04 tm51104en03gla3 procedures

UMTS Functionalities and Procedures

TM51104EN03GLA3 © 2010 Nokia Siemens Networks

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Contents

1 Radio Resource Management 3 1.1 Introduction 4 1.2 Radio Resource Control 6 1.3 Admission Control 10 1.4 Packet Scheduler 16 1.5 Load Control 18 1.6 Code Allocation 26 1.7 Channelization code planning 26 1.8 Scrambling code planning 28 1.9 Power Control 28 1.10 Handover Control 32 1.11 Diversity 54 2 Mobility Management 59 2.1 Introduction 60 2.2 Location Update 62 2.3 Paging 70 2.4 Roaming Case 72 3 Session Management 75 3.1 Initial Access to the Network 76 3.2 Authentication and Ciphering 78 4 Procedure Examples 85 4.1 Elementary Procedures 86 4.2 Paging 88 4.3 RRM Procedure Examples 115 4.4 MM Procedure Examples 128 5 Exercises 133



TM51104EN03GLA3

© 2010 Nokia Siemens Networks 2



3

1 Radio Resource Management


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1.1 Introduction The Radio Network Controller (RNC) has similar functionality as the BSC in the GSM BSS but there are also a few differences. Unlike the GSM systems, the Radio Access Network (RAN) has RNC-RNC interface (Iur), which enables the RNC to maintain Radio Resource Management (RRM) independently. The wideband switching in the RNC makes the element structure of RNC remarkably different to element structure of BSC in GSM BSS during implementation.

The RNC has different functions to control the radio resource connection. The functions are divided into network and connection based functions. The connection based functions are related to task that RRM performs on an active bearer connection, whereas the network based functions are used continuously in a cell for all allocations. The examples of network based functions are Admission Control (AC), Load Control (LC), Resource Manager (RM), and Packet Scheduler (PS). AC and LC are used to manage the amount of power being transmitted and the number of subscribers in a cell. This control is important when introducing new bearer allocations into the network. The RM is responsible for the allocation of the bearer.



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© Nokia Siemens Networks

Interface Units

(Wideband)Switching

Interface Units

ControlUnits

O&MInterface

RadioResource

Mgmt.

Iub

Iu

Iur

other

RNC

Core

Network

BS

Network

Management

RNC

Fig. 1 General Diagram of RNC

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Network Based Functions

LC

Connection Based Functions

PS

RM

AC

PC

HC

• Packet Scheduler (PS)

• Resource Manager (RM)

• Admission Control (AC)

• Load Control (LC)

• Code Allocation

• Power Control (PC)

• Handover Control, Micro Diversity (HC)

Fig. 2 Radio Resource Management Functions


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1.2 Radio Resource Control The RRC has two main states, idle and connected. From the UE to the network connection point of view, the RRC changes its state from idle to connect. For any activity between the UE and the network, the RRC-connected state can be considered as a prerequisite. When there is no RRC connection between the mobile and the network, but the mobile is switched on, the mobile is considered to be in an idle mode. It means that the mobile is listening to one base station and is in readiness to start a connection, or is waiting to be paged. When a dedicated channel is provided to the subscriber, for example, for video, the subscriber is considered to be in the Cell_DCH state. (The DCH is derived from the name of the channel in the air interface). In this state the UE is sending measurement reports to the network, which enables the system to control the dedicated bearer and perform handovers. If the mobile is only sending small pieces of information to the network, for example irregular Internet based traffic or for signaling, then the RRC can be in a mode known as Cell_FACH (the FACH stands for Forward Access Channel) and is different from the previous state as UE is not using dedicated channel. The network does not perform handovers as the mobile moves from one cell to another. The UE just informs the network about its current location. Depending on the bearer we have and how it is being used, the RNC will move the RRC between the different states. In addition to the Cell_FACH, if the network finds that the bearer is not being used for a long time, it can move the connection to a Cell_PCH mode (Paging Channel), where the mobile is still known to a cell level but can only be reached via the PCH. In this state the UE is using a Discontinuous Repetition Function (DRX) to save battery. Again, unlike in the Cell_DCH, as the subscriber moves, the mobile informs the RNC which cell it has moved to. The final state is the URA_PCH. This state is similar to the Cell_PCH. But, instead of monitoring the connection on a cell level, it is now on a RNC level. URA stands for UTRA Registration Area and the UE monitors the broadcast channel for URA identities. The RRC as an entity consists of two items, Medium Access Control (MAC) and Radio Link Control (RLC). Together these two are also called as Layer 2 processing.



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LARARA

URA

URA

URAURA

URAURA

URA

RRC Idle• no UE infos in UTRAN,

only in CN• no signaling possible

RRC Connected• UE position known in UTRAN

• Signaling connection existsCell_DCH

• DCH allocated• UE’s cell known

Cell_FACH• common channels used

• UE’s cell known

Cell_PCH• “Standby” (DRX)• UE’s cell known• UE to be paged

URA_PCH• “Standby” (DRX)• only URA known• UE to be paged

Establish / ReleaseRRC Connection

TS 25.331TS 25.331

RNC(stores UEs Cell/URA)

UE

Prerequisite forexchange of

User Data / Signaling

Prerequisite forexchange of

User Data / Signaling

URAUTRAN

Registration Area

• set of cells• operator defined• for RRC“Standby”

DCH: Dedicated ChannelFACH: Forward Access Channel PCH: Paging ChannelRRC: Radio Resource Control

RRC ConnectionRNC:

RRC States &

Location Information

Fig. 3 Radio Resource Control States


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The Physical Layer, in the layer 1 offers Transport Channels to the MAC layer. There are different types of transport channels with different characteristics depending on the transmission. Common transport channels can be shared by multiple handsets (for example, FACH, RACH, DSCH, BCH, and PCH). Dedicated transport channels (DCH) are assigned to only one handset at a time. The transmission functions of the physical layer include channel coding and interleaving, multiplexing of transport channels, mapping to physical channels, spreading, modulation and power amplification, with corresponding functions for reception. A frequency and a code characterize a physical channel. The MAC protocol, in the layer 1 offers logical channels to the layers above. The logical channels carry different types of information, which include Dedicated Control Channel (DCCH), Common Control Channel (CCCH), Dedicated Traffic Channel (DTCH), Common Traffic Channel (CTCH), Broadcast Control Channel (BCCH) and the Paging Control Channel (PCCH). The MAC layer performs scheduling and mapping of logical channel data onto the transport channels provided by the physical layer. For common transport channels, the MAC layer adds addressing information to distinguish data flows intended for different handsets. One major difference to GSM is the possibility to dynamically switch one logical channel (data flow) onto different transport channel types, based on the activity of the subscriber. The Radio Link Control (RLC) protocol, a layer 2 protocol, operates in one of three modes: transparent, unacknowledged, or acknowledged mode. It performs segmentation/re-assembly functions and, in acknowledged mode, provides an assured mode delivery service by use of retransmission. RLC provides a service for the RRC signaling to both, the Signaling Radio Bearer and for the user data transfer, the Radio Access Bearer. Above these layers the Radio Resource Control (RRC) protocol, in the layer 3 provides control of the handset from the RNC. It includes functions to control radio bearers, physical channels, mapping of the different channel types, handover, measurement and other mobility procedures.



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Physical ChannelsPhysical Channels

Transport ChannelsTransport Channels

Logical ChannelsLogical Channels

RLCRLC

Medium Access ControlMedium Access Control

Physical LayerPhysical Layer

RLCRLCRLCRLC

MAC for

Common

Channels

RRC

Signaling

Iu

Iub/Iur

CS RAB

(speech)

PS RAB

(data)

• Selection of the data to be inserted in the radio frame

• Selection of common or dedicated channels

• Multiplexing of logical channels into same transport channels

• Ciphering to Real Time data

• Segmentation

• Retransmission across the air

• Ciphering of Non Real Time data

• Buffering

Fig. 4 Layer 2 Processing


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1.3 Admission Control WCDMA radio access has several limiting factors, some of them being absolute and others environment-dependent. The most important and the most difficult is to control the interference occurring in the radio path. Due to the nature and basic characteristics of WCDMA, every UE accessing the network generates a signal. Simultaneously, the signals generated by the UE can be interpreted to be interference from the other UEs point of view. When the WCDMA network is planned, one of the basic criteria for planning is to define the acceptable interference level, with which the network is expected to function correctly. This planning based value and the actual signals the UE transmit set practical limits for the Uu interface capacity. To be more specific, a value called Signal-to-Interference Ratio (SIR) is used in this context. Based on radio network planning, the network is, in theory, able to stand as maximum one SIR of certain size within one cell. That is, in the BTS receiver, the interference and the signal must have a certain level of power difference in order to extract one signal out from the other signals using the same carrier. If the power distance between interfering components and the signal is too small, the BTS is not able to extract an individual signal out from the carrier anymore. Every UE having a bearer active through this cell “consumes” a part of the SIR. The cell is used up to its maximum level when the BTS receiver is not able to extract the interference from the carrier. The main task of admission control is to estimate whether a new call can have access to the system without sacrificing the bearer requirements of existing calls. Thus the AC algorithm should predict the load of the cell if the new call is admitted. It should be noted that the availability of the terrestrial transmission resources is verified, too, meaning that there is no limiting factor in the rest of the UTRAN either. Based on the admission control, the RNC either grants or rejects the access. The SIR or Interference Margin has direct relationship with the cell load. If we express the cell load with a Load_Factor (from 0 to 1, equals cell percentage load, that is, 10 % load gives value 0.1) and mark the Interference Margin with I, it leads to the following equation:



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Radio Access Bearers in Uu Interface

UuIn

terfa

ce B

andw

idth

Admission Control

SIR Allowed Range

Fig. 5 Admission Control


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Based on the graph, it is fairly easy to indicate that when the cell load exceeds 70 %, the interference in that cell will be very difficult to control. This is why the WCDMA radio network is normally dimensioned with expected capacity equivalent to Load Factor value 0.5 (50 %). This value has a safety margin in it and the network will behave as expected.



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0

5

10

15

20

25

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9

Interference Margin (dB) and Load Factor

Inte

rfe

renc

e M

arg

in (

dB

)

Load Factor

Fig. 6 Interference Margin as a function of cell load


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Planning Uplink Admission Control The RNC controls the interference on the uplink, and the parameters are used to act as boundaries. The UL interference power, which determines the maximum limit where the cell is considered to be at maximum load. From the graph depicting, interference margin as a function of a cell load, a realistic value to represent a sensible load can be arrived at The value is known as the PRX_Target (PRX stands for Receive Power level.) value.

The area from 0 to this value is known as the planned load. Once the load is approaching this value, the Traffic Handovers (TRHO) are performed.

As UMTS traffic is variable and constantly changing, it is possible that the traffic admission may exceed the PRX_Target. To handle this situation, a second level of value known as the PRX_TARGET_BS is used by the BTS to stop situations of congestion. Once this value is reached, the BTS takes actions to reduce the load in the cell.



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Load Factor

Interference Margin (dB)

Prx_target_BS

Prx_target

TRHO_threshold

Prx_offsetMarginal Load Area

Planned Load Area

Planned Uplink Interference Power

Defines the limit (the first UL overload threshold) for UL interference power, after which the BS starts its load control action to prevent overload.

