01- Ran Interfaces Protocols

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RNC Architecture and functionality- RAN Interfaces & Protocols

RNC Architecture and functionality-RAN Interfaces & Protocols


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Table of Contents:

1. RAN Interfaces & Protocols .....................................................................................82. UTRAN interfaces....................................................................................................82.1. Protocol Structure of UTRAN interfaces ............................................................92.1.1. Control Plane and User Plane..................................................................92.1.2. Radio vs. Transport Network Layer........................................................102.1.3. Bearer Concept......................................................................................11

2.1.4. ATM for Transport..................................................................................122.1.5. ATM / AAL5............................................................................................132.1.6. ATM / AAL2............................................................................................142.1.7. AAL5 vs. AAL2.......................................................................................152.1.8. Transport Network Control Plane...........................................................162.1.9. AAL5 in User Plane................................................................................172.1.10. Convergence Protocols...................................................

.......................182.2. Exercise..........................................................................................................193. Iub Interface ..........................................................................................................193.1. Interface Protocol Structure.............................................................................203.1.1. NBAP Signalling Protocol.......................................................................203.1.2. Common NBAP Procedures...................................................................213.1.3. Dedicated NBAP Procedures.................................................................223.1.4. Frame Protocol.......................................................................................233.1.5. User Plane Transport Channels.............................................................243.1.6. NBAP over AAL5....................................................................................253.2. Exercise..........................................................................................................264. Iur Interface...........................................................................................................274.1. Interface Protocol Structure.............................................................................27

4.1.1. RNSAP Signalling Protocol ....................................................................274.1.2. RRC Signalling Protocol..................................................


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.......................284.1.3. Serving RNC vs. Drift RNC.....................................................................294.1.4. Interface Protocol Structure....................................................................304.2. Exercise..........................................................................................................31

5. Iu-Cs/Ps Interface..................................................................................................325.1. Interface Protocol Structure.............................................................................325.1.1. RANAP Signalling Protocol ....................................................................325.1.2. RANAP (cont.)........................................................................................335.1.3. Iu-CS and Iu-PS Control Plane...............................................................345.1.4. Iu-CS User Plane...................................................................................35

5.1.5. Iu-PS User Plane ...................................................................................365.1.6. IP overIP...............................................................................................375.1.7. GPRS Tunneling Protocol (GTP)............................................................385.1.8. Convergence Protocols..........................................................................395.1.9. 3GPP Release 4 Architecture.................................................................405.2. Exercise..........................................................................................................416. Iu-BC Interface..............................................................

........................................416.1. Interface Protocol Structure.............................................................................426.1.1. Iu-BC Interface Protocol Structure..........................................................426.1.2. SABP.....................................................................................................436.1.3. TCP........................................................................................................446.2. Sab service Animation.....................................................................................45


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6.2.1. RNC can serve up to four CBCs.............................................................45

6.2.2. Message broadcasting...........................................................................466.3. Exercise..........................................................................................................477. UU Interface..........................................................................................................487.1. Interface Protocol Structure.............................................................................487.1.1. MAC Protocol.........................................................................................487.1.2. Transport Channels................................................................................49

7.1.3. RLC Protocol..........................................................................................507.1.4. RRC Signalling Protocol.........................................................................517.1.5. User data ...............................................................................................528. SS7 Signalling.......................................................................................................538.1. Broadband SS7 Protocol Structure..................................................................548.1.1. Iub Protocol Structure (1).......................................................................548.1.2. Iub Protocol Structure (2)...............................................

........................558.1.3. Iur Protocol Structure (1)........................................................................568.1.3.1. Iur Protocol Structure (2) ......................................................................578.1.4. Iur Protocol Structure (3)........................................................................588.1.5. Iu Protocol Structure ..............................................................................598.2. Exercise..........................................................................................................609. O&M and Supporting Interfaces.............................................................................619.1. Interface Protocol Structure.............................................................................619.2. Exercise..........................................................................................................6410. RRC Connection Set-up ....................................................................................6510.1. RRC Connection Setup Procedure.............................................................6510.1.1. RRC connection is initiated by UE..........................................................6510.1.2. RRC Connection Request......................................................................66

10.1.3. Admission Control..................................................................................6710.1.4. Setting up a Dedicated Channel..........................................


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...................6810.1.5. Setting up a Dedicated Channel (2)........................................................6910.1.6. Setting up a Dedicated Channel (3)........................................................7010.1.7. RRC Connection Setup Message...........................................................71

10.1.8. RRC Connection Setup Complete..........................................................7110.1.9. Radio Bearer Management ....................................................................7210.2. Exercise.....................................................................................................7311. Non-Access Stratum Signalling..........................................................................7311.1. Example: Mobile Originated Call................................................................7411.1.1. RRC Connection Must Exist...................................................................74

11.1.2. Initial NAS Message...............................................................................7511.1.3. Further NAS Messages..........................................................................7611.1.4. Access Stratum Signalling Required ......................................................7711.1.5. Mobile Originated Call............................................................................7811.2. Example: Mobile Terminated Call...............................................................7911.2.1. Paging....................................................................................................7911.2.2. RRC Connection Must Exist...............................................

....................8011.2.3. Initial NAS Message...............................................................................8111.2.4. Mobile Terminated Call ..........................................................................8212. Radio Access Bearer Set-up..............................................................................8312.1. RAB Setup Procedure................................................................................8412.1.1. RAB Assignment Request......................................................................8412.1.2. Admission Control..................................................................................85


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12.1.3. AAL2 Data Bearer at Iu-CS....................................................................86

12.1.4. DCH Reconfiguration (1)........................................................................8612.1.5. DCH Reconfiguration (2)........................................................................8712.1.6. AAL2 Data Bearer at Iub........................................................................8812.1.7. Data Bearer Synchronisation..................................................................8912.1.8. New Radio Link Configuration................................................................9012.1.9. RB Setup Message................................................................................90

12.1.10. RB Setup Completion...........................................................................9112.2. Exercise.....................................................................................................9213. Soft Handover Signalling ...................................................................................9213.1. Example: Branch Addition..........................................................................9313.1.1. Measurement Report..............................................................................9313.1.2. Handover Control...................................................................................9413.1.3. Radio Link Setup Request Iur............................................

