements

• UTRAN

• USIM



47

5.3 The Circuit Switched Domain & GSM Core Network Elements

The GSM core network elements form the basis for the circuit-switched domain inUMTS, albeit with some enhancements to support the higher data rates and otherrequirements of UMTS services. The entities specific to CS domain are: MSC, GMSC,VLR.

MSC The MSC provides the interface between the radio system and fixed network,performing all necessary functions to handle CS services to and from mobileterminals. As such, an MSC will interface with several base stations.

In effect it is an exchange which performs switching and signalling functions formobiles within its designated area of control. It needs to take into account theallocation of radio resources and the mobile nature of users, which impact thelocation registration & handover between cells.

Gateway MSC The GMSC provides routing to the appropriate MSC where a mobile terminal islocated, after having interfaced with the databases within the home environment.

VLRThe Visitor Location Register is used by an MSC to retrieve information for mobilestations currently in its area. A mobile terminal registers as it enters the area, at whichpoint the VLR and HLR (Home Location Register – see below) exchange informationon the subscriber and his/her service capabilities. It is the VLR which tracks thecurrent location of the terminal, although the HLR will know on which VLR thesubscriber is registered.

An additional element which is required in UMTS is:

Interworking Function (IWF) In generic terms, an IWF provides the functionality to allow interworking of differingnetworks such as ISDN, PSTN and PDNs (i.e. protocol conversion). A new elementrequired for the CS part of the core network in UMTS is such an interworking functionto provide protocol conversion between the A (GSM) and Iu-CS (UMTS) interfaceswhere the radio network joins the Core Network. This requirement is in order to enablethe core network to operate with both the existing 2G and new UMTS radio access.

UTRAN

UMTS ME GSM ME

GSM/EDGE BSS

MSCVLR

GMSC

IWF

IuCS

A

PSTN, ISDN, etc…

New in UMTS

GSM Phase 2+

Fig. 24 – Circuit-Switched Domain




49

5.4 The GSM Location Registers

The core GSM elements also include some further databases, which are carriedforward into UMTS with appropriate modifications as required:

HLRThe Home Location register contains subscriber information, and is the register towhich a subscriber is assigned. It will also contain information enabling charging andpacket routing of messages to the area where the mobile is currently registered(for GPRS support), plus various location-service related information if that is alsosupported.

Subscriber information consist of:

• the IMSI (international Mobile Subscriber ID)

• Mobile Station ISDN numbers

• Packet Data Protocol Addresses for GPRS

• LMU indicator for location services

• information on service access/restrictions

Authentication Centre (AuC) The AuC stores data for each subscriber to allow the IMSI to be authenticated and toallow ciphering of communication over the radio path. In short, it allows the mobile touse the network. The data required for these two processes is transmitted via theHLR to the VLR, MSC and SGSN as required.

Equipment ID Register (EIR) The EIR is responsible for storing the International Mobile Equipment IDs (IMEIs) inthe GSM system. These classify equipment as white, grey or blacklisted, and soenable service to be prevented to stolen or uncertified terminals.

This set of registers can be grouped together for simplicity to define the “HomeSubscriber Server” (HSS).

EIR

HLR

AUC

HSS

PSTN, ISDN etc…

UTRANGSM/

EDGE BSS

MSCVLR

GMSC

IWF

IuCS

A

UMTS ME GSM ME

Fig. 25 – Location Registers




51

5.5 The Packet-Switched Domain & GPRS Core Network Elements

Since the circuit-switched side of the network is limited to 64kb/s by its ISDN-basedswitching capability, whereas GPRS allows direct interconnect with data networks ofmuch higher bit-rates, GPRS is a prerequisite for the introduction of UMTS.

Entities specific to PS are the GPRS-specific entities, the SGSN (serving GPRSsupport node) and GGSN (gateway GSN).

The GGSN and SGSN have comparable functions and architectural positions as theGMSC and MSC/VLR in the circuit-switched domain. They are IP routers, which allowdirect transmission between mobile terminals and data networks such as the Internet,Intranets, X25 and so on.