Fig. 7 Admission Control on the Uplink


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1.4 Packet Scheduler Packet scheduler is a general feature, which handles scheduling radio resources for Non-Real-Time (NRT) radio access bearers for both uplink and downlink directions. Packet access is implemented for both dedicated (DCH) and common control transport channels Random Access and Forward Access Channels (RACH/FACH). Packet scheduler makes the decision of the used channel type for the downlink direction. For uplink direction the decision of the used channel type is made by the UE. Following figure illustrates function of the packet scheduler. In Release 3, the actions of the packet scheduler are driven by the load control function. The gap between Real-Time (RT) traffic and the load target of the cell can be filled by the packet scheduler. As IPv6 is implemented, and Quality of Service (QoS) becomes a key part of the interface, the scheduler no longer sees the traffic as real-time and non-real-time, but instead uses a priority system on the packets being transmitted.



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Packet Service Session

Packet Call

Reading Time

Packet Size Packet Arrival Interval

time

Fig. 8 Packet scheduler function


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1.5 Load Control The Radio Resource Management mechanisms Admission Control, Packet Scheduler and Load Control are important components when controlling the load in the UTRAN network. The purpose of load control is to optimize the capacity of a cell and prevent an overload situation to maintain the stability of the system. Load control consists of Admission Control (AC) algorithms, Packet Scheduler (PS) algorithms, and Load Control (LC), which updates the load status of the cell based on resource measurements and estimations provided by AC and PS. Following figure illustrates how the load control works logically in the RNC.



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LC

AC

PS

Load Change Info

Load Status

NRT Load

Fig. 9 Load Control


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If the system is overloaded, LC returns the system to normal load state in a fast and controlled way. LC can be divided into two functions:

1. Preventive control, which guards the system from overload.

2. Overload control, which returns the system from an overload state to normal state.

Interference is the main resource criteria for the CDMA system the following load control measures are practiced:

1. UL total received wideband interference power.

2. DL total transmission power.

3. One RNC on cell basis periodically under.

RRM acts according to these measurements and parameters set by radio network planning. Following figure illustrates Capacity in the Uplink Limited by Interference:



21

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The further away users are moving

The more users are connected

The more transmission power are required to achieve certain quality

Finally the capacity is filled.

Fig. 10 Capacity in the Uplink Limited by Interference


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In the downlink, the capacity is limited by the transmission power of the site. As more subscribers are added to a cell and as the subscribers move further away from the site towards the cell edge, the more power is needed to achieve a certain quality. At this point the capacity of a cell is filled in the downlink because the power/signal quality to assure a quality connection is not enough. In the uplink, the capacity is limited to amount of power or interference that is present in a cell. Therefore, the loading of a cell is based on the combination of these two directions. Following figure illustrates the Model of Cell Loading against Traffic Profile.



23

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Power

Load Target

Overload

Estimated capacity of NRT traffic.

Measured load caused by non-controllable load.

Fig. 11 Model of Cell Loading Against Traffic Profile


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The traffic in a cell can be categorized by priority, depending on the traffic type, for example, Conversational, Streaming, Interactive, and Background. The categories can be subdivided into Real Time (RT) and Non-Real Time (NRT) traffic. The figure above displays a simple example of Release 99 implementation of real time and non-real time services. In practice, the real time traffic is given priority over non-real time traffic. The packets are scheduled to fill in the gaps between the real-time traffic and the load target. The traffic profile in UMTS is variable, therefore, overload values are used. As in displayed in the figure above, the system can handle moments of traffic peaks, but if the traffic is constantly above a certain limit, then certain load reduction measures, such as handovers, are taken.

The parameters on the right-hand side specify the behavior of the load control. When the load on the cell is not much, the AC allocates real time bearers, and the PS is flexible with the packet load. When the load increases, no more real time bearers are allocated and the PS does not increase the load.

If the load continues to grow, or at the most stay the same (remember, that the subscribers are still moving, which is affecting the power levels), then the LC takes actions, such as traffic reason handovers. The PS decreases the bit rate of the non-real time bearers in an effort to decrease load simultaneously. Under extreme conditions of the cell being overloaded, the LC may take actions, such as dropping the NRT bearers.



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Interference Margin

Load Factor

Admission Control

Load Control

Packet Scheduler

Ptx_threshold or Prx_threshold

P_CellMax

Ptx_target or Prx_target

Ptx_target + Ptx_offset or Prx_target + Prx_offset

Overload actions

PS decreases bitrate and drops NRT

bearers

PS decreases bitrates of NRT

bearers

PS doesn’t increase NRT load but can change NRT

bitrates

PS increases the amount of NRT bearers

Load preventive LC

actions

No actions

No actions

AC doesn’t admit new

bearers

AC doesn’t admit new

bearers

AC admits RT bearers normally

Fig. 12 How the AC, LC and PS work together


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1.6 Code Allocation This section aims to further specify the properties and usage of the scrambling and channelization codes in Radio Resource Management (RRM). The different codes used in WCDMA were briefly explained in the chapter earlier this section aims to further specify the properties and usage of the scrambling and channelization codes in RRM. Both scrambling and channelization codes used in the Uu interface connections are maintained by the RNC. In principle, the codes could be maintained by the BTS, but then the system would experience lack of radio resource control such as soft handovers. When the codes are maintained by the RNC, it is easier to allocate Iub data ports for multi path connections. The Uu interface requires two kinds of codes for proper functionality. A part of the codes used must correlate with each other to a certain extent, and the others must be orthogonal. Every cell uses one scrambling code. As you already know, this code acts like a cell ID. Under every scrambling code the RNC has a set of channelization codes. This set is the same under every scrambling code must first find the correct scrambling code value first in order to access the cell. When a connection between the UE and the network is established, the channels used must be separated. The channelization codes are used for this purpose. The information sent over the Uu interface is spread with a spreading code per channel. Spreading code by definition is the same as scrambling code x channelization code.

1.7 Channelization code planning The codes used in Uu interface can be handled in a code tree, where branches are consequently blocked when a certain code on a certain spreading factor level is taken into use. When having plenty of simultaneous connections, with multiple radio links, multiple channels, and multiple codes, the code tree may easily become fragmented. Fragmentation means the phenomenon where the probability of the blocked branch of the code tree increases too much and thus it starts to prevent new accesses to the system. For example, if an active call uses high bit rate over the Uu interface, the spreading factor value in use is small. It furthermore means that a very high-level branch of the code tree is blocked. When this call is finished and simultaneously new calls access the system, the blocked code tree branch is not “released” before the new accesses. In this situation the system wastes capacity because the code channels allocated for new calls are not necessarily allocated in the best possible way. The channelization code used has the same length as the base band data. As a part of the spreading operation, the base band data and the code are combined and spread. The result is a fixed length code that is then scrambled. A low data rate communication can be spread much more over the bandwidth, which also means that a high spreading factor is used.



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CC1,0 = (1)

CC2,1 = (1,-1)

CC2,0 = (1,1)

CC4,0 = (1,1,1,1)

CC4,1 = (1,1,-1,-1)

CC4,2 = (1,-1,1,-1)

CC4,3 = (1,-1,-1,1)

CC8,0 = (1,1,1,1,1,1,1,1)

CC8,1 = (1,1,1,1,-1,-1,-1,-1)

CC8,2 = (1,1,-1,-1,1,1,-1,-1)

CC8,3 = (1,1,-1,-1,-1,-1,1,1)

CC8,4 = (1,-1,1,-1,1,-1,1,-1)

CC8,5 = (1,-1,1,-1,-1,1,-1,1)

CC8,6 = (1,-1,-1,1,1,-1,-1,1)

CC8,7 = (1,-1,-1,1,-1,1,1,-1)

Spreading Factor:

SF=1 SF=2 SF=3 SF=4

Example of code allocation

Fig. 13 Channelization codes

As codes are released in different branches, the tree can become fragmented and the RNC should always try to reorganize the tree to make the best use of the resources. Therefore, in UMTS networks, it is possible that the channelization codes could change during a connection.

Also, if the scrambling code in the uplink is being used by another person in another RNC as the subscriber performs a soft handover, the handover is refused and the serving RNC must allocate a new scrambling code to the subscriber.


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1.8 Scrambling code planning

There are totally 512 downlink scrambling codes used, eight in each of the 64 code groups. All the cells that the UE is able to measure in one location should have different scrambling codes. To ensure this, different scrambling code groups in the neighboring base stations should be used. The code group allocation is performed during the network planning. The corresponding functionality should be present in the network planning tool. The number of re-uses could be 64, as there are 64 code groups. The scrambling code group planning for different frequency carriers can be done independently.

1.9 Power Control

In the UTRAN, the power control and accuracy is extremely essential (unlike in GSM networks). The reasons can be as follows:

The mobiles transmit simultaneously in time (not in different timeslots like in GSM).

The UTRAN uses often only one frequency, which means that the frequency re-use factor will be 1.

Any inaccuracy in power control immediately increases interference, which then decreases the capacity of the network.

Following figure shows the Power and Distance.



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P1P2

Fig. 14 Power and Distance


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The physical facts in the UTRAN with regard to the radio path and the distance are similar to the GSM, but because of the three reasons stated above, the power control mechanism must be accurate and fast. As a result, the WCDMA power control mechanism differs from the GSM mechanism.

Following figure shows the different WCDMA Power Control mechanisms.

1.9.1 Open Loop Power Control

When the UE accesses to the network, the initial level for accessing is based on an estimate. This estimate in turn is based on the signal level received from the Node B when the UE is in the idle mode and the downlink power level that the UE detects from the physical channel PICH. In other words, when in an idle mode, the UE receives information about the used and allowed power levels from the Pilot Channel of the cell. In addition, the UE evaluates the path loss occurring compared to the figures received from the CCH-1. Based on this difference, the UE is able to estimate the correct-enough power level to initialize the connection.

1.9.2 Closed Loop Power Control

When the radio connection is established, the power control method is changed. During the connection, the method used is called the closed loop power control. Within this method, the Node commands the UE either to increase or to decrease its transmission power with the pace of 1.5 kHz (1500 times per second) in the FDD mode. The decision whether to increase or decrease the power is based on the received SIR estimated by the Node B.

1.9.3 Outer Loop Power Control

Due to the macro diversity, the UE is simultaneously attached to the network through more than one cell. The RNC must be aware of the current radio link conditions and quality. The RNC knows the allowed power levels of the cell and target SIR. In order to maintain the quality of the radio link, the RNC uses this power control method to adjust the SIR of the connection. By doing this, the network is able to compensate changes in the air interface propagation conditions and to achieve the target quality for the connection. The target quality can be measured with the help of Bit Error Ratio (BER) and Frame Error Ratio (FER) observations.



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Open Loop Power Control

Closed Loop Power Control

Outer Loop Power Control

UE NodeB RNC

Fig. 15 WCDMA Power Control Mechanisms


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1.10 Handover Control

UMTS handovers can be intra-system, (inside the WCDMA radio network) or inter-system (from WCDMA to GSM 900/1800). The Inter-System Handovers (ISHO) are of the traditional type, which are also used in GSM. The ISHO are also known as a hard handover, because the UE does not maintain simultaneous connections, in practice it breaks the old connection and then establishes a new connection. Following figure illustrates The Intra-System and Inter-System Handovers. The intra-system handovers of UMTS are classified as being inside the same WCDMA band (intra-frequency) or being from one frequency band to another. The inter-frequency handover could be a handover from one cell layer to another. The inter-frequency handovers are hard handovers and are similar to the inter-system handovers. The intra-frequency handovers, on the other hand, could be so-called soft handovers. In a soft handover, the signal is received in both the new and the old channel for a period of time. One characteristic of a UMTS network is that the network will communicate with the UE through different base stations (Node Bs). An active set is a list of cells, through which the UE has a connection to the network, that is, through which the radio link set-up has been made.