..................9413.1.4. Admission Control..................................................................................9513.1.5. Radio Link Setup Request Iub.............................................................9613.1.6. Radio Link Setup Response Iub..........................................................9713.1.7. Radio Link Setup Response Iur...........................................................9713.1.8. AAL2 Data Bearer Setup........................................................................9813.1.9. Active Set Update Message...................................................................9913.1.10. Active Set Update Complete...............................................................10013.2. Radio Bearer vs. Radio Link.....................................................................10013.2.1. Radio Bearer vs. Radio Link.................................................................10013.2.2. Radio Bearer........................................................................................10113.2.3. Radio Link............................................................................................10213.2.4. Soft Handover......................................................................................103

14. RAN Protocol Exercise ....................................................................................104Table of Figures:


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Figure 1...........................................................................................................................8Figure 2...........................................................................................................................9Figure 3.........................................................................................................................10

Figure 4.........................................................................................................................11Figure 5.........................................................................................................................12Figure 6.........................................................................................................................13Figure 7.........................................................................................................................14Figure 8.........................................................................................................................15Figure 9.........................................................................................................................16

Figure 10.......................................................................................................................17Figure 11.......................................................................................................................18Figure 12.......................................................................................................................19Figure 13.......................................................................................................................20Figure 14.......................................................................................................................21Figure 15.......................................................................................................................22Figure 16.......................................................................

................................................23Figure 17.......................................................................................................................24Figure 18.......................................................................................................................25Figure 19.......................................................................................................................26


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Figure 20.......................................................................................................................27

Figure 21.......................................................................................................................28Figure 22.......................................................................................................................29Figure 23.......................................................................................................................30Figure 24.......................................................................................................................31Figure 25.......................................................................................................................32Figure 26.......................................................................................................................33


................................................40Figure 34.......................................................................................................................41Figure 35.......................................................................................................................42Figure 36.......................................................................................................................43Figure 37.......................................................................................................................44Figure 38.......................................................................................................................45Figure 39.......................................................................................................................46Figure 40.......................................................................................................................47Figure 41.......................................................................................................................48Figure 42.......................................................................................................................49Figure 43.......................................................................................................................50Figure 44.......................................................................................................................50Figure 45.......................................................................................................................51

Figure 46.......................................................................................................................52Figure 47.......................................................................


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................................................53Figure 48.......................................................................................................................54Figure 49.......................................................................................................................55Figure 50.......................................................................................................................56



................................................68Figure 64.......................................................................................................................69Figure 65.......................................................................................................................70Figure 66.......................................................................................................................70Figure 67.......................................................................................................................71Figure 68.......................................................................................................................72Figure 69.......................................................................................................................72


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Figure 70.......................................................................................................................74



................................................87Figure 84.......................................................................................................................88Figure 85.......................................................................................................................89Figure 86.......................................................................................................................89Figure 87.......................................................................................................................90Figure 88.......................................................................................................................91Figure 89.......................................................................................................................92Figure 90.......................................................................................................................93Figure 91.......................................................................................................................94Figure 92.......................................................................................................................94Figure 93.......................................................................................................................95Figure 94.......................................................................................................................96Figure 95.......................................................................................................................96

Figure 96.......................................................................................................................97Figure 97.......................................................................


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................................................98Figure 98.......................................................................................................................99Figure 99.......................................................................................................................99Figure 100...................................................................................................................100

Figure 101...................................................................................................................101Figure 102...................................................................................................................102Figure 103...................................................................................................................103Figure 104...................................................................................................................104Figure 105...................................................................................................................104


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1. RAN Interfaces & ProtocolsFigure 1.

Objectives

To be able to describe the interfaces in the Nokia Radio Access Network and explainthe key functionality of the related protocols. To understand the use of RAN protocols within the call establishment proceduresLearning time

3 6 hours

Target group

All who want to understand the basics of RAN interfaces

Pre-requisites

2G Systra or similar knowledge Basic understanding of 3G, UTRAN and WCDMA is beneficial2. UTRAN interfacesIn the UMTS Terrestrial Radio Access Network (UTRAN), there are two internalinterfaces:

Iub interface between the RNC and the base transceiver station Iur interface between two RNC nodes involved in a soft handover.


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Between the radio access network and the core network, there are three interfaces:

Iu-CS interface between RNC and MSC Iu-PS interface between RNC and SGSN Iu-BC interface between RNC and Cell Broadcast Center.This page explains the generic protocol structure of the five UTRAN interfaces.Thefollowing pages describe the protocol structure of each interface in more detail.

Figure 2.

2.1. Protocol Structure of UTRAN interfaces2.1.1. Control Plane and User PlaneUTRAN interfaces are vertically divided into a control plane and a user plane.

In the control plane, signalling messages are carried over the interface using aspecificapplication protocol.

In the user plane, different kinds of user data is carried over the interface for instanceencoded speech, IP traffic, or various kinds of multimedia information.


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Also, higher layer signalling traffic for instance RRC messages between the RNCand

user equipment are carried in the user plane.

Figure 3.

2.1.2. Radio vs. Transport Network LayerUTRAN interfaces are horizontally divided into a radio network layer and a transportnetwork layer.

The radio network layer contains the application protocols in the control planeand data

streams carrying user data in the user plane.

The transport network layer offers various signalling bearers and data bearers forreliable transport of the radio network layer information.


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Figure 4.

2.1.3. Bearer ConceptIn 3G terms, a bearer is a transport entity with specified capacity and Qualityof Service(QoS) attributes.

As you can see from the figure on the right, many kinds of bearers have been specified,such as radio access bearers and radio bearers note that these are two differenttypesof bearers.

The UTRAN transport network layer is concerned with bearer management at the low

estlevel.


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Figure 5.