The SGSN includes a location register function which stores subscription informationand location information for packet-switched services for each subscriber registeredin the SGSN.

The GGSN stores subscription information and routing information for eachsubscriber for which the GGSN has at least one PDP context active. This informationis used to tunnel packet data destined for a GPRS terminal through to the SGSNwhere this terminal is registered.

Once again, a new interworking function is required in order that the SGSN cancommunicate both with the new UTRAN and the existing GSM BSS.

UTRAN

UMTS ME GSM ME

Gb

IuPS

GSM/EDGE BSS

SGSN

IWF

GGSN

HSS

Internet, X25 etc…

EIR

HLR

AUC

IWF

MSCVLR

GSMC

Fig. 26 – Packet-Switched Domain




53

5.6 Operation in the Core Network CS Domain

5.6.1 User and Control Information in the CS Domain

Within the CS Domain of the Core Network, MSCs provide the switching functionalityand control for setting up, tearing down and supervising circuits, as well as somesupport for supplementary services. In addition, the HLR and SCP provide support forMobility and Operator Specific Services respectively. The VLR also provides supportfor mobility and is co-located with the MSC. This is illustrated in Fig 4.

User data is transferred between MSCs, and between the GMSC and externalnetwork via traffic circuits, without further protocols being added (although overheadis introduced at the physical level, the amount and format being dependent on thetransmission system being used).

The control information is passed within Signalling System Number 7 protocols, andmakes use of the lower layer signalling network (which would usually share the sametransmission infrastructure as the user data).

HLR

SCP

MSC/VLR

MSC

MSC

Circuit ( User Data)

Core Network

User Data

Traffic Channel Set Up, Clear Down and Supervision (includes Supplementary Service Support)

Service Control

Mobility Management

Signalling (Control Information)Circuits (User Data)

GMSC

1

2

3

4

1

2

3

4

Fig. 27 – User and Control Information in the CS Domain




55

5.6.2 Signalling in the CS Domain – SS7 Overview

In the circuit-switched domain, MSCs are switching centres which hold all theswitching functions needed to support mobiles in their area, routing transmissionpaths for both the actual user data, and the signalling messages needed to controlservices. They may also hold interworking functions required to interwork with othernetworks such as the PSTN.

These MSCs are connected to each other and to the HLR and other databases usinga variation of the ITU standardised SS7 (signalling system 7). SS7 is a “common-channel” signalling system, and the circuit-switched user data network is actuallyseparated from the packet-switched SS7 signalling network. SS7 operates using aspecified stack of protocols.

SS7 provides call control by exchanging control messaging between the MSCs andfixed network switches. This may be via direct paths or via signalling transfer points(STPs), designed to route packets across this network.

A third entity, the service control point (SCP), is a database which may controlinformation relevant to routing, for example translating a freephone (0800) numberinto a routing address within the network. No longer do individual switches need tobe modified to introduce a new service. Instead, such changes are made to the SCPelements within the signalling network, which controls the user data switchingperformed at the MSCs/Switches. The SCPs and MSCs/Switches communicate viaa standardised interface, and (if it has this functionality), the MSC/Switched is termeda Service Switching Point (SSP).

In the UMTS CS domain, the MSCs correspond to the SS7 SSPs, providing bothbasic switching, plus access points to supplementary and advanced IN services.Location registers, such as the HLR and VLR, and additional service elements suchas the CAMEL Service Environment, are similar in functionality to the SCPs.

Where remote data bases are being accessed, a Signalling Point Relay (SPR) allowsthe SS7 entity’s unique global address to be translated into the simpler format (pointcodes) used in the individual SS7 packet-switched networks (called the MessageTransfer Port), as the signalling message is passed from one network to anotheren-route to its remote destination.