33

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GSM900/1800

WCDMA TDDWCDMA TDD

WCDMA FDD

Inter-System

Intra-System

Inter-System

BSC

RNC

Fig. 16 Intra-System and Inter-System Handovers


TM51104EN03GLA3


1.10.1 Soft Handover

One characteristic of a UMTS network is that the network will communicate with the UE through different base stations (Node Bs). An active set is a list of cells, through which the UE has a connection to the network, that is, through which the radio link set-up has been made. This is, the UE may have active radio connection between itself and the network through three cells simultaneously. In soft handover, the UE is connected to at least two Node Bs at the same time. In the uplink direction, the two signals come via the base stations to the RNC. In the RNC the signal to be transported forward to the core network is selected. The selection is done frame by frame for the speech, and in smaller blocks for data. In the downlink direction, the UE uses the RAKE receiver to combine signals from two different base stations.



35

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CN

RNC

Frame Selection / Duplication

Frame Reliability Info

Frame Reliability Info

Fig. 17 Soft Handover and Active Node B Set


TM51104EN03GLA3


Soft handover is performed between two cells belonging to different Node Bs but not necessarily to the same RNC. The source and target cell of the soft handover has the same frequency. In case of a circuit switched call, the terminal performs soft handovers at all times if the radio network environment has small cells.



37

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Frequency f1Frequency f1

Fig. 18 Soft Handover


TM51104EN03GLA3


If the subscriber moves from cell site 1, Node B1 to cell site 2, Node B2, first the UE has a connection through Node B1. The power level and Signal to Interference Ratio decreases as the UE moves towards Node B2. At some point the Node B2 signal is high enough and the UE starts to talk via both Node B1 and Node B2. The signal via Node B2 gets clearer and the signal via Node B1 gets worse. Therefore, when the UE talks through two Node Bs, we have macro diversity.



39

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Soft Handover Window

BS1 BS2

Power

BS1 BS2

Connect to BS1 Add BS2 Drop BS1 Connect to BS2

Fig. 19 Soft Handover (UE moving from BS1 to BS2)


TM51104EN03GLA3


1.10.2 Softer Handover

In softer handover, the Node B transmits through one sector, but receives from both the sectors. In this case, the UE has active uplink radio connections with the network through two cells populating the same Node B.



41

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Sector 1

f1

Sector 2

f1

Sector 3

f1

Multipath signal through Sector 3

Multipath signal through Sector 1

Fig. 20 Softer Handover


TM51104EN03GLA3


1.10.3 Hard Handover

UMTS Handover The UMTS hard handover is a ‘GSM-like’ handover performed between two WCDMA frequencies. In case of a hard handover, the connection through the old cell is cleared and the connection with the radio network continues through the new cell. Hard handover should be avoided if possible because it often results in increased interference.



43

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Fig. 21 UMTS Handover


TM51104EN03GLA3


The hard handover performed if the Iur interface is not available. For example, between the RNCs coming from two manufacturers.



45

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RNC RNC

Iur

Iub Iub

Fig. 22 Intra-Frequency Handover


TM51104EN03GLA3


1.10.4 Inter-System Handover

The possible co-existence of the different radio accesses in the UMTS network, the UE should be able to fluently change the radio access technology when required. In order to cater this situation, the 3GPP Specifications identify the combination of UMTS and GSM as one source for inter-system handovers. The possibility to perform an inter-system handover is enabled in the UMTS by a special functioning mode, slotted mode. When the UE uses Uu interface in the slotted mode, the contents of the Uu interface frame are compressed in order to open a time window, through which the UE is able to peek and decode the GSM BCCH information.

In addition, both the WCDMA RAN and GSM BSS must be able to send the identity information of the other on the BCCH and BCH channels to enable the UE to perform the decoding properly.



47

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only

GSMUMTS

Fig. 23 UMTS/GSM Inter-System Handover


TM51104EN03GLA3


Inter-System Handover from GSM

The handover between GSM and UTRAN can be performed for a number of reasons, for example, to provide specific high bit rate services. The handover is possible because a dual mode UE receives the UTRAN neighbor cell parameters on GSM system information messages. The parameters that enable the UE to measure the neighboring UTRA FDD cell are: downlink centre frequency, downlink bandwidth (currently only 5 MHz), downlink scrambling code, or scrambling code group for the CPICH, and reference time difference for the UTRA cell.

The sequence of events in the intersystem handover to BSS to UTRAN is as follows:

1. The UE/MS creates a measurement report that the BSC evaluates to make the handover decision.

2. The reservation messages are sent to the UTRAN if the BSC decides to hand over to a UTRA cell resource,

3. The UTRAN acknowledges the resource reservation and provides a UTRAN handover command.

4. The BSC sends the GSM intersystem handover command to the UE. In this command, a UMTS Handover to UTRAN command is included, which contains all the information needed to set up a connection to the UTRA cell. The message contains reference number to UTRA parameters not the real values to cater to a situation when the configuration information is more than a GSM message can handle.

5. The UE completes the procedure of Handover to UTRAN by a sending complete message to the RNC.

6. Finally, the RNC commands resources to be released by the BSC.



49

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UE GSM BSS MSC UTRAN

GSM BCCH or SACCH: System Information

GSM SACCH: Measurement Report

Resource Reservation

Resource Reservation Acknowledge and Handover CommandGSM DCCH:

Inter-System Handover Command

UTRAN DCCH or DCH: Handover to UTRAN Complete

Release Resources

Fig. 24 Inter-System Handover from BSS to UTRAN


TM51104EN03GLA3


Inter-System Handover from UTRAN The handover between GSM and UTRAN can be performed for a number of reasons, for example, to provide specific high bit rate services. The handover is possible because a dual mode UE receives the UTRAN neighbor cell parameters on GSM system information messages. The parameters that enable the UE to measure the neighboring UTRA FDD cell are: downlink centre frequency, downlink bandwidth (currently only 5 MHz), downlink scrambling code, or scrambling code group for the CPICH, and reference time difference for the UTRA cell.

The sequence of events in the intersystem handover to UTRAN to BSS is as follows:

1. Based on the measurement report including both UTRAN and BSS values the RNC makes the handover decision.

2. Resource reservation messages are sent to the BSC.

3. The BSC acknowledges the resource reservation and includes a GSM handover command.

4. The RNC sends an Intersystem handover command message to the UE. In this message, the GSM Handover command is included.

5. The UE switches to GSM RR protocol and sends the GSM handover access message to the BSC.

6. The BSC finally initiates resource release with message to the UTRAN.



51

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UE GSM BSSMSCUTRAN

UTRAN BCCH: System Information orUTRAN DCCH:Measurement Control

UTRAN DCCH or DCH: Measurement Report

Resource Reservation

Resource Reservation Acknowledge and Handover CommandUTRAN DCCH: Inter-System Handover Command

GSM DCCH: Handover Access

Release Resources

RNC

Fig. 25 Inter-System Handover from UTRAN to BSS


TM51104EN03GLA3


Handover Decision Mechanism During the connection, the UE continuously measures some parameters such as signal strength, quality, and interference, concerning the neighboring cells and reports the status of these items to the network up to the RNC. These parameters are measured from the neighboring cells PICHs. The RNC checks whether the values indicated in the measurement reports trigger any criteria set. As the result of the trigger, the new Node B is added to the Active Set. An Active Set is a list of cells, through which the UE has a connection to the network or through which the radio link set-up is established. The minimum size and maximum size of the Active Set is one cell and three cells respectively. The UE can have active radio connection with the network through three cells simultaneously



53

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Measurement Reports

Handover Algorithm

Criteria Fulfilled?

Activate New BS

Update Active Set

• Signal Strength

• Signal Quality

• Interference

Procedures Functional Split

Created and collected by UE

and BS

Investigated by RNC

Commanded by RNC executed by

UE

Measurement Phase

Decision Phase

Execution Phase

Yes

No

Fig. 26 Handover Decision Mechanism


TM51104EN03GLA3


1.11 Diversity

1.11.1 Micro Diversity

With reference to the soft handover and active set, the two terms that describe the handling of the multi path components are micro diversity and macro diversity. Micro diversity means the situation where the propagating multi path components are combined in the Node B.

WCDMA utilizes the Multi path Propagation, which means that the Node B receiver is able to determine, differentiate and sum up several signals received from the radio path. The receiver used for Multi path Propagation is equipment called a RAKE receiver.

In Multi path Propagation, a signal sent to the radio path is reflected from, for example, ground, water and buildings and the sent signal is displayed as many copies at the receiving end. Each one of these signals reaching the receiver at a different phase and time. The micro-diversity functionality at the Node B level combines (sums up) different signal paths received from one cell and, in case of sectorized Node B, the outcomes from different sectors (softer handover).



55

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Uplink

UE BS RAKE

Receiver

Same signal propagating in different ways in the radio path

Summed signal

(Micro) Diversity Point

Fig. 27 Micro Diversity


TM51104EN03GLA3


1.11.2 Macro Diversity

As the UE may use cells belonging to different Node Bs or even different RNCs, the macro-diversity functionality also exists on the RNC level. The figure depicting Macro Diversity in RNC presents a case in which the UE has a 3-cell active set in use and one of those cells is connected to another RNC. In the case, the Node Bs performs summing of the signal concerning their own radio paths. At the RNC level, the serving RNC evaluates the frames coming from the Node Bs and chooses the best signal to send towards the CN domains.

In case of the soft and softer handovers, subjective call quality will be better when the final signal is constructed from several sources (multi path).

In GSM, the subjective call quality depends on the transmission power used. In other words, the more power the better quality.

In UMTS, the terminals cannot use much power because if they do the transmission levels that are very high will start blocking the other users away.

Therefore, the better way to gain better subjective call quality is to utilize the MultiPath Propagation.

The soft and softer handovers consume radio access capacity because the UE is occupying more than one radio link connection in the Uu interface. The added capacity gained from the interference reduction is also bigger and as a result the system capacity is increased if soft and softer handovers are used.



57

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Core Network

Active Set

RNC

RNC

BS/NodeB

BS/NodeB

BS/NodeB

UE

(Macro) Diversity Point

Fig. 28 Macro Diversity in RNC


TM51104EN03GLA3




59

2 Mobility Management


TM51104EN03GLA3


2.1 Introduction There are several different mobility management procedures. Following is a short list of UMTS specified procedures:

• Paging (CS)

• Paging (PS)

• Location update (CS)

• Cell attach/detach (PS)

• IMSI attach/detach (CS)

• Routing area update (PS)

• UE identity checking (CS/PS)

• Ciphering procedure (CS/PS)

• Location registration (CS and PS)

• Authentication procedure (CS/PS)

• Location info retrieval (CS and PS)

• UE hardware (IMEI) checking (CS/PS)

As the user terminals are not fixed to certain positions, the network must keep track of the location of the mobile. The system must at least be able to know the geographical area in which the subscriber is located. As in GSM networks, UMTS has a cellular architecture that allows the network to identify the subscriber. Therefore, the network maintains information about the location of a subscriber, and the procedures are specified to allow a constant updating of the databases as the subscriber moves around the network, and also from one network to another.