2.1.4. ATM for TransportAsynchronous Transfer Mode (ATM) forms the basis of the transport network layerandis used for all transport -both user data and signalling - over all UTRAN interfaces.


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Figure 6.

2.1.5. ATM / AAL5Signalling messages are carried over ATM using ATM Adaptation Layer 5 (AAL5), whichin practice means chopping up a message and carrying it within successive ATM cells.


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Figure 7.

2.1.6. ATM / AAL2Information in the user plane is also carried over ATM, but employing ATM AdaptationLayer 2 (AAL2).

AAL2 offers multiplexing of bytes of data from different sources (or to differentdestinations) within a single ATM cell.

In this way the available transport network resources in other words ATM links aremore efficiently utilised than when using AAL5 transport.At the same time, AAL2

guarantees low delay, which is important for real-time services.


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Figure 8.

2.1.7. AAL5 vs. AAL2In summary, the ATM adaptation protocols AAL5 and AAL2 are compared in the figureon the right.


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Figure 9.

2.1.8. Transport Network Control PlaneThe transport layer network contains its own control plane, which carries an applicationprotocol denoted Q.2630.1. The main task of this protocol more generally referred toas the Access Link Control Application Part (ALCAP) is the dynamic management ofthe data bearers, in other words dynamic management of the AAL2 connections carryingthe data streams in the user plane. By contrast, the signalling bearers in the controlplane, as well as in the transport network control plane, are always set up by O

peration& Maintenance (O&M) actions.


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Figure 10.

2.1.9. AAL5 in User PlaneIn the Iu-PS and Iu-BC interfaces, data bearers are based on AAL5 transport.

The reason that AAL5 was selected instead of AAL2 was that IP-over-ATM solutionsaregenerally based on AAL5.

The AAL5 bearers in the user plane are always set up by O&M actions, in the samewayas the signalling bearers based on AAL5. Consequently, the transport network controlplane is not needed in this case.


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Figure 11.

2.1.10. Convergence ProtocolsFinally, convergence protocols adapt the application protocols in the control plane andALCAP in the transport network control plane to the lower protocol layers, AAL5andATM.

The convergence protocols are described in more detail on the page SS7 Signalling.


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Figure 12.

2.2. Exercise3. Iub Interface


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The logical interface between an RNC and a base transceiver station, which is Node B in

3G terminology, is called Iub.

Although this interface is defined in the 3GPP Technical Specifications, it is partly aNokia proprietary interface.

The control plane is concerned with Node B Application Part (NBAP) signalling betweenthe RNC and base transceiver station.

In the user plane, information carried in the various transport channels is packed into the

payload of Frame Protocol (FP) frames. Both the user plane data and the signalling iscarried over ATM connections.

Figure 13.

3.1. Interface Protocol Structure3.1.1. NBAP Signalling ProtocolThe application protocol Node B Application Part in the control plane is used forsignalling between the RNC and the base transceiver station. This protocol can be

divided into common NBAP procedures and dedicated NBAP procedures.


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Common NBAP procedures include various functions that are not related to a specific

mobile user.

In contrast, dedicated NBAP procedures are concerned with the signalling related to aspecific mobile user.

Figure 14.

3.1.2. Common NBAP ProceduresCommon NBAP procedures include cell configuration, traffic management of commontransport channels, initialisation and reporting of cell or base station specifi

cmeasurements, and fault management.

Last but not least, an important task is to set up the first radio link for a specific userequipment. A traffic termination point must first be assigned at the base station for thisuser before further user-related signalling using dedicated NBAP procedures is possible.

Only one signalling connection is needed at the Iub interface for handling commonNBAP procedures.


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Figure 15.

3.1.3. Dedicated NBAP ProceduresAfter the first radio link to a certain user has been set up, all subsequent NBAP signallingrelated to this user takes place using NBAP dedicated procedures.

Such signalling includes the addition, release and reconfiguration of radio links for thisuser, the handling of dedicated transport channels, the handling of softer combining,initialisation and reporting of radio link specific measurements, and radio linkfaultmanagement.


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Figure 16

3.1.4. Frame ProtocolThe Frame Protocol (FP) is employed for the transfer of the data streams on topofAAL2. The Frame Protocol header contains control information for transport channelsynchronisation and outer loop power control.

The Frame Protocol header contains control information for transport channelsynchronisation and outer loop power control.

Furthermore, in the uplink direction, signal quality information for macrodiversity

combining is sent together with the signal replicas received via the various radiochannels. Based on this information, the best signal can be selected by the Serving RNCfor further transmission towards the core network.


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Figure 17

3.1.5. User Plane Transport ChannelsThe user data carried in various transport channels is packed into the payload of FrameProtocol frames and carried over AAL2 connections in the user plane.


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Figure 18.

3.1.6. NBAP over AAL5In the control plane, the Node B Application Part signalling protocol is carriedon top ofthe Signalling ATM Adaptation Layer (SAAL) using AAL5 transport.


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Figure 19.

3.2. Exercise


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4. Iur InterfaceThe logical interface between two RNC nodes is called Iur. This interface is nee

dedduring soft handover.

In the control plane, the application protocol Radio Network System ApplicationPart(RNSAP) carries signalling information between the RNC nodes.

The protocol structure in the user plane is similar to the user plane protocol structure ofthe Iub interface.

Figure 20.

4.1. Interface Protocol Structure4.1.1. RNSAP Signalling ProtocolThe application protocol at the Iur interface, the Radio Network System Application Part(RNSAP), takes care of setting up and releasing transport connections between RNCnodes.

This is necessary for implementing inter-RNC soft handovers, in other words handoversinvolving base transceiver stations that are connected to different RNC nodes.


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Figure 21.

4.1.2. RRC Signalling ProtocolThe signalling protocol Radio Resource Control (RRC) carries signalling informationbetween the Serving RNC (SRNC) and the user equipment.

Note that RRC signalling messages are carried transparently via the Drift RNC (DRNC)and the base transceiver station.

But what is the difference between Serving RNC and Drift RNC?