Databases(SCP, HLR, CSE)

MSC(SSP)

MSC(SSP)

STP

STP

SS7 NETWORK

Use

r D

ata

Fig. 28 – SS7 within the Core Network CS Domain




57

5.7 Operation in the Core Network PS Domain

5.7.1 User and Control Information in the PS Domain

The Packet-Switched domain uses packets of information to carry both user data andthe control information for the user data between GSNs. This means a common set ofpacket protocols can be defined to allow this exchange of information to take place.The intermediate routers handle the information in the same way, simply routing it onto its “final” destination (the SGSN or GGSN).

For mobility control and provision of operator specific services, the GSNscommunicate with the HLR and SCP respectively using the standard techniquesfound within the Circuit-Switched domain.

HLR

SCP

SGSN GGSN

Signalling (Control Information)Packets of Data(Control and UserInformation)

Router

Router

Packets (U ser & Control Data)

Core Network

User Data

Control Data

Service Control

Mobility Management

1

2

3

4

1 2

4

3

Fig. 29 – User and Control Information in the PS Domain




59

5.8 OTHER NETWORK ENTITIES

5.8.1 Some other entities for specific services

Although the above are the basic and key components for GSM and GPRS basedsystems, and hence the basis for the first release of UMTS, it should be noted thatthere are various other network elements which relate to the delivery of specificservices. Notably

For SMS:Two elements are important in the delivery of SMS. Firstly, the SMS Gateway MSCacts as an interface between the SMSC (Short Message Service Centre) and thePLMN for the delivery of messages. Its counterpart is the SMS Interworking MSC,which acts in reverse, as an interface between the PLMN and SMSC for thesubmission of messages. These connect to the serving MSC/GSN.

For Location Services:The Serving Mobile Location Centre (SMLC) manages the overall scheduling ofresources to perform positioning. In UMTS this functionality is integrated into theServing RNC, an element of the UTRAN.

For CAMEL: To support CAMEL features, including Service Control Functions, Switching Functionsand so on, the CAMEL Service Environment (CSE) is defined and lies within the HomeNetwork.

For Cell Broadcast:The Cell Broadcast Centre (CBC) manages Cell Broadcast messages and determinesdelivery parameters. The CBC attaches directly to the RNC via the Interface IuBC, forwhich a mandatory logical interface protocol is defined.

Other entities include:

• the Group Call Register Entity, holding information on Voice Group Call or VoiceBroadcast Services

• the Shared Interworking Function, providing interworking for data/fax calls

• various Number Portability entities

However, detailed discussion of these further entities is beyond the scope of thisoverview course.

Fig. 30 – Specific Services and Support Elements


SMS

• Short Message Service Centre

• Connects to the UMTS Core Network via SMSGateway/Interworking MSC

Location Services

• Serving Mobile Location Centre (SMLC)

• Located within the UTRAN Radio Network Controller

CAMEL

• Camel Service Environment

• Located within the Home Network

Cell Broadcast

• Cell Broadcast Centre

• Connects directly to the UTRAN Radio NetworkController


6. IN/CAMEL IN UMTS

6.1 Intelligent Networks

Intelligent Networks originally provided advanced features such as freephone, callingcard and so on, by providing intelligence within databases which could translatethese dialled numbers into standard routing numbers within networks. These earlyservices were soon followed up by further advanced services based on thisintelligence, incorporating interaction with the user to further customise services.

Traditionally, switching equipment would need to be upgraded each time a newservice was required. IN separates service intelligence and switching, such that toimplement any defined “Capability Set” of services, upgrades to switches arerequired, but the addition of the actual services within this capability set do notrequire switch upgrades. This means that new services can be quicker and cheaperto install, and that service creation and switching is split into two markets, therebyincreasing vendor competition.

IN can provide such services only when there is an exchange of data betweenthe switch and an application or database which has knowledge about numbertranslation or other features. Most INs, including GSM Phase 2+ networks, use lowerlayer SS7 protocols to enable the Switches (known as Service Switching Points, orSSPs) to communicate with databases known as Service Control Points (or SCPs).

The application or database must reside in the IN, and a standardised protocol layerknown as INAP is used to enable interaction between the SSP and SCP. INAP liesabove the internationally standard protocols which form the SS7 signalling system,incorporating MTP, SCCP and TCAP.