The HLR is the central database that stores information on the subscriber, such as the IMSI and MSISDN. The HLR also stores information about the serving MSC and SGSN of the subscriber.

In addition, the HLR stores information on the subscriber's service profile. In other words, the record of the different services, for example, tele services, supplementary, and packet services, that the subscriber can or cannot use. Therefore, if the network needs to locate the subscriber in case of a mobile terminated call, or if the network needs to check if the subscriber is valid, then all requests are sent to the HLR.



61

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Supplementary Services Locating a

Subscriber

Location Updating

Service Information

Radio Access

Core Network

Foreign Network

UE

RNC

HLR, AC, EIR

MSC

SGSN

GGSN

MSC

Fig. 29 Role of the HLR


TM51104EN03GLA3


2.2 Location Update

As the network maintains three layers of information on the subscriber's location, LA, RA, and URA, there are multiple procedures used to track the movement of the subscriber. The following are three basic types of location update procedures:

1. Location registration (power on/cell attach)

2. Movement between areas

3. Periodic update



63

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RNC

RNC

MSC

VLR

HLR

SGSN

LAC=1

LAC=2

When switching mobile ON/OFF

(IMSI attach/detach)

When moving to other area

(Cell, LA, RA, or URA level) Periodically over time if the mobile doesn’t perform location

update

RNC keeps track of the mobiles within the URAs. It also informs Core Network when subscriber changes LA and/or RA

VLR contains the subscriber’s location and also subscriber data as a copy from HLR.

SGSN keeps track of the subscribers.

HLR keeps the subscriber’s MSC/VLR area.

Fig. 30 Location Update Generic Procedures


TM51104EN03GLA3


As the RNC receives a location update message, it takes responsibility for informing the core network. The RNC updates its own information about the subscriber within the URA and informs the SGSN and VLR, respectively, if the routing area or location area has also changed. The reason for updating a location is that the VLR and SGSN databases of the network are only temporary. Depending on the parameters that the operator use, the information is only stored for a certain time. If there are no updates, it is assumed that the information is old. Therefore, avoid storing a large amount of useless data in the network, the information is removed.

2.2.1 Location Area Procedure

A Location registration or IMSI attach takes place when a UE is turned on and informs the VLR that it is now back in service to receive the calls. The network then sends the UE two numbers that are stored in the USIM or SIM card of the UE. These two numbers are the current LAI and the TMSI. The network sends the LAI through the control channels of the air interface. The TMSI, temporary identity, which regularly changes, is used for security purposes, so that the IMSI of a subscriber does not have to be transmitted over the air interface. Every time the mobile receives data through the control channels, it reads the LAI and compares it with the LAI stored in its USIM card. A generic location update is performed if they are different. The mobile starts the location update process by accessing the MSC or VLR that sent the location data.

A channel request message sent contains the subscriber identity, which can be the IMSI or TMSI, and the LAI stored in the USIM card. When the target MSC or VLR receives the request, it reads the old LAI, which identifies the MSC or VLR that has served the mobile up to this point. A signaling connection is established between the two MSCs or VLRs and the subscriber's IMSI is transferred from the old MSC to the new MSC. Using this IMSI, the new MSC requests the subscriber data from the HLR and then updates the VLR and HLR after successful authentication.

Periodic location update is carried out when the network does not receive any location update request from the mobile in a specified time. This kind of situation is created when a mobile is switched on but no traffic is carried and the mobile is only reading and measuring the information sent by the network. If the subscriber is moving within a single location area, the MS does not need to send the location update request.

A timer controls the periodic updates and the operator of the VLR sets the timer value. The network broadcasts this timer value so that a UE knows the periodic location update timer values.

The location registration procedure is similar for both CS and PS Domains. In case of PS Domain, the MSCs or VLRs are replaced with SGSNs. When the VLR or SGSN is changed, the new VLR or SGSN sends information about this change to the HLR. The HLR responds by sending the subscriber information to the VLR or SGSN. Any earlier location information of the subscriber present in the HLR is cancelled.



65

2.2.2 IMSI Attach/Detach Procedure

In the CS Domain, the UE may have the following two states: 1. Attached 2. Detached In the attached state, the UE is able to handle transactions and is active in the network. The UE continuously analyses its radio environment for LAC and cell identities being visible. When the UE is switched off or detached, it stores the latest radio environment information into its memory and informs the network that is now being switched off. The VLR stores this state change and does not try to reach the UE for mobile terminated transaction. When the UE is switched on again, it first checks whether the radio environment matches the one it has on its memory. If the radio environment matches, the UE informs the VLR that it is now attached again and is able to handle transactions. If the radio environment does not match, the UE performs a location area update.


TM51104EN03GLA3


2.2.3 Routing Area Update

As a procedure, the routing area update is very similar to the location update and serves the same purpose. Periodic routing area update is used for checking that a UE that has not performed any routing area updates for a period is still reachable. The UE performs a cell update and cell reselection when it changes cell within a routing area in Ready mode. This could be compared to a handover in UMTS or GSM for PS connections. Cell update and routing area updates halt possible reception or sending of data. In such cases, there is a possibility of buffering data in the Serving SGSN. When the UE changes cells between the different routing areas, it performs a routing area update. The two types of routing area updates as follows:

1. Intra-SGSN routing area update - One SGSN can manage many routing areas. If the new routing area is managed by the same SGSN as the old one, an intra-SGSN routing area update is performed.

2. Inter-SGSN routing area update - If the new routing area is managed by a different SGSN, an inter-SGSN routing area update is performed. The old SGSN then forwards user packets to the new SGSN.



67

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RA2

RA1

New Cell

Old Cell

RNC

RNC

SGSN

Fig. 31 Routing Area Update


TM51104EN03GLA3


2.2.4 Cell Attach/Detach

In the core network packet domain, the MM-state changes during the PS connection. The MM-state mostly depends on the activity of the connection. This means that when there are packets to send or receive, the MM-state of the connection is MM-connected. When there is nothing to transmit, the MM-state of the connection is MM-idle. The MM-detached state has the same meaning in both the CN domains.

In order to utilize the 3G network resources, such as radio bandwidth effectively, the MM-state management is not enough for the PS traffic. In PS traffic, the traffic delivered can be presented as occasional packet bursts. Between these bursts, the connection is not used, which leads to a situation where it is reasonable to cut the connection through the network in order to make the network resources available for other active connections. The method to suppress the packet connection, but at the same time retain the necessary information in both ends of the connection is called cell attach / detach.



69

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CS & PSService States

TS 23.060,23.121

TS 23.060,23.121Detached:

• CS: UE not reachable; no LUPs• PS: UE not reachable, no RUPs

Idle:• CS: UE reachable for Paging; LUPs: LAI in VLR • PS: UE reachable for Paging; RUPs: RAI in SGSN

Connected: • CS: UE - CN signalling connection; no LUPs

• PS: UE - CN signalling connection; RUPs

Attach /Detach

Signalling ConnectionEstablish / Release

Detach

RNC

MSC/VLR(UE Service State; LAI)

SGSN(UE Service State; RAI)

HLR(VLR/SGSN

address)

IMSI Attach / LUP

GPRS Attach / RUP

Iu(CS)

Iu(PS)

D

Gr

UMTS Mobility

Management

Fig. 32 Mobility Management State Diagram


TM51104EN03GLA3


2.3 Paging

In UMTS, the RNC can handle simultaneous CS and PS connections to the subscriber. Both domains use the LA and RA respectively to track the subscriber's location. The RNC must track the URA in which the subscriber is. In networks where the RNCs are connected through Iur interfaces as opposed to the MSC controlling handovers, the subscribers drift through the radio network passing from one RNC to another. Therefore, the serving RNC must identify in which URA a subscriber is located when it receives traffic for itself in a circuit switched connection.

From the HLR, the network is able to determine area or routing area where the subscriber is located. The network, for example, MSC will contact the MSC or SGSN serving that area and request contact to the mobile. The VLR or SGSN will then send a paging message, which contains the ID of the subscriber on a dedicated channel in the air interface. A mobile in idle mode is always listening to this channel.

If the mobile is able to detect that the network is trying to contact it, the mobile will request access to the network to gain a signaling channel and determine what the network is asking for, for example, set up a call or receive the SMS.

In GSM, the VLR or SGSN asks every cell in a certain location area to send the same paging message. In UMTS, if the subscriber is known to be located in a certain URA, the RNC can intelligently page for the subscriber in the URA, therefore, reducing the signaling in the network.



71

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LA2/RA2

LA1/RA1

RNC

RNC

SGSN

Paging

Paging

MSC

Paging Response

Fig. 33 Paging in the network


TM51104EN03GLA3


2.4 Roaming Case

When a subscriber is in a foreign network, the procedures are the same. When the subscriber registers in the visiting network, it will in turn contact the home network. A part of the IMSI code specifies the home network. If the two operators have a roaming agreement and the subscriber is valid, the subscriber information is copied into the serving VLR of the MSC and the information on the subscriber is stored in the HLR.

Every VLR in the world has a unique address. As a subscriber moves from one network to another, the location updating proceeds as normal. The HLR is always informed of the unique VLR, in which the subscriber was last seen.

If a subscriber is roaming in another network and the network needs to contact the subscriber to receive a video call, the location of the subscriber is checked from the HLR. The HLR will then contact the serving MSC to check if the subscriber is still located in the VLR. This is called the HLR request. Then, the information is returned to the MSC and a call is routed to the foreign MSC to begin the paging process. Even if the calling subscriber is located in the foreign network, the call still has to be initially placed back to the home MSC.



73

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UE

BS

BS

RNC MSC/VLR

SGSN

HLR

BS

BS

RNC MSC/VLR

SGSN

HLR

Visiting Network

Home Network

(1) IMSI Attach, request access

(2) Update Location

(3) Copy subscriber data to VLR

(4) Confirm access

Fig. 34 Roaming in another network


TM51104EN03GLA3




75

3 Session Management


TM51104EN03GLA3


In the previous topic we looked at the mobility management and how the network keeps track of the location of the subscriber and the procedures it performs. In this topic we will look at how the mobile is able to access the network and to obtain a bearer. We will also cover two simplified cases of how real time and non-real time bearers are set up in the network. There are procedures used to obtain a bearer through the network and terminal is also capable of determining one network from another. Each country has its own MCC and each operator within a country has a unique MNC. This information is broadcasted by every cell in the network. Therefore, when the mobile is activated, it is able to distinguish between operators by checking this information. Through co-operation of the operators, the frequencies and codes used and shared in inter-boarder areas are selected for network planning to reduce conflict.

3.1 Initial Access to the Network When the mobile is switched on, it starts the network selection procedure. The mobile is aware of the possible frequencies that are available in UMTS and all the possible codes that are used by the cells. First, the mobile checks the last frequency and code used to identify the cell, to check if it is still valid. If the cell cannot be found, the mobile starts applying each code to each possible frequency in an attempt to detect a signal that indicates the presence of a cell. Once the scanning process is over, the mobile selects its home network as the first choice. The information of this is on the SIM. If the home network is not present, then it can choose a preferred network, which is usually set by the home network operator. If the preferred network is not available, the mobile randomly selects another network that provides the adequate signal level. The procedure of network selection is usually performed automatically, but it can also be made manually. On the selection of the network, the mobile will request a location update or IMSI attach for its position. The RNC will then request for the location update. If the home network is present the first time, the update is made. For the update, the information on the subscriber is copied to the serving VLR for the MSC area and the current information on the subscriber is updated to the HLR. The subscriber is also registered into the current URA.