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Figure 22.

4.1.3. Serving RNC vs. Drift RNCThe Serving RNC (SRNC) manages the radio connections between the radio accessnetwork and the user equipment. The SRNC also handles the macrodiversity combiningin the case of a soft handover.

The connection to the core network over the Iu interface is always via the SRNC.

The Drift RNC (DRNC) does not process Frame Protocol frames in any way, but insteadrelays the frames transparently to the destination the SRNC or the user equipmen

t.


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Figure 23.

4.1.4. Interface Protocol StructureThe protocol structure of the Iur interface is practically the same as that of the Iubinterface. See the page Iub Interface.

However, there are some differences in the convergence protocols. See the page SS7Signalling.


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Figure 24.

4.2. Exercise


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5. Iu-Cs/Ps InterfaceThe interface between the RNC and circuit-switched core network is called Iu-CS.

Theinterface between the RNC and packed-switched core network is called Iu-PS.

In the control plane, the application protocol RANAP (Radio Access Network ApplicationPart) carries signalling information over the Iu-CS and Iu-PS interfaces.

The Iu-CS user plane is based on AAL2 connections, whereas the Iu-PS user planeisbased on IP-over-ATM (IPoA) and AAL5 transport.

Figure 25.

5.1. Interface Protocol Structure5.1.1. RANAP Signalling ProtocolThe main task of RANAP is to set up and release dedicated connections, with definedcapacity and QoS performance, between the user equipment and the core network.Such a dedicated connection is called Radio Access Bearer (RAB).


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RANAP signalling is also required during an intersystem or intrasystem handoverif the

RNS Relocation procedure is involved in this handover. RNS Relocation means thatadifferent RNC becomes the Serving RNC.

Figure 26.

5.1.2. RANAP (cont.)RANAP also carries the signalling between the core network and RNC needed forimplementing location services, for downloading security keys to the radio accessnetwork, and for paging mobile users.

Finally, like the signalling protocol RRC between UE and RNC, RANAP carries higher-layer or Non Access Stratum (NAS) signalling information for various Call Control(CC), Mobility Management (MM) and Session Management (SM) tasks. This signallinginformation is carried transparently via the RNC and thus has no significance for theRNC.


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Figure 27.

5.1.3. Iu-CS and Iu-PS Control PlaneThe protocol structure of the control plane is similar to the control plane protocolstructure of the Iub and Iur interfaces.

The signalling bearers are based on the Signalling ATM Adaptation Layer (SAAL) whichuses AAL5 transport.


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Figure 28.

5.1.4. Iu-CS User PlaneLike the Iub and Iur user planes, the Iu-CS user plane is based on AAL2 connectionsthat are set up by means of the ALCAP protocol.

On top of AAL2, the user data is carried within Iu User Plane Protocol (Iu-UP) frames.In Support Mode, the Iu-UP frame size can change during the connection. This feature isuseful for transport of AMR encoded speech.

In Transparent Mode, Iu-UP does not offer any services.


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Figure 29.

5.1.5. Iu-PS User PlaneAll user plane traffic at the Iu-PS interface is multiplexed on to one or several AAL5permanent virtual circuits.

The permanent AAL5 connections are set up by Operation & Maintenance actions; sothe transport network control plane is not required here.

Transport in the transport network layer is based on IP-over-ATM.

The user data usually in form of IP packets is carried in the GTP-U payload with

outusing any Iu-UP services.


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Figure 30.

5.1.6. IP over IPIt should be noted that the Internet Protocol (IP) is located in two different layers in theIu-PS user plane protocol stack.

The Iu-PS user data typically consists of IP packets that are packed into the GTP-Upayload.

In addition, the transport network layer uses IP-over-ATM technology for carrying theGTP-U protocol data units over the Iu-PS interface.


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Figure 31.

5.1.7. GPRS Tunneling Protocol (GTP)At the Iu-PS interface, a connection-oriented data bearer is obtained by formingatunnel between the RNC and SGSN. This tunnel is, in fact, an extension of a similartunnel in the packet-switched core network between SGSN and GGSN.

The tunnel identifier required for setting up a GTP-U tunnel is provided in a RANAPmessage during Radio Access Bearer (RAB) establishment.

In order to fulfill certain QoS requirements, separate GTP-U tunnels are establi

shed fornon-real-time (NRT) and real-time (RT) traffic.


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Figure 32.

5.1.8. Convergence ProtocolsThe convergence protocols at the Iu-PS interface are the same as those used at the Iurinterface.

The convergence protocols at the Iu-CS interface are also the same, with the exceptionthat the IP-based protocol stack is not supported.

Convergence protocols are explained in more detail on the page SS7 Signalling.


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Figure 33.

5.1.9. 3GPP Release 4 ArchitectureIn 3GPP Release 4, as far as the circuit-switched core network is concerned, MobileSwitching Center (MSC) network elements are replaced with Media Gateway (MGW)and MSC Server nodes.

The user plane data originates or terminates in the MGW.

However, the RANAP signalling in the control plane traverses the MGW and originatesor terminates in the MSC Server.

On the other hand, ALCAP signalling originates or terminates in the MGW.


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Figure 34.

5.2. Exercise6. Iu-BC InterfaceThe interface between the RNC and a Cell Broadcast Center (CBC) is called Iu-BC.Thisinterface is required for implementing the Service Area Broadcast (SAB) service.


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This service corresponds to the SMS Cell Broadcast Service (CBS) defined for GSM.

SAB is a service that enables a provider of information to submit short messagesforbroadcasting within a specified area of the mobile network. These messages couldbeused for notification of, for example, PLMN news, emergencies, traffic reports,roadaccidents, delayed trains, weather reports, theatre programmes, telephone numbers ortariffs.

Figure 35.

6.1. Interface Protocol Structure6.1.1. Iu-BC Interface Protocol StructureThe protocol stack at the Iu-BC interface is shown in the figure on the right.