The intelligent applications which control IN services are defined by the operator, andare not themselves standardised. This means that IN offers a route to operatordifferentiation, but equally that in many cases the same services cannot be offeredoutside the network of that operator.


61

Switching&

Service Control

Switching

ServiceControl Point

ServiceSwitching Point

Intelligent Applications

SS7 (INAP)

IN:

Pre-IN:

ServiceCreation

Tools

Fig. 31 – Intelligent Networks



6.2 CAMEL

CAMEL (Customised Application for Mobile network Enhanced Logic) is a featuredesigned to provide support for services of operators which are not standardisedservices (e.g. operator-specific IN services), even when subscribers are roamingoutside the home network. CAMEL is a network feature, not a supplementary service.

In order for CAMEL to function, information exchange is required between the Homeand Visited networks, and subscribers who have access to CAMEL services aremarked within each network.

The concept is basically that of IN, in that it is the MSCs (now termed SSPs within theCAMEL context) which communicate with the SCP. The big difference is that the MSCand SCP may well be in different networks (the SCP will be located in the subscriber’shome network for home network operator specific service support).

Due to different networks being involved, the CAMEL standard is more tightly definedthan IN capability set 1 (IN CS-1), although it is still seen as an extension of CS-1. It isspecified within GSM Phase 2+, but is a core feature of the Virtual Home Environment(VHE) concept of UMTS.

GPRS and Circuit-Switched connections are both supported by CAMEL.


63

CAMELService

EnvironmentSCP

HOME UMTS

NETWORK

SERVING UMTS

NETWORK

GatewayMSC/GSN

ServingMSC/GSN

(SSP)

Cam

el In

tera

ctio

ns

Traffic

Cha

nnel

Fig. 32 – CAMEL Within UMTS



7. CHARGING


65

CS Domain:

• Time

• Location

• Number of Channels

PS Domain:

• Time

• Location

• QoS

• Data volume

Fig. 33 – Charging



8. CORE NETWORK TRANSPORT

The CS Domain will in most cases be brought forward directly from an operator’sexisting GSM Phase 2+ core network. Such networks commonly use PDH or SDH,although there is no standard specified for this.

Equally in the PS domain, no standard transport is specified – any IP network can liebetween the GGSN and SGSN. ATM is a common choice, since it is designed forrobust support of packet networks.

ATM is already specified for transport within the UTRAN. It seems likely that both corenetwork domains may also migrate to ATM in UMTS networks, particularly whenoperators seek to combine the transport systems of the CS and PS domains, a majorgoal behind the network architectures proposed in Release 4 and beyond.

Such an upgrade will require the addition of an interworking function within the MSCsto support ATM-PSTN interworking, and provide support for the ATM protocol stack.

However currently, the choice of transport layers remains up to the operator, and isnot defined in the standards.


67

Mobility & Service

Databases

PS DOMAIN

UTRAN

• PDH or SDH

• Any IPNetwork

• Commonly uses ATM

CS DOMAIN

• ATM Based

• Goal is to combine PS & CS Transport Systems (Release 4 Architecture)

Fig. 34 – Transport in the Core Network



9. MOBILE IP

9.1 Mobile IP: Basics

Mobile IP is an ongoing standardisation project within the IETF (Internet EngineeringTask Force), who are now also a market representation partner within 3GPP.

The aim of Mobile IP is to enable a mobile to communicate using the same IPaddress at all times, regardless of the IP network through which it accesses theInternet. If this were not the case, then active TCP sessions would be broken eachtime the mobile wanted to access through a different network (e.g. UMTS vs. LAN),meaning that guarantees of service quality, and a “seamless view” for the application,would not be possible.

Mobile IP provides a mechanism whereby a mobile station is given a permanent IPhome address, which belongs within its original home network. If accessing throughthis home network, it will therefore just act like any non-mobile station and can bereached through normal IP routing.