77

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UE

Operator A

Frequency 1RNC MSC/VLR

SGSN

HLR

Operator A

Frequency 1

Operator B

Frequency 2

Operator C

Frequency 3

Networks transmit information about themselves (MCC + MNC)

Fig. 35 Initial Network Access to the Network


TM51104EN03GLA3


3.2 Authentication and Ciphering The most important security features in the access security of UMTS are as follows:

• Mutual authentication of the user and the network

• Use of temporary identities

• Radio access network encryption

• Protection of signaling integrity inside UTRAN

Note that publicly available cryptographic algorithms are used for encryption and integrity protection. Algorithms for mutual authentication are operator-specific. The SN checks the identity of the subscriber as in GSM by a technique called challenge-and-response. , A new feature in UMTS is that the terminal checks that the SN is authorized by the home network and the mobile is connected to a legitimate network. The security is based on the Quintet, UMTS authentication vector, which is temporary authentication and key agreement data that enables a VLR or SGSN to engage in UMTS AKA with a particular user. A quintet consists of five elements as follows:

1. A Network Challenge Random Access Number (RAND)

2. An Expected User Response (XRES)

3. A Cipher Key (CK)

4. An Integrity Key (IK)

5. A Network Authentication Token (AUTN)

The cornerstone of the authentication mechanism is a master key, K, which is shared between the USIM of the subscriber and the home network database. This is a permanent secret with the length of 128 bits. The key, K, is never transferred out from the two locations and the subscriber has no knowledge of the master key. At the same time, with mutual authentication, keys for encryption and integrity checking are derived. These are temporary keys with the same length of 128 bits. New keys are derived from the permanent key, K, during every authentication event. It is a basic principle in cryptography to limit the use of permanent keys to minimum and instead derive temporary keys from it for protection of bulk data. In the Authentication and Key Agreement (AKA) mechanism, the authentication procedure is started after the user is identified in the serving network. The identification occurs when the identity of the user, which can be the permanent identity, IMSI, or temporary identity, TMSI, is transmitted to VLR or SGSN. Then, the VLR and SGSN send an authentication data request to the AuC in the home network. The AuC contains master keys of the users and based on the knowledge of IMSI the AuC is able to generate authentication vectors for the user. The generation process contains executions of several cryptographic algorithms. The generated vectors are sent back to VLR or SGSN in the authentication data response. These control messages are carried on the MAP protocol.



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USIM

AuthenticationData Request [IMSI]

AuthenticationData Response

[AV(1..n)]

Authentication Request[Authentication Parameter]

IMSI K;f1...f5

Authentication Vector/ Quintet

K: secret KeySQN: Sequence Numberf1...f5: message authentication /

key generating Functions

Ksecret Key

128 bit length

Visited PLMN Home PLMN

Authentication ResponseNetwork

Authentication UserAuthentication

Authentication Principles

SGSN/VLR

HLR/AuC

Fig. 36 Authentication Principles


TM51104EN03GLA3


In the serving network, one authentication vector is needed for each authentication instance, for example, for each run of the authentication procedure.

In other words, the potentially long distance signaling between SN and the AuC is not needed for every authentication event and the authentication events can be performed independently without the user actions after the initial registration. The VLR or SGSN can obtain new authentication vectors from AuC before the number of stored vectors runs out.

The serving network, VLR or SGSN sends a user authentication request to the terminal. This message contains two parameters from the authentication vector, called RAND and AUTN. These parameters are transferred into the USIM that exists inside a tamper-resistant environment, for example, in the UMTS IC card (UICC). The USIM contains the master key, K, and uses it with the parameters

RAND and AUTN as inputs for carrying out a computation that resembles the generation of authentication vectors in AuC. This process also contains executions of several algorithms similar to the corresponding AuC computation.

As the result of the computation, USIM is able to verify whether the parameter AUTN was indeed generated in AuC and the computed parameter RES is sent back to VLR or SGSN in the user authentication response. Now, the VLR or SGSN is able to compare user response RES with the expected response XRES, which is part of the authentication vector. In case the responses match, authentication ends positively.



81


RANDRandom Number

128 bit

XRESExpected Response

32 - 128 bit

CKCipher Key

128 bit

IKIntegrity Key

128 bit

AUTNAuthentication Token

48 + 16 + 64 bit

USIM

Authentication Request[RAND(i), AUTN(i)]

Authentication Response[RES(i)]

User Authentication:Compare

XRES(i) & RES(i)

• generate RES(i) = f2(RAND(i),K)

• AUTN(i) for Network Authentication

randomly generated,i.e. non-predictable

Used for dataencryption

Used forintegrity check

RES: Response

• consisting of 3 parts• Used for network

authenticationUsed for userauthentication

Authentication Vector AV

SGSN/VLR(store AV(1..n))

Fig. 37 Authentication Vector


AuC

AV =

RAND GeneratorDatabase

(IMSI;K)

AMFAuthentication &key Management

Field

SQN Generator

SQNSequence Number

RANDRandom Number

Ksecret Key

f1 f2 f3 f4 f5

MACMessage Authentication

Code→ Network Authentication


→ UserAuthentication

CKCipher Key

→ Ciphering

IKIntegrity Key→ Ciphering

AKAnonymity Key

→ SQN Anonymity

RANDRandom number


CKCipher Key

IKIntegrity Key

AUTNAuthentication Token

SQN ⊕ AK48 bit

AMF16 bit

MAC64 bit

AMF→ selection of f1-5 version→ different f1-5 versions possible

(operator-dependent)

AV Generation

Fig. 38 Authentication Vector Generation


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The keys for radio access network encryption and integrity protection, CK and IK, are created as a by-product in the authentication process. These temporary keys are included in the authentication vector and are transferred to the VLR or SGSN.

These keys are later transferred further into the RNC in the radio access network when the encryption and integrity protection start. Simultaneously, the USIM is also able to compute CK and IK after it has obtained RAND and verified it through AUTN. The temporary keys are subsequently transferred from USIM to the mobile equipment where the encryption and integrity protection algorithms are implemented. The Sequence Number (SQN) is a counter. There are two SQNMS and SQNHE respectively to support network authentication. The sequence number SQNHE is an individual counter for each user and the sequence number SQNMS denotes the highest sequence number the USIM has accepted.



83


USIM Authentication Request[RAND(i), AUTN(i)]

Authentication Response[RES(i)] Compare:

• XRES(i) = RES(i) ? User Authentication

Generate: • RES• XMAC• CK• IK

RAND SQN ⊕ AK AMF MAC AUTNK

f5 AK

f1f2f3f4

⊕

SQN

IK CK RES XMAC

CipheringIntegrityCheck

to network→ User

Authentication

XMAC = MAC ?→ Network

Authentication

XMAC:Expected MessageAuthentication CodeAK: Anonymity Key

AMF:Authentication &key ManagementField

or Authentication Reject[XMAC ≠ MAC]

Authentication in the USIM

VLR / SGSN(stores AV(1..n))

Fig. 39 Ciphering in UMTS/UTRAN


UE or S-RNC

UE

f8 (UEA)CKCipher Key

COUNT-C Bearerradio bearer id.

Lengthlength indicator

Keystream block

UL = 0DL = 1

CipherSequence No.

indicate lengthof required

keystream blockDirectiondirection bit

CKPS & CKCS

1 Bearer parameter /user radio bearer

Keystream block⊕Plain text block = ciphered text block

ciphered text block Plain text blockKeystream block⊕ =

“cipher sequence”

not in case ofemergency calls

S-RNC

Ciphering

UMTS Encryption Algorithm UEA

Fig. 40 Ciphering in UMTS/UTRAN


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85

4 Procedure Examples


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In order to allow the reader to accomplish a network-wide view of some of the most important UMTS functionalities discussed in the preceding chapters we present a few examples of UMT system procedures. These procedures illustrate the co-ordination of actions carried out by a network entities and the role of the UMTS protocols in controlling this co-ordination.

4.1 Elementary Procedures

In this chapter a basic model for system-wide procedures is presented by modeling each system procedure as a kind of communication-intensive transaction. The concept of transaction is used in order to underline the fact that these system-wide procedures are executed into well-defined completion and that subsequent procedures represent fairly independent instances of communication.

The layering and interworking of protocols makes it possible to distinguish a set of elementary procedures, which can be used as building blocks in the design of network transactions. Depending on a transaction and its type - whether it is mobile terminated or mobile originated - some parameters and messages vary inside the elementary procedures and some elementary procedures may or may not be used.

The UMTS protocol interworking model is needed here in the sense that different procedures make use of different layers in the UMTS network. The transport network is used in every procedure: signaling transport is always required and user data transport is required by many transactions. Radio network functions are needed whenever access network services are required within an elementary procedure. The overall control of system transactions is done by the system network protocols, which invoke the elementary procedures in a stepwise manner and determine how the transaction is proceeding through different steps.

Basically any network transaction can be divided into eight steps as presented in following figure. For each step an elementary procedure can be distinguished.

Paging is a mobility management procedure used when searching certain subscriber from the network coverage area. This procedure is only executed if the transaction originates from the network side. The remaining seven steps are the same whether the transaction is originated or terminated by the UE.

Radio Resource Control (RRC) connection set-up is an elementary procedure containing activities and message flow to establish a radio control connection between the terminal and radio access network. The details of this procedure vary depending on the case.



87


Paging

RRC Connection Setup

Transaction Reasoning

Authentication and Security

Transaction Setup and RAB Allocation

Transaction

Transaction Clearing and RAB Release

RRC Connection Release

Radio Network (C)

System Network (C)

Radio Network (C)

Radio Network (C)

System Network (C)

Radio Network (C)

System Network (C)

Radio Network (C)

System Network (C)

Radio Network (C/U)

System Network (C/U)

Radio Network (C)

System Network (C)

Radio Network (C)

Fig. 41 Basic model of UMTS network transactions

Transaction reasoning is an elementary procedure where the terminal indicates to the Core Network (CN) which kind of transaction is requested. Based on the reasoning information the CN may decide how to proceed with the transaction or decide to terminate the execution of the transaction. After these steps the authentication and security procedure normally takes place. This elementary procedure authenticates the UMTS subscriber and network to each other and later activates the necessary security mechanisms for the access network connection. Transaction set-up with radio access bearer allocation is an elementary procedure which allocates the actual communication resources for the transaction. Therefore the details of this procedure depend on the type of the transaction (circuit or packet switched). The elementary procedure transaction is the phase where the user plane connection exists, i.e. the UE has a UMTS bearer connection active across the whole UMTS network. Transaction clearing with radio access bearer release is an elementary procedure used for releasing the network resources related to the transaction. The RRC connection release is an elementary procedure containing mechanisms with which the radio control connection between the UE and the access network is released.


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4.2 Paging

The paging procedure is needed whenever a UE terminating transaction is to be performed. Unlike in GSM, UMTS contains two types of paging methods, Type 1 and Type 2. Paging Type 1 is used by the CN domains and this is the “conventional” way to use paging. Paging is part of the radio network control plane and it is delivered over the Iu Interface by using the AP Paging message. The CN domain initiating paging addresses it to those LAs/RAs, in which the desired UE has last reported its location, i.e. performed either location update or routing area update.