The Service Area Broadcast Protocol (SABP) carries both the user data and theassociated signalling over the Iu-BC interface.Consequently, there is no clear separation between control plane and user plane.


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Figure 36.

6.1.2. SABPThe protocol SABP performs the following tasks:

Distribution of messages to the correct service areas and stopping thebroadcasting of specific messages if requestedSending of the load status of the service area(s) to the CBC when requestedEnding the broadcasting in one or more service areas, as ordered by the CBCReporting of errors to the CBC.


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Figure 37.

6.1.3. TCPAt the Iu-BC interface, the Transmission Control Protocol (TCP) is employed instead ofthe User Datagram Protocol (UDP) used at the Iu-PS interface.

TCP is a connection-oriented protocol that offers flow control - which means temporarilysuspending transmission in case of congestion - and error control, which in thiscase isretransmission of erroneous packets.


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Figure 38.

6.2. Sab service Animation6.2.1. RNC can serve up to four CBCsThe RNC can be connected to a maximum of four cell broadcast centers, for examplesituated in the four core networks of four different operators.


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Figure 39.

6.2.2. Message broadcastingThe broadcast message is sent from the CBC over the Iu-BC interface to the RNC usingthe SABP protocol.

In the RNC, the message is delivered to the correct Broadcast/Multicast ControlProtocol(BMC) entity, where it is stored.

This BMC entity corresponds to a specific service area in which the message willbebroadcast.

The message is broadcast within the service area using the BMC protocol.


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Figure 40.

6.3. Exercise


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7. UU InterfaceThe logical interface between the base station and the user equipment in other w

ordsthe air interface or radio interface is called Uu in the 3GPP specifications.

The physical protocol layer of this interface is based on WCDMA technology andphysical channels that are multiplexed in the so-called code division domain.

So far as the architecture and operation of the RNC is concerned, this basic WCDMAtechnology has little significance. We shall concentrate instead on the higher protocollayers, since the RNC has to handle the processing of these protocols.

Figure 41.

7.1. Interface Protocol Structure7.1.1. MAC ProtocolThe main task of the Medium Access Control protocol is to handle the mapping betweenlogical channels and transport channels, and for selecting an appropriate transportformat for each transport channel.


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Figure 42

7.1.2. Transport ChannelsThe concept of transport channel is central to WCDMA. The transport channel defineshow the higher-layer data is carried over the air interface. For instance, the higher-layerdata can be split up into several bit streams that are carried in parallel overthe transportchannel.

As shown on the right, there are five kinds of transport channel available for carryinguser data and control information between the Nokia RNC and the user equipment.


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Figure 43

7.1.3. RLC ProtocolThe Radio Link Control protocol handles, for instance segmentation and reassembly ofhigher-layer data and retransmission of data segments that contain bit errors.

Figure 44


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7.1.4. RRC Signalling ProtocolThe signalling protocol Radio Resource Control carries signalling information be

tweenthe Serving RNC and the user equipment.

Note that RRC signalling messages are carried transparently via the base stationandthe Drift RNC in the case of a soft handover.

Figure 45

The Radio Resource Control protocol basically offers two functions:

1) RRC participates in various radio bearer management tasks in coordination with theNBAP protocol operating over the Iub interface.

2) RRC also carries higher-layer signalling information for various call control, mobilitymanagement and session management tasks. This so-called Non Access Stratumsignalling information is carried transparently between the user equipment and the corenetwork using the RRC and RANAP protocols.


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Figure 46

7.1.5. User dataFinally, the Uu interface also carries the user data for instance coded speech or IPtraffic between the UE and Serving RNC.

The user traffic is carried within a transport channel which could be a bi-directionalDedicated Channel (DCH), a downlink Forward Access Channel (FACH) or an uplinkRandom Access Channel (RACH).

The Forward Access Channel and Random Access Channel can be used for carryingsmall amounts of data in the downlink and uplink direction, respectively. In thi

s case, softhandover is not used.


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Figure 47

8. SS7 SignallingSignalling in the PSTN (Public Switched Telephone Network) and the circuit switchedcore of mobile networks is based on a high-performance and reliable common channelsignalling system called SS7 (Signalling System no. 7).

The SS7 protocol stack consists of the Message Transfer Part (MTP) that offers reliablesignalling transport between exchanges in circuit switched networks, the SignallingConnection Control Part (SCCP) that also offers signalling connections to networ

k nodesother than exchanges, and the application protocol that contains the actual signallingmessages.

It is also possible to use IP-based transport protocols instead of MTP or use themtogether with MTP.


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Figure 48.

8.1. Broadband SS7 Protocol Structure8.1.1. Iub Protocol Structure (1)The control plane protocol stack at the Iub interface does not contain any SS7-relatedparts. Let us still look at the protocols between the ATM layer and the signallingprotocols NBAP and ALCAP, since these protocols are also partly used at the other RANinterfaces:

The Service Specific Connection Oriented Protocol (SSCOP) guarantees error-freeand

in-sequence delivery of signalling messages.

AAL5 alone is not capable of offering error-free and in-sequence message delivery.


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Figure 49.

8.1.2. Iub Protocol Structure (2)At the User-Network Interface (UNI), the Service Specific Coordination Function(SSCF)has very little functionality.

In the transport network control plane, ALCAP is used for managing AAL2 connectionsin the user plane.

The main task of the protocol Q.2150.2 is to re-establish a connection if SSCOPhasreleased the connection.


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Figure 50.

8.1.3. Iur Protocol Structure (1)Like the Iub interface, the control plane of the Iur interface is based on AAL5andSSCOP. However, unlike the Iub interface, the Iur interface makes use of the SS7protocol SCCP. For this reason, the signalling protocol structure is referred toasbroadband SS7 broadband since ATM is used at the lower layers.

The SCCP protocol offers direct signalling connections for each active UE. In otherwords, SCCP connections are used for differentiating the signalling transactions

intended for different subscribers.


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Figure 51.

8.1.3.1. Iur Protocol Structure (2)The Service Specific Coordination Function (SSCF) at the Network Node Interface(NNI)allows an ATM link to be seamlessly incorporated into an existing SS7 stack, employingMTP3b as its network layer.