However when it accesses through some visited network, it is assigned a “care ofaddress” (COA) which belongs to this visited network, and which identifies the currentlocation of the mobile. Since other stations do not know the location of the mobile,they will send packets to its permanent home address, where the packets arereceived by a router which is assigned the status of the “home agent” (HA).

This HA forwards packets onto the mobile station using tunnelling, having previouslybeen provided with the COA by the mobile. The mobile station can answer directly tothe other station, although using its home address rather than the COA as the sourceaddress for the message. Any time the mobile station moves to attach via a differentIP subnetwork, it will register its new COA with its HA.


69

HomeAgent

“COA”

COA – “Care of Address”

(Uses Home Address, not COA)

Can Answer Direct

CalledCaller

“Permanent(Home) Address”

Visited Network

Fig. 35 – Mobile IP



9.2 Mobile IPv4 vs. IPv6

In the case of IPv4, a COA address will most likely be a router, called the foreignagent (FA), which will have the functionality to enable it to forward messages onto the mobile station. A single COA may apply to more than one mobile station.Overlaying Mobile IP onto a GPRS/UMTS network means enabling the GGSN to havethis FA functionality, able to set up a PDP context for the mobile station, and tunnelPDUs from the GGSN towards the user.

A key advantage in moving to IPv6 is that the number of IP addresses availablebecomes effectively unlimited. It is possible to assign mobile stations a direct COA,using some form of automatic assignment mechanism. Messages from the homeagent can be tunnelled directly towards the mobile station.

If the mobile station has a direct COA, then the core network tunnelling provided byGTP becomes redundant, since data can be tunnelled directly from source to user.Indeed it will be possible to combine the GGSN and SGSN into a single InternetGPRS Support node (IGSN), which acts as the FA and marks the end of the UMTS-specific network. The IGSN would need to support current SGSN functionality,supporting MAP communication with UMTS location registers, plus of course supportMobile IP and any accounting procedures required by an FA.

In effect, an ultimate scenario is that Mobile IP may handle mobility management andtunnelling within the PS domain core network.


71

a) IPv4 Plus GPRS/UMTS

HA

“COA”

PDP/GTP

PDP/GTP

Visited Network

GGSN/FA

SGSN

HA

“COA”“COA”

Visited Network

IGSN

b) IPv6 Plus GPRS/UMTS

COA – “Care of Address”HA – Home AgentFA – Foreign AgentIGSN – Internet GPRS

Support Node

Fig. 36 – Mobile IP Evolution



10. RELEASE 4

10.1 Release 4 – Control & Data Separation in the CS domain

In particular, Release 4 introduces the concept of separation of the control and userplanes (i.e. signalling & user data transport). Whereas Release ’99 is based on re-usingand extending the equipment from GSM core networks, Release ’00 makes the firstmove towards implementing a full IP core network, and introduces new multimediaserver elements.

The advantage of this new approach is in moving towards a scenario needing onlya single transport network for both voice and data (i.e. both the circuit and packetswitched domains), so the same IP or ATM based interface can be employed totransport packets between the Radio Access Network and the external PSTN orIP networks.

In particular, the first step is that the circuit-switched domain is evolved by splitting theMSC into two entities, a Media Gateway which handles actual user data transport(“transport plane”), and the MSC server, which lies within the “control plane”, and isinvolved in signalling and control of the Media Gateway. Non IP-native terminals(e.g. legacy GSM handsets) are handled by the MSC servers.


73

UTRAN PSTNMSC GMSC

1. Release ’99 – CS Domain

2. Release 4 – CS Domain

HSS

IucsSignalling &User Data

UTRAN PSTNMGW MGW

MSCServer

GMSCServer

HSS

User Data

Control

Signalling

Control

Fig. 37 – Evolution of Circuit Switched Domain



10.2 The IP Multimedia Subsystem

In the packet switched domain, transport continues between the Serving SGN andGateway GSN, but a new subsystem, the IP Multimedia subsystem, is introduced intothe core network. This system is introduced in order to enable support for IPtelephony a well as IP multimedia applications, direct to multimedia, IP-addressable terminals.