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RANAP: Paging

RRC: Paging Type 1

UECore Network Domain

RNC

RRC Connection Setup

Fig. 42 Paging Type 1


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An RANAP paging message contains two mandatory parameters, the requesting CN domain and IMSI. From a system security point of view, the unnecessary transfer of IMSI over the Uu reference point without encryption is not desired and because of this the RNC may or may not perform various conversions for the IMSI. For example, the RNC may issue a RRC Paging Type 1 message containing Radio Network Temporary Identity (RNTI) instead of IMSI. This value has been assigned when the UE is attached to the network or performed the previous transaction.

Typically the receipt and recognition of the RRC paging Type 1 message by the UE leads to the establishment of an RRC connection, which is needed to carry on the transaction, which was initiated by the paging.

In addition to the previously described use, the RRC paging Type 1 has some RAN specific uses. When the UE is not in idle mode and changes in the system information parameters need to be made known to the UE, the RNC may use RRC paging Type 1 to "awake" the UE and thus make it able to read the revised system information or certain parts of it.

If the UE has a packet switched connection without any activity the RNC uses RRC paging Type 1 to inform the UE that the packet switched connection should be reactivated. In this case there are some packet data the CN desires to deliver to the UE. This forces the UE to perform cell update and after this the RNC is able to route the data packets to the correct UE.

UMTS terminals are meant to be able to handle various connections simultaneously. This is, they may have more than one connection at a time with CN domains. When the UE already has a connection with one CN domain and another connection is established with the same domain the CN sends RANAP paging like in previous cases. The paging sent to the UE by the SRNC is now RRC paging Type 2.

The separation between paging Type 1 and Type 2 is done in the RNC, the RANAP paging always looks the same. The difference between Type 1 and Type 2 is that Type 1 is used for paging those UEs which are in idle mode, cell-PCH or URA-PCH state and hence it could be either for certain UEs or for all UEs within cell(s). Type 2, on the other hand is used for paging those UEs, which are in cell-DCH or Cell-FACH state, therefore it is always dedicated and addressed to one UE only.



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RANAP: Paging

RRC: Paging Type 2

UECore Network Domain

RNC

Active connection exists

Fig. 43 Paging Type 2


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93

4.2.1 RRC Connection Setup

Following figure illustrates the principle of how the radio connection between the UE and the RNC is established over the Uu interface and the access domain internal interface Iub. The RRC connection set-up always starts from the UE with the message RRC Connection request sent over CCCH (Common Control Channel). Referring to Chapter 3, the CCCH in the uplink direction means RACK (random access) and actually the RRC connection request content is coming from the UE over physical RACH as a result of the initial access procedure. This message arrives to the RNC over the Iub RACH data port. Upon receiving this message the RRC entity at the RNC side changes its state from IDLE to CONNECTED Cell_FACH or Cell_DCH. Then the RNC communicates towards the UE over common control channels and from those, the used ones are FACH and RACH, respectively. The RRC connection request message contains plenty of information related to the requested radio connection, terminal and subscriber identity; in this message the UE sends to the network, for instance, IMSI or TMSI, IMEI, location area identity and routing area identity. The RRC connection request must indicate which of these values are inserted in the message and also contain those values accordingly. In addition to these identities the RRC connection request contains the reason why the radio connection is requested. There are various reasons and the following list shows some of them:

• Originating conversational call

• Originating streaming call

• Originating interactive call

• Originating background call

• Terminating conversational call

• Terminating streaming call

• Terminating interactive call

• Terminating background call

• Emergency call

• High priority signaling

• Low priority signaling

• Call re-establishment

As can be seen from the list, the RRC connection request already indicates what kind of QoS will be requested when the transaction proceeds. Emergency call is separated because it will be treated differently over the network than normal transactions. If the subsequent transaction will be only signaling, this is indicated, too.


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Depending on the request reason, the RNC makes a decision whether to allocate dedicated or common resources to this transaction. Based on this decision-making, the SRNC allocates RNTI and other resources to this transaction.

The Iub interface is "opened" when the RNC sends a NBAP radio link set-up message to the BS. This message contains transport format description, power control information and code information, which is the uplink scrambling code for WCDMA-FDD. The BS acknowledges this message by sending a NBAP radio link set-up response message. This message informs the RNC about transport layer addressing information (AAL2 address) and gives also some reference information for establishment of an Iub bearer in the transport network.

The serving RNC starts the Iub bearer establishment according to the information received from the BS. This procedure is carried out by the internal control plane of the transport network at the Iub interface. The established Iub bearer is bound together with the DCH (Dedicated Channel) assigned to the transaction. After this the frame protocol connection over Iub is synchronized with message exchange. If we suppose that a dedicated radio connection using DCH is set up here, then it is the dedicated channel frame protocol for Iub.

When the Iub communication is ready the RNC sends the RRC connection set-up message to the UE over common control channels (FACH in this case). In this message the SRNC informs the UE about the transport format, power control and codes, which in the case of WCDMA-FDD is a downlink scrambling code. The UE confirms RRC connection establishment by sending the message RRC connection set-up complete.



95

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RRC: Connection Request

NBAP: RL Setup

RRC: Connection Setup

UE BS RNC

Uu Iub

NBAP: RL Setup Response

Iub Bearer Establishment

FP: Downlink Synchronization

FP: Uplink Synchronization

RRC: Connection Setup Complete

Fig. 44 RRC Connection Setup


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4.2.2 Transaction Reasoning

When the RRC connection has been established the UE sends the message RRC initial direct transfer. RRC initial direct transfer carries in its payload the first system network message of the transaction from UE to the network.

Upon receiving this information the RNC adds some more parameters to it and forwards this combination to the appropriate CN domain as a RANAP UE initial message. The RANAP UE initial message contains in its payload the contents of the original RRC direct transfer message together with the first system network message generated by the UE.

The contents of the UE initial message provide the CN with lot of information about the transaction initiated by the UE. Among such information is the (claimed) identity of the UMTS subscriber (TMSI or IMSI), his/her current location area and the kind of transaction requested. All this information is used by the CN node (MSC/VLR or SGSN) to decide, how to proceed with the transaction request.



97

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UE RNCMSC/VLR or SGSN

RANAP: UE Initial Message

RRC: Initial Direct Transfer

Fig. 45 Transaction reasoning


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4.2.3 Authentication

During the RRC connection establishment the UE already informed the RNC with the UE classmark parameter about its capabilities in many respects, one of those being the security algorithms supported by the UE.

The UE and the network authenticate each other by sending the authentication request message in the payload of RANAP and RRC direct transfer messages to the UE. After executing the authentication algorithms on USIM the UE responds with an authentication response message again in the payload of RRC and RANAP direct transfer messages. In this dialogue the RNC acts as a relay forwarding the contents of RANAP direct transfer to RRC direct transfer and vice versa.

With a RANAP security mode command message the related CN domain indicates to UTRAN that the transaction should be encrypted. This message indicates the selected security algorithms and delivers the integrity and encryption keys to UTRAN.

Based on this information the RNC commands the UE to start encryption with the corresponding keys and algorithms by sending the RRC security mode command. By issuing an RRC security mode complete message the UE indicates that it has successfully turned on the selected integrity protection algorithm and encryption algorithm for protection of further communication in this transaction. The RNC still has to inform the related CN domain about the procedure completion with the message RANAP security mode complete.



99

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UE RNC MSC/VLR or SGSN

RANAP: Direct Transfer (Authentication Request)

RRC: Direct Transfer (Authentication Request)

RRC: Direct Transfer (Authentication Response)

RANAP: Direct Transfer (Authentication Response)

RANAP: Security Mode Command

RRC: Security Mode Command

RRC: Security Mode Complete

RANAP: Security Mode Complete

Fig. 46 Authentication and Security Control


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4.2.4 Transaction Setup with RAB

So far, the contents of the elementary procedures have been similar and thus independent on the CN domains. Transaction set-up with radio access bearer allocation is the first step where the different nature of CN domains has to be taken into account.

In the case of circuit switched transaction the actual set-up information is delivered through RRC/RANAP direct transfer messages. These messages may carry in their payload the Call Control (CC) set-up message as shown in Figure 46. The CC set-up message identifies the transaction and indicates the QoS requirements, i.e. what kind of bearer is required for the service with the following parameters:

• Transaction identifier (TI)

• Stream identifier

• Traffic class

• Asymmetry indicator

• Maximum bitrate

• Guaranteed bitrate

With the TI value the UE and the CN node are able to separate calls from each other. Each call has its own TI value. The stream identifier recognizes the bearer used for this call. If the stream identifier value does not exist yet, it means that the CC protocol in UE likes to establish a new bearer. The stream identifier value could be in use already. In that case the CC protocol likes to establish a multicall using an existing bearer given by the stream identifier. Upon receiving the CC set-up message the MSC/VLR starts some activities. At first, the MSC/VLR checks whether the UE and current subscription have rights to perform the requested operation. If yes, the MSC/VLR starts RAB allocation by assigning a unique RAB ID and requesting a RAB with given QoS parameters to be set-up by sending the message RANAP RAB assignment request over the Iu interface. When the RNC receives the RANAP RAB assignment request it starts activities for radio bearer allocation by first checking if there are enough resources available to satisfy the requested QoS. If yes, the radio bearer is allocated according to the request. If not, the RNC may select either to continue the allocation with lowered QoS values (for instance the maximum bitrate for the bearer could be lowered) or it may select to queue the request until radio resources become available.



101

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UE RNC MSC/VLR

RRC: Direct Transfer (CC: Setup)

RANAP: RAB Assignment Request

RRC: Radio Bearer Setup

RRC: Radio Bearer Setup Complete

RANAP: RAB Assignment Response

RANAP: Direct Transfer (CC: Setup)

Radio Bearer Establishment

Iu CS Bearer Establishment

RANAP: Direct Transfer(CC: Call Proceeding)

RRC: Direct Transfer(CC: Call Proceeding)

Fig. 47 Transaction Setup with RAB allocation (CS)


TM51104EN03GLA3


The RNC informs the UE about the bearer allocation by sending the message RRC radio bearer set-up to the UE. When the UE receives this message it is able to combine the information it originally sent to the network in its CC set-up message with the received radio bearer identifier. After this the UE can route the user data traffic (user plane) to the correct radio bearer. As soon as the UE is able to receive data from the new radio bearer it acknowledges this by sending the RRC radio bearer set-up complete message to the RNC. The RNC must also establish a Iu bearer for the new transaction.

After this the RNC indicates that RAB has now been allocated by sending the message RANAP RAB assignment response to MSC/VLR. If the RNC made exceptions from the QoS values requested by the MSC/VLR, the changes are indicated in this message.

Now the RAB is established and the procedure continues on the CC protocol level. In case the RAB is to be established for a packet switched transaction, the procedure looks pretty much the same and the differences are within the messages. Instead of the CC protocol the UE now uses the SM protocol. The UE sends the SM activate PDF context request message to the network as an RRC direct transfer message and the RNC relays this as a RANAP direct transfer message to the SGSN in the CN.

Radio bearer allocation is similar to circuit switched traffic except that the parameters describing the QoS properties of the bearer are different. For example, guaranteed bitrate is an essential parameter in the case of a packet switched RAB but is not significant with circuit switched RABs.