The broadband version of Message Transfer Part Layer 3 (MTP3b) is used fortransporting the SCCP packets over several parallel signalling links. If one ofthesignalling links fails, the MTP3b protocol directs the traffic to the remaininglinks.

Finally, Q.2150.1 offers a similar functionality to the protocol Q.2150.2 whichis used atthe Iub interface.


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Figure 52.

8.1.4. Iur Protocol Structure (3)It is possible to replace the conventional convergence protocol stack between AAL5and SCCP with a similar protocol stack based on IP over ATM transport.

The message-oriented and reliable Stream Control Transmission Protocol (SCTP) isanew alternative to the unreliable UDP and the reliable but slow TCP protocol. SCTP isdescribed in IETF RFC 3286.

The MTP3 User Adaptation (M3UA) protocol supports the transport of SCCP messages

over IP using the services of SCTP. M3UA is described in RFC 3332.

A similar IP-based protocol stack can also be used for carrying ALCAP signalling.


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Figure 53.

8.1.5. Iu Protocol StructureThe convergence protocol structure at the Iu-CS interface is exactly the same asthat ofthe Iur interface with one exception: the IP-based transport alternative is notused.

At the Iu-PS interface, the convergence protocol structure is also the same. However,remember that the transport network control plane is not used. Naturally, the IP-basedtransport alternative is available at this packet-oriented interface.


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Figure 54.

8.2. Exercise


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9. O&M and Supporting InterfacesThe NWI3 network management interface connects Nokia NetAct (network and service

management system) to an RNC and, via a mediator implemented in the RNC, to a BTS.The ATM Cross-Connect (AXC) at the BTS is managed directly from Nokia NetAct.

Instead of the network management interface, the telecom related radio networkconnection and performance management for a base station is handled with the IubNBAP interface, which is not a network management interface.

Figure 55.

9.1. Interface Protocol StructureNWI3 is a Nokia Proprietary interface that connects Network Management Systems withmediators such as NEMU in RNC or network elements with integrated mediationfunction such as ATM Cross Connects.

The network management interface for network elements in WCDMA RAN containfunctionality for performance, configuration, fault, hardware, and software management.


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Configuration management includes management of configurable parameters andnetwork topology.

Figure 56.

The NWI3 network management interface between the RNC and Nokia NetAct is basedon TCP/IP communication protocols.

The RNC provides a LAN interface (Ethernet) or IP-over-ATM (IPoA) connection toNokia NetAct. The application level communication mechanism is based on CommonObject Request Broker Architecture or CORBA.


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Figure 57.

CORBA is an architecture and specification for creating, distributing, and managingdistributed program objects in a network.

The essential concept in CORBA is the Object Request Broker or ORB that allowsthe usage of distributed objects using the General Inter-ORB Protocol (GIOP) and, forthe Internet, its Internet Inter-ORB Protocol (IIOP).

Figure 58.The CORBA based NetAct interface is mediated in the RNC to the BTS O&M protocol.


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The network management interface for the BTS is a TCP/IP connection. The O&Mmessages to the BTS are passed via the IP router in the AXC.

NetAct offers a means for the BTS software handling, fault management, reset from theGUI, BTS connection establishment indication and BTS hardware handling.

Figure 59.

9.2. Exercise


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10. RRC Connection Set-upThe purpose of RRC connection setup is to establish an RRC connection for variou

skinds of signalling between the RNC and a certain UE.

Not more than one RRC connection at a time can be established between the UE andthe RNC.

An RRC connection is always initiated by the UE, and the release of the connection isalways initiated from the network side.

An important task during RRC connection setup is to establish a dedicated contro

lchannel (DCCH) over which the subsequent signalling can be transported.

Figure 60.

10.1. RRC Connection Setup Procedure10.1.1. RRC connection is initiated by UEThe establishment of an RRC connection is always initiated by the UE.

This is obviously true for mobile originated transactions.

This is also true for mobile terminated transactions for instance an incoming ca

ll. In thiscase the network informs the UE about the incoming call by means of paging - viaacommon paging channel - after which the UE initiates the RRC connection establishmentprocedure as described in the following paragraphs.


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Figure 61.

10.1.2. RRC Connection RequestThe UE leaves the idle mode and initiates set-up of an RRC connection by sendingtheRRC Connection Request message over the Random Access Channel (RACH) to theRNC.

Rather complex random access signalling takes place in the lower protocol layersbeforethe RRC Connection Request message is received at the base station.

The RRC Connection Request message contains the initial UE identifier, theestablishment cause, and the initial UE capability.


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Figure 62.

10.1.3. Admission ControlAfter receiving the RRC Connection Request message, the RNC performs admissioncontrol.

If the cell is not in an overload condition, the request to set up the RRC connection isaccepted. During admission control, the RNC allocates a Radio Network TemporaryIdentity as well as radio resources for this connection.


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Figure 63.

10.1.4. Setting up a Dedicated ChannelIf the RRC Connection Request message indicates that the RNC should allocate adedicated control channel (DCCH) to the UE, the NBAP message Radio Link SetupRequest is sent to the base transceiver station. This message includes importantinformation required for setting up the dedicated control channel.


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Figure 64.

10.1.5. Setting up a Dedicated Channel (2)After the BTS has allocated the required radio resources, it can receive information overthe air interface related to this dedicated channel.

The BTS returns the NBAP message Radio Link Setup Response, which includestransport layer addressing information.


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Figure 65.

10.1.6. Setting up a Dedicated Channel (3)Using the AAL2 binding identity provided by the BTS, the RNC establishes the AAL2data bearer over the Iub interface using the ALCAP protocol.

After some additional signalling between the BTS and RNC in order to achieve perfectsynchronisation, the dedicated control channel is ready for use, meaning that the BTS isready to transmit and receive information over the air interface.

Figure 66.