The use of IPv6 is mandatory within this IP Multimedia subsystem, and control of thesystem is centred around a new element, the Multimedia Call Server (CSCF – CallState Control Function). The CSCF is a SIP server, creating a SIP session to themobile terminal.

A Media Gateway takes care of the transformation of user and signalling trafficbetween this packet-based domain and the PSTN world, or the IP MultimediaSubsystem can of course connect directly into an external IP network via the GGSN.

Other new elements include the Media Gateway Control Function (MGCF) and theMedia Resource Function (MRF).


75

UTRANInternetetc…

PSTNetc…

SGSN GGSN

CSCF(SIP

Server)

MRF

MGCF MGW

HSS

Signalling OnlyUser Data

Fig. 38 – IP Multimedia Subsystem



10.3 New Domain Concept in Release 4 and Beyond

Taking the new architectural evolutions into account, it is now possible to summarisethe new UMTS. The Radio Access and User Domains remain structurally the same.

The Core Network remains divided into circuit and packet switched domains, with theIP multimedia Core Network subsystem newly added.

It is also useful to define the “service subsystem”, which can link into each of thecore network domains and the IP multimedia subsystem by means of a “servicecontrol point” to the Open Service Architecture.

All these various systems of course must continue to interact with the HomeSubscriber Server, HSS.

For the sake of clarity, not all the elements or interfaces within and between thevarious elements are shown.


77

RadioAccess

HSS

OSA

Services Subsystem IP Multimedia CN Subsystem

CS Domain PS Domain

UserDomain

GGSN

SGSN

CSCFSCP

MSCServer

Fig. 39 – Domains in All-IP UMTS



11. NETWORK EVOLUTION

11.1 3GPP Release ’00/Release 4

Release ’00 (Release 4) and future planned releases of UMTS concentrate on theevolution of the core network architecture defined in Release ’99. In particular, thelong-term aim is to move towards an architecture which is “all IP”.

The phases of standardisation can be summarised as shown opposite.

This reiterates the major changes as follows, from GSM as the original starting point:

• GPRS introduced a packet switched domain into the system, and allowed IP services tunnelled directly to ISPs (single media IP).

• UMTS Release ’99 added a brand new radio interface.

• UMTS Release ’00/4 and future releases, will integrate the packet and circuit-switched domains in the core network, adding new server elements to achieve this,and ultimately enabling true IP Multimedia services to be offered.


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Services

CoreNetwork

AirInterface

IP Multimedia

IP MultimediaSubsystem

GSNs Continue

Tunnels to ISPs – PS Single Media

CS

PS Uses GSNs

CS Uses MSCs

GSM EDGE

GSM GPRS R’99 R’00(R4)

UTRAN

Servers Replace MSCs

Fig. 40 – Phases of Standards



ANNEX 1DOMAINS


A.1 DOMAINS IN UMTS

A.1.1 Domain StructureThe physical architecture in UMTS is modelled using the concept of domains,where areas of the network are identified as separate entities, with each beingmade up of the physical elements in that part of the network. Standard interfacesconnect the different domains together.

The domains are organised hierarchically such that the first split simply describesa User Equipment Domain and an Infrastructure Domain. However, these arefurther broken down into the USIM Domain and Mobile Equipment Domain (forthe User Equipment Domain), and Access Network Domain and Core NetworkDomain (for the Infrastructure Domain). The Core Network Domain is then furtherbroken down into Serving, Transit and Home Network Domains.

In general, it is fairly clear what each domain represents.


82

SERVING NETWORK

Zu

YuIuUuCu

Serving NetworkDomain

Core Network DomainAccessNetworkDomain

Infrastructure Domain

MobileEquipment

Domain

User EquipmentDomain

USIMDomain

Transit NetworkDomain

TRANSPORTNETWORK

HOMENETWORK

HomeNetworkDomain

Fig. A.1 – Domains in UMTS


the umts architecture

Documents

umts network

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umts domains

core network architecture

umts system

core network cs domain

access domain

network entities