After the RAB has been established the CN domain confirms the packet session establishment by sending an SM activate PDF context accept message. In terms of Session Management (SM) this means that the SM state is now active and the UE and the CN domain are able to exchange packet switched data.



103

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UE RNC SGSN

RRC: Direct Transfer (SM: Activate PDP Context Req.)

RANAP: RAB Assignment Request

RRC: Radio Bearer Setup

RRC: Radio Bearer Setup Complete

RANAP: RAB Assignment Response

RANAP: Direct Transfer (SM: Activate PDP Context Req.)

Radio Bearer Establishment

Iu PS Bearer Establishment

RANAP: Direct Transfer(SM: Activate PDP Context Accept)

RRC: Direct Transfer(SM: Activate PDP Context Accept)

Fig. 48 Transaction set-up with RAB allocation (PS)


TM51104EN03GLA3


4.2.5 Transaction

In the most typical cases this phase of the transaction contains an active user plane connection. Examples of this kind of transactions are given later in this chapter. However, if the RRC connection was requested for a pure signaling purpose like an MM activity, no user plane bearers are needed. Instead, a signaling connection to perform the MM activity is used here.

4.2.6 Transaction Clearing and RAB release

In the most typical cases this phase of the transaction contains an active user plane connection. Examples of this kind of transactions are given later in this chapter. However, if the RRC connection was requested for a pure signaling purpose like an MM activity, no user plane bearers are needed. Instead, a signaling connection to perform the MM activity is used here.

The UE still has an RRC connection to the UTRAN and any other RABs for the same UE may still exist. In the general case with a single RANAP RAB assignment request message the CN may cause a number of RABs to be created, deleted or modified. If all the RABs between the UE and CN domain are released at the same time and also the related radio resources are deallocated, this can be done by using RANAP



105

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UE RNC MSC/VLR

RRC: Direct Transfer (CC: Disconnect)

RANAP: Direct Transfer(CC: Release)

RRC: Direct Transfer(CC: Release)

RRC: Direct Transfer(CC: Release Complete)

RANAP: Direct Transfer(CC: Release Complete)

RANAP: Direct Transfer (CC: Disconnect)

RANAP: RAB Assignment Request (Release)

RRC: Radio Bearer Release

RRC: Radio Bearer Release CompleteRANAP: RAB Assignment Response

(Release)

Fig. 49 Transaction clearing with RAB Release (Circuit Switched)


TM51104EN03GLA3


Iu release command as shown in following figure. Upon receiving this message the RNC starts RRC radio bearer release over the Iub interface. Also all possible radio bearers established through DRNCs are released.

When the radio bearers are released the RRC connection is also released. After all radio bearers and the RRC connection are released the RNC informs the CN domain with a RANAP Iu release complete message.

released. RAB release is done in the same manner as in the case of circuit switched connection. When the RAB has been released the SGSN indicates that packet switched connection is now deactivated by sending the SM deactivate PDP context accept message to the UE.



107

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UE RNC MSC/VLR

RRC: Direct Transfer (CC: Disconnect)

RANAP: Direct Transfer(CC: Release)RRC: Direct Transfer

(CC: Release)

RRC: Direct Transfer(CC: Release Complete) RANAP: Direct Transfer

(CC: Release Complete)

RANAP: Direct Transfer (CC: Disconnect)

RANAP: Iu Release Command


RRC: Radio Bearer Release Complete

RANAP: Iu Release Complete

Clearing of RRC Connection

Fig. 50 Transaction clearing with Iu release (Circuit Switched)


TM51104EN03GLA3


In the case of packet switched connections the user plane and RAB(s) are cleared in the same way as in the case of circuit switched connections. Again the differences are in the message parameter level. Following figure illustrates the case where the packet switched connection is closed by deactivation of the PDF content.

The closing of a packet switched connection is one of the SM protocol tasks where the SM changes its state from active to inactive. The signaling procedure used for this state transition is PDF context deactivation. When the UE desires to do this, it sends an RRC direct transfer message containing SM deactivate POP context request to the SRNC. The SRNC further relays it as a RANAP direct transfer message to the CN PS domain.

PDP context deactivation request indicates to the SGSN that the RAB allocated for this packet switched connection is not needed any more and thus it should be released.



109

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UE RNC SGSN

RRC: Direct Transfer (SM: Deactivate PDP Context Req.)

RANAP: RAB Assignment Request (Release)


RRC: Radio Bearer Release CompleteRANAP: RAB Assignment Response

(Release)

RANAP: Direct Transfer (SM: Deactivate PDP Context Req.)

RANAP: Direct Transfer(SM: Deact. PDP Context Accept)

RRC: Direct Transfer(SM: Deact. PDP Context Accept)

Fig. 51 Transaction clearing with RAB release (Packet Switched)


TM51104EN03GLA3


After this procedure the UE still has an RRC connection with the network and it may have other circuit switched and packet switched connections open. If all of the packet switched connections are to be terminated and also related radio resources de-allocated, the system uses the RANAP Iu release command like in the case of circuit switched connections.



111

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UE RNC SGSN

RRC: Direct Transfer (SM: Deactivate PDP Context Req.)

RRC: Radio Bearer Release Complete


RANAP: Direct Transfer (SM: Deactivate PDP Context Req.)


RRC: Direct Transfer (SM: Deactivate PDP Context Accept)

RANAP: Direct Transfer (SM: Deactivate PDP Context Accept)


Clearing of RRC Connection

Fig. 52 Transaction clearing with RANAP lu release (Packet Switched


TM51104EN03GLA3


4.2.7 RRC Connection Release

The RRC connection release procedure, which is shown in following figure, is always started by the RNC. The RNC identifies which RRC connection is to be released and then sends this information to the UE in the message RRC connection release. The UE acknowledges that the RRC connection has been released by sending the message RRC connection release complete.

After this the RNC starts to clear the lub interface resources. This is done by exchange of NBAP radio link deletion and NBAP radio link deletion response messages. When the radio link deletion is agreed on the NBAP level, the transport data bearer at the lub interface is released. Any radio links established over a DRNC also has to be released by using the lur subprocedures.



113

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RRC: Connection Release Complete

NBAP: RL Deletion

UE BS RNC

Uu Iub

NBAP: RL Deletion Response

Iub Bearer Release

RRC: Connection Release

Fig. 53 RRC connection release


TM51104EN03GLA3




115

4.3 RRM Procedure Examples

The RRM is a collection of algorithms needed to establish and maintain a good quality radio path between the RNC and the UE. Thus, basically every algorithm the RRM contains is present in any transaction the UTRAN is involved in. The RRM algorithms are handover algorithm, power control algorithm, admission control and packet scheduling as well as code management. From these, power control, admission control and code management are used "continuously", i.e. from the very beginning of the transaction and they are involved in every phase of a transaction. Packet scheduling is used in the context of packet switched transactions and handovers are applied for both circuit and packet switched transactions whenever needed.

The RRM entities located in UE and RNC communicate on radio resource management between themselves by using the RRC protocol. Therefore the establishment of an RRC connection, is a must for every transaction. However, it is important to keep in mind, that for any UE one RRC connection with UTRAN is enough no matter how many radio bearers are simultaneously open for various system-wide transactions.

In this subsection we present some examples of handovers. The handover type to be explained first is soft handover. After soft handover examples, we give an overview of the SRNS relocation procedure, where UE is not involved in the procedure. The last RRM procedure example is the inter-system handover case where the UE performs handover from WCDMA to GSM radio access.


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4.3.1 Soft Handover – Link Addition and Link Deletion

When the UE has a service in use the RRC connection with the UTRAN exists and is active. In this case, the UE continuously measures the radio connection and sends measurement reports to the SRNC.

The handover algorithm located in the SRNC averages and investigates the contents of the received measurement reports. Based on the results the SRNC realizes that the UE has measured a cell located in BS 2 to have radio conditions fulfilling handover criteria defined in the SRNC. Based on the radio network information stored in the SRNC database the SRNC finds out that the target cell in BS 2 does not belong to the same RNS.

The SRNC starts arrangements on the UTRAN side by requesting through the lur interface the DRNC to set-up a new radio link. This is done by sending a RNSAP RL set-up request message. This triggers the DRNC to establish a radio link over the lub interface between the DRNC and BS 2 with NBAP protocol exchange.

After these steps the lub and lur bearers are established and frame protocols are synchronized in the downlink and uplink directions between the SRNC and BS 2. The frame protocols in the lub and Iur interfaces implement the radio network user plane and carry actual user data flow. In this example we assumed that the used service is a voice call. Thus, the frame protocol used in this example is the Iub/Iur Dedicated Channel (DCH) frame protocol. When the SRNC has received the FP uplink synchronization, it sends a RRC active set update message to the UE. In this message the SRNC indicates to the UE that a new radio link has been added to the active set of the connection through the cell located in the BS 2 and that the connection can be taken into use. The UE acknowledges this by responding with RRC active set update complete.

From the bearer architecture point of view, this example illustrates a situation where the UE has active, circuit switched RAB. This RAB is realized within UTRAN by the Iu bearer located between the SRNC and CN and one radio bearer going from the SRNC to the UE through a cell located in BS 1. The procedure described in following figure is, in the sense of QoS and bearers, a case where the SRNC adds an additional radio bearer to the existing connection. The RAB and Iu bearer remain unchanged. When the frame protocols are synchronized the SRNC performs RAB mapping to the radio bearers.



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RRC: Measurement Report

RNSAP: RL Setup Request

UE BS1 Serving RNC

Uu Iub

NBAP: RL Setup Response

Iub Bearer Setup

Drift RNCBS2Iub Iur

NBAP: RL Setup

RNSAP: RL Setup Response

Iur Bearer Setup

FP Downlink Synchronization

FP Uplink Synchronization

RRC: Active Set Update Complete

RRC: Active Set Update (RL Addition)

Fig. 54 Soft handover - link addition


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When the UE moves in the network during the transaction, it comes to the point where the SRNC finds out from the received measurement reports that a radio connection carrying the radio bearer through a cell located in BS 2 does not fulfill the criteria set to the radio connection any more. When this happens, the SRNC indicates to the UE that this particular radio connection can be removed from the active set. This is done by sending again an RRC active set update message to the UE indicating the radio connection to be removed. The UE acknowledges the radio connection removal by sending an RRC active set update complete message to the SRNC. Upon receiving the UE's acknowledgement the SRNC can now start the radio link deletion between itself and the BS 2. This is done by using a RNSAP RL deletion request over the Iur interface, which in turn triggers the DRNC to delete the radio link in the Iub interface with NBAP protocol exchange. When the radio link has been deleted both in Iub and Iur, the related Iub and Iur bearers are also released. The message flow related to a soft handover with a radio link deletion is illustrated in following figure.

From the bearer architecture point of view this radio link deletion example illustrates the case, where the SRNC removes one radio bearer but RAB between the CN and UE remains.



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RRC: Measurement Report

RNSAP: RL Deletion Request

UE BS1 Serving RNC

Uu Iub

NBAP: RL Deletion Response

Iub Bearer Release

Drift RNCBS2Iub Iur

NBAP: RL Deletion Request

RNSAP: RL Deletion Response

Iur Bearer Release

RRC: Active Set Update Complete

RRC: Active Set Update (RL Deletion of BS2)

Fig. 55 Soft handover - link deletion


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4.3.2 SRNS Relocation – Circuit Switched

The role of Serving RNC (SRNC) means the RNC maintains the Iu bearer and performs RAB mapping to the radio bearers with respect to a single active UE. Since the radio link components of the radio bearers may continuously change due to soft handover, the SRNC may end up in the situation, where it does not manage any radio links with its own BSs directly over the Iub any more. When this happens, the RAB mapping functionality should be changed to another RNC which is in a better position to perform it. Since the RAB mapping requires the existence of the Iu bearer in the same RNC, the CN connection will change, too.