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10.1.7. RRC Connection Setup MessageAfter the dedicated control channel has been set up in the radio access network,

anRRC Connection Setup message is sent to the UE over the Forward Access Channel(FACH).

This message includes among others, the following information: The initial UE identifier,the Radio Network Temporary Identity, and the various parameters related to thededicated control channel that has been set up.

Now the UE is also ready to transmit and receive information over the dedicatedcontrolchannel.

Figure 67.

10.1.8. RRC Connection Setup CompleteThe BTS achieves uplink synchronisation and notifies the RNC with an NBAP RadioLink Restore Indication message.

The user equipment acknowledges successful setup of the RRC connection by sendingan RRC Connection Setup Complete message over the recently established dedicatedcontrol channel to the RNC.


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Figure 68.

10.1.9. Radio Bearer ManagementThe RRC connection setup procedure automatically creates three optionally four signalling radio bearers between the UE and the RNC. These bearers can later bereconfigured or deleted.

RRC signalling is also used for creating and releasing radio bearers for the transport ofuser data between the UE and the RNC.

Figure 69.


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10.2. Exercise11. Non-Access Stratum Signalling

A Non Access Stratum (NAS) signalling connection could be requested by the userterminal by itself for example to initiate a certain service or could be the result ofpaging from the core network.

Both cases are described in the examples below.


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Figure 70.

11.1. Example: Mobile Originated Call11.1.1. RRC Connection Must ExistIn this example, a mobile user wishes to call in other words set up a circuit switchedconnection to another user located, for example, in another radio access network.

If the UE is in idle mode, an RRC connection must first be established. This isexplainedon the page RRC Connection Setup.


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Figure 71.

11.1.2. Initial NAS MessageThe UE sends an RRC Initial Direct Transfer message to the RNC over the DedicatedControl Channel (DCCH) established during RRC connection setup.

This RRC message carries the initial NAS message, in this case ConnectionManagement Service Request indicating to the destination the MSC that the userwishes to place a call.

The initial NAS message is then carried without modification from the RNC to theMSC inthe RANAP Initial UE Message.


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Figure 72.

11.1.3. Further NAS MessagesAll subsequent Non Access Stratum messages are then carried between the MSC andthe UE, packed within RANAP and RRC Direct Transfer messages.


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Figure 73.

11.1.4. Access Stratum Signalling RequiredHowever, during the setup of a mobile call, there are two signalling functions that involveAccess Stratum signalling.

One is related to ciphering control. Ciphering is employed between the UE and the RNC,not between the UE and the MSC.

The other signalling transaction in which the radio access network is involved is theestablishment of the radio access bearer carrying the speech. This is described

on thepage Radio Access Bearer Setup.


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Figure 74.

11.1.5. Mobile Originated CallIn summary, setting up a mobile call typically involves the signalling at the originatingend as shown in the figure.

The labelled boxes represent radio access network specific signalling functions.

If a mobile user answers an incoming call, the situation is slightly different.This is shownin the second signalling example.


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Figure 75.

11.2. Example: Mobile Terminated Call11.2.1. PagingIn the case of a mobile terminated call, the called user must be notified abouttheincoming call. For this purpose, the MSC initiates the paging procedure.

The RNC broadcasts the RRC Paging message over the paging channel (PCH).


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Figure 76.

11.2.2. RRC Connection Must ExistBefore the UE can respond to the paging, an RRC connection must exist between theUE and the RNC.

If the UE is in idle mode, the RRC connection must first be established. This isexplainedon the page RRC Connection Setup.


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Figure 77.

11.2.3. Initial NAS MessageThe UE sends an RRC Initial Direct Transfer message to the RNC over the DedicatedControl Channel (DCCH) established during RRC connection setup.

This RRC message carries the initial NAS message, in this case Paging Responseindicating to the destination the MSC that the user is responding to the pagingmessage.

The initial NAS message is then carried without modification from the RNC to theMSC inthe RANAP Initial UE Message.


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Figure 78.

11.2.4. Mobile Terminated CallThe subsequent signalling is quite similar to that of a mobile-originated call.

In summary, setting up a mobile call typically involves the signalling at the terminatingend as shown in the figure.


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Figure 79.

12. Radio Access Bearer Set-upThe Radio Access Bearer (RAB) defines the transmission characteristics for the userplane data. The RAB parameters describe the traffic class, symmetry properties andtransmission characteristics such as capacity and permissible delay. For circuit-switcheddata, the RAB is usually established over a dedicated channel as in the RAB SetupProcedure example below.

The RAB establishment is initiated by the core network. The RNC, after checking

theavailability of air interface resources, maps the RAB parameters into the appropriatetransport format and physical channel characteristics.


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Figure 80.

12.1. RAB Setup Procedure12.1.1. RAB Assignment RequestIt is always the core network that initiates the establishment of a radio accessbearer.

For this purpose, the MSC or the SGSN in the case of a packet connection sends aRANAP RAB Assignment Request message to the RNC.

This message includes various radio access bearer parameters and transport layeraddressing information for establishing the Iu-CS data bearer.


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Figure 81.

12.1.2. Admission ControlAfter receiving the RAB Assignment Request message, the RNC performs admissioncontrol to determine whether the request can be fulfilled or not.


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12.1.3. AAL2 Data Bearer at Iu-CSUsing the AAL2 address provided by the MSC in the RAB Assignment Request

message, the RNC establishes the AAL2 data bearer over the Iu-CS interface usingtheALCAP protocol.

This step is not, of course, required if a packet-based radio access bearer is establishedbetween the user equipment and the packet switched domain of the core network.

Figure 82.

12.1.4. DCH Reconfiguration (1)

The RNC requests the base transceiver station to prepare reconfiguration of thededicated channel (DCH) that will carry the radio access bearer by sending an NBAPRadio Link Reconfiguration Prepare message to the BTS.

This message contains important information for the BTS, so that it can allocatetherequired transport resources over the air interface.


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Figure 83.

12.1.5. DCH Reconfiguration (2)The base transceiver station allocates the transport resources and notifies theRNC thatthe preparation is ready by sending an NBAP Radio Link Reconfiguration Readymessage.

This message contains the selected downlink channelisation code and transport layeraddressing information.


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Figure 84.

12.1.6. AAL2 Data Bearer at IubUsing the AAL2 address provided by the BTS in the Radio Link Reconfiguration Readymessage, the RNC establishes the AAL2 data bearer over the Iub interface using theALCAP protocol.


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Figure 85.

12.1.7. Data Bearer SynchronisationIn the case of a soft handover, after all the AAL2 data bearers between the basetransceiver stations and the serving RNC have been established as previouslyexplained, the base transceiver stations and the serving RNC must establishsynchronism for the downlink and uplink data bearers, respectively, by means oftheexchange of appropriate Frame Protocol frames.

Figure 86.


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12.1.8. New Radio Link ConfigurationReturning to the case of one radio link in the active set for the sake of simple

illustration the RNC sends an NBAP Radio Link Reconfiguration Commit message tothe BTS to order it to switch to the new configuration for the radio link.

Figure 87.

12.1.9. RB Setup MessageThe RNC sends the RRC message Radio Bearer Setup to the user equipment over anexisting dedicated control channel (DCCH).

This message contains parameters related to the dedicated channel that has been

setup for carrying the radio access bearer.


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Figure 88.

12.1.10. RB Setup CompletionThe user equipment returns the RRC message Radio Bearer Setup Complete to theRNC over the dedicated control channel (DCCH).

Finally, the RNC sends the RANAP message RAB Assignment Response indicating tothe MSC that radio access bearer establishment was successful.


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Figure 89.

12.2. Exercise13. Soft Handover SignallingA basic WCDMA function is macro diversity. This means that a terminal can beconnected to the network through several radio links, also called branches, duringa so-called soft handover.

In practice, the RNCs and base stations that are involved in the soft handover need tobe controlled in a centralised manner. The central control point is the Serving RNC


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(SRNC). If other RNCs are involved in the soft handover, they are called Drift RNCs

(DRNC).

The number of radio links involved in a soft handover at any point in time is calledActive Set. Radio links can be added or deleted depending on their signal quality.

Figure 90.

13.1. Example: Branch Addition13.1.1. Measurement Report

he user equipment continuously measures the downlink quality of signals receivedfrombase transceiver stations located nearby.

When certain reporting criteria set by the RNC are met, the user equipment sendsanRRC Measurement Report message to the Serving RNC over the Dedicated ControlChannel.


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Figure 91.

13.1.2. Handover ControlThe Handover Control entity in the Serving RNC analyses the measurement report anddecides to add a new radio link to the Active Set.

Figure 92.

13.1.3. Radio Link Setup Request IurThe Serving RNC requests radio resources from the Drift RNC by sending an RNSAPRadio Link Setup Request message.


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The parameters of this message should be quite familiar after having studied thepages

RRC Connection Setup and Radio Access Bearer Setup.

If this is the first radio link via the Drift RNC for this UE, a new Iur signalling connectionmust be established. This new Iur connection will be used for all further RNSAPsignalling related to this UE.

Figure 93.

13.1.4. Admission ControlThe Drift RNC performs admission control to determine whether or not the request

canbe fulfilled.


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Figure 94.

13.1.5. Radio Link Setup Request IubIf the requested radio resources are available, the Drift RNC sends an NBAP RadioLinkSetup Request message to the base transceiver station.

The parameters of this message are shown in the figure.

Figure 95.


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13.1.6. Radio Link Setup Response IubAfter the BTS has allocated the required radio resources, it can receive informa

tion overthe air interface related to this radio link.

The BTS returns the NBAP message Radio Link Setup Response which alsocontains transport layer addressing information to the Drift RNC.

Figure 96.

13.1.7. Radio Link Setup Response IurThe Drift RNC returns the RNSAP Radio Link Setup Response message to the ServingRNC.


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Figure 97.

13.1.8. AAL2 Data Bearer SetupUsing the AAL2 address provided by the BTS and the Drift RNC, respectively, theDriftRNC and Serving RNC establish the AAL2 data bearers over the Iub and Iur interfacesusing the ALCAP protocol.

After some additional signalling between the base transceiver station and the ServingRNC in order to achieve synchronisation with respect to the other existing radiolinks, thebase transceiver station is ready to transmit information over the air interface

.


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Figure 98.

13.1.9. Active Set Update MessageThe Serving RNC has all the necessary information for sending the RRC Active SetUpdate message to the user equipment over the existing Dedicated Control Channel.

Now the user equipment is also ready to receive information over the new radio link.

Figure 99.


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13.1.10. Active Set Update CompleteThe user equipment acknowledges receipt of the information with the RRC Active Se

tUpdate Complete message over the Dedicated Control Channel.

Figure 100.

13.2. Radio Bearer vs. Radio Link13.2.1. Radio Bearer vs. Radio LinkRadio bearers and radio links are not the same thing. Let us look at the differencebetween a radio bearer and a radio link.


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Figure 101.

13.2.2. Radio BearerRadio bearers carry signalling or user data between the user equipment and the ServingRNC.

A radio bearer is controlled by the Serving RNC responsible for managing the radioaccess bearer with which the radio bearer is associated. Several radio bearers can beestablished at the same time towards a certain user equipment.

There are two types of radio bearers: signalling radio bearers and radio bearers

fortransport of user data.


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Figure 102.

13.2.3. Radio LinkEach radio bearer utilises one or several radio links for physical transport over the airinterface. In fact, there are also situations where a radio link can carry several radiobearers. If several radio links are used, this is called macro diversity or softhandover.


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Figure 103.

13.2.4. Soft HandoverSoft handover means that at least two radio links are involved in the handover.

The radio links can exist via one base station, via several base stations belonging to thesame RNC, or via several base stations belonging to different RNCs resulting intheServing RNC / Drift RNC concept.


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Figure 104.

14. RAN Protocol ExerciseFigure 105


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01- Ran Interfaces Protocols