The RRM procedure where the Iu bearer termination is changed from one RNC to another (and at the same time the radio bearers) is called SRNS relocation. The message flow of this procedure is illustrated in the following figure. In this figure, the RNC1 is the original SRNC and the RNC2 is the RNC, which will adopt the SRNC functionality.

The SRNS relocation procedure can be done in two different ways: either the UE is actively involved in the procedure or it is not. The example presented here illustrates the situation where the UE is not involved.

When the original SRNC (RNC 1) realizes that the SRNC functionality should be transferred to a better RNC candidate (RNC 2), it starts this procedure by contacting the relevant CN domain with a RANAP relocation required message. This message contains the reason for the relocation, target RNS identification and UE classmark information. Based on the target RNS identification the CN domain is able to route this query further onto the target RNS. This re-routed query is the RANAP relocation request message. In addition to other relocation related information, the RANAP relocation request contains information about bearer contexts with which the CN at the same time effectively moves the RAB and RRC connection end-point from the original SRNC (RNC1) towards the target SRNC (RNC2).

If the target RNC is able to provide resources to handle the incoming SRNC functionality it acknowledges the request back to the CN, which relays this acknowledgement to the source RNS. From the RNC1 point of view this acknowledgement is a command to start relocation, since everything should be now ready in the target RNC. Upon receiving the RANAP relocation command the original SRNC (RNC1) starts to forward data to the target SRNC (RNC2). The expected SRNC (RNC2) realizes the incoming data and informs the CN domain about the detection of SRNS relocation with the message RANAP relocation detect. Optionally, the source SRNC (RNC1) may send a RNSAP SRNC relocation commit message to the target SRNC (RNC2) over the Iur interface.

In the case of SRNC relocation where the UE is not involved, the UE is not aware of the message flow described above. If there is a need to perform any RRC procedures, they are performed after the RANAP relocation detect message. UTRAN may, for instance, send an RRC UTRAN mobility information message to the UE. With this message the UTRAN updates C-RNTI and U-RNTI and possibly other relevant MM information at the UE.


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When all relevant RRC procedures are performed the new SRNC (RNC2) informs the CN that SRNC relocation procedure is now complete by sending the message RANAP relocation complete. This then triggers the CN to release resources related to the old SRNC (RNC1) by issuing a RANAP Iu release message, which is acknowledged by the old SRNC (RNC1). Now the user data flows will go through the new serving RNC and, according to its role, it acts as the macro diversity combination point for the UE and also controls all RRC activities for this UE.



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RANAP: Relocation Command

RANAP: Relocation Required

(Optional) RNSAP: SRNC Relocation Commit

RANAP: Relocation Complete

User Data flow through RNC1

RANAP: Relocation Request

RANAP: Relocation Request Ack.

Data Forwarding

RNC1 to RNC2

RANAP: Relocation Detect

RRC Procedures



User Data flow through RNC2

UE RNC1 MSC/VLR RNC2

Fig. 56 SRNS relocation - circuit switched (UE not involved)


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125

4.3.3 Inter-System Handover from UMTS to GSM – Circuit Switched

The 3GPP specifications define that a handover between two radio accesses is a mandatory requirement for the system. From the UMTS point of view this means that the system must be able to perform handover between UTRAN and a GSM BSS.

Inter-system handover between UTRAN and GSM BSS is a special case of the SRNS relocation procedure as can be seen from following figure. Actually RANAP carries the same kind of information as the BSSMAP protocol in the GSM network and also much more. If the handover is performed from UTRAN to a GSM BSS, the UTRAN side messages are exactly the same as in the case of SRNS relocation but the contents of those messages vary. For example, the message RANAP relocation required contains the target RNC ID in the SRNC relocation case. When the target is on the GSM side, the target RNC ID is replaced with cell global identity, which is a parameter more familiar to the GSM BSC.

Unlike the SRNS relocation, the inter-system handover is a procedure where the UE is always involved, since the UE must perform Radio Resource (RR) activities in order to access the GSM BSS; note that the GSM BSS is not able to handle any kind of context information valid in UTRAN.

The UE must have the possibility to measure the GSM cells surrounding the current UTRAN cell(s). This is made possible in UTRAN by using the slotted mode procedure. Slotted mode gives the UE some time to perform measurements on the GSM band and thus detect any possible handover candidate(s). Information about these GSM cell candidates is delivered to the RNC with measurement reports just like that of the UTRAN cells.

As soon as the RNC realizes that a GSM cell is the best candidate and no suitable UTRAN cells are available, the SRNC starts to require relocation from the CN. The CN checks the contents of the message RANAP relocation required and finds out that the target cell for this handover belongs to a GSM BSS. Thus, the CN sends the message GSM BSSMAP handover required to the target BSC on the GSM network side. This triggers the target BSC to set up Traffic Channel (TCH) for the connection to be handed over to it. When the TCH allocation has been successfully performed within the GSM BSS the handover request is acknowledged back to the CN. The CN relays this message back to the SRNC as a RANAP relocation command message. The RANAP relocation command starts the relocation procedure within the SRNC. Since this is the case where the UE is involved the SRNC commands the UE to perform inter- system handover by sending the message RRC handover from UTRAN command. This message contains information about the target system and it also may carry in its payload any additional information related to the inter-system handover.

Upon receiving the RRC handover from the UTRAN command the UE checks whether the message indicated any time to perform the handover (default is immediately) and starts handover actions accordingly. Since the target radio access is GSM, the UE sends a GSM RR handover access message to the target cell in the GSM BSS. When the GSM BSS target cell detects this, it indicates to the BSC that


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the UE is accessing the GSM BSS. The GSM BSC in turn informs the CN about the incoming UE by sending the message GSM BSSMAP handover detect.

As an acknowledgement to the GSM RR handover access the UE receives GSM RR physical info from the GSM BSS cell. This message contains information with which the UE is able to start to use the GSM radio access, for example, channel descriptions are sent here to the UE.

Finally, when the UE has successfully accessed the GSM BSS cell, the UE sends a GSM RR handover complete to the GSM BSC. The BSC in turn relays the same information to the CN thus indicating that the UE has now entered into the GSM BSS and the inter-system handover is successfully completed in this respect. Since the UE does not use UTRAN resources any more, all resources related to the UE can be released and the CN issues a RANAP lu release command. The RANAP lu release command in turn triggers the RNC to clear the RRC connection and all the resources related to the UE are thus released. After this is done the RNC confirms the release to the CN with the message RANAP lu release complete.



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UE BSCMSCRNC

User Data flow through RNC

RRC: Measurement Report RANAP: Relocation

Required BSSMAP: Handover Required

TCH Allocation

BSSMAP: Handover Required Ack.RANAP: Relocation

CommandBSSMAP: TCH Assign

Command

GSM RR: Handover Access

BSSMAP: Handover Detect

GSM RR: Physical Info

GSM RR: Handover Complete

BSSMAP: Handover CompleteRANAP: Iu Release

Command


User Data flow through BSC

Fig. 57 Inter-system handover from UMTS to GSM - circuit switched


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4.4 MM Procedure Examples When compared to GSM, the MM behaves mostly the same way in UMTS but there are, however, some differences. MM as an entity covers all procedures, methods and identities required to maintain knowledge about the UE's location when it is moving in the network. Due to the existence of packet switched transactions a state model for MM was introduced. In GSM networks MM is completely handled between the MS and NSS. In UMTS networks most of the MM functions are equally handled between the UE and the CN but not all. The RNC partially handles the UE's movement within the radio access network using RRC procedures for this purpose. The MM activities handled by the RNC are cell and URA (UTRAN Registration Area) updates

4.4.1 Cell Update

During a circuit switched transaction, the CN knows the location of a UE with the accuracy of a location area and the RNC knows the location of the UE with the accuracy of a cell or – to be more precise - cells if the UE is in soft handover state. The cell information is continuously changing during the transaction. The pace of change depends on radio network conditions and whether the UE is moving and with what speed.

In the case of packet switched transactions the location information is distributed in such a way that the CN PS domain knows the location of the UE with the accuracy of location area and routing area and the RNC knows the location of the UE with the accuracy of a cell. Because packet switched connection is discontinuous in nature, the cell information in the RNC will actually be out-of-date when the data connection between the UE and the CN is temporarily closed. Hence, the packet switched connection is enabled from the UE and the CN point of view but the RNC is not able to route the data packets in this state. If the UE desires to send data packets and the current cell is different than it was earlier, the UE performs an RRC cell update procedure to inform the RNC about the current cell. After this basically any RRC activity is possible, for instance, a radio bearer can be established.



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RRC: Cell Update Confirm

UE RNC

Information Exchange between UE and RNC

RRC: Cell Update

Fig. 58 Cell update procedure

If, on the other hand, it is the CN PS domain that has some data packets to be sent to the UE, the CN PS domain first issues a RANAP paging to make the RNC send a RRC paging Type 1 to the UE. Upon receiving this the UE checks whether the cell update procedure is needed. If yes, the UE performs cell update and after that it is ready to start packet reception.

To summaries, the various reasons for a UE to perform a cell update are as follows:

• Cell reselection

• Periodic cell update

• Uplink data transmission

• Paging response

• Re-entered service area

• Radio link failure

• Unrecoverable RLC error

In association with a cell update procedure RNC may allocate a new RNTI to the RRC connection.


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4.4.2 URA Update

The RNC maintains registration of the current URA for each UE. a URA consists of number of cells belonging to either one RNC or several RNCs. Like LA and RA, the URA is one of those identities the UE stores in USIM. When switched on, the UE continuously monitors the received URA identity. If the received URA differs from the one stored in USIM the UE performs an RRC URA update procedure.

Other conditions, which trigger a URA update procedure, are, when the periodic URA update timer has expired or the UE has re-entered a network service area. As in the case of cell update, the RNC changes the RNTI allocated for the UE.



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RRC: URA Update Confirm

UE RNC

Information Exchange between UE and RNC

RRC: URA Update

Fig. 59 URA update procedure


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133

5 Exercises


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1. What is the name of the interface between 2 RNCs?

Iu-b

Iu-r

Iu-cs

Iu-ps

2. In the state URA_PCH, the connection is monitored?

On cell level

On site level

On RNC level

On URA level

3. What is the purpose of Admission Control?

To estimate whether a new call can have access to the system or not

To optimize the capacity of a cell and prevent an overload situation

To make the decision of the used channel type for the downlink direction

To return the system to normal load state in a fast and controlled way

4. How many downlink scrambling codes are available?

8

64

128

512

5. In power control what is the accuracy of decreasing and increasing power?

15 times per second

150 times per second





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6. Soft Handover is performed between two cells?

Belonging to different Node Bs but not necessarily to the same RNC

Belonging to different Node Bs and the same RNC

Belonging to the same RNC

Belonging to the same Node B 7. Micro Diversity is?

Micro diversity means the situation where the propagating multi path components are combined in the Node B.

Micro diversity means the situation where the propagating multi path components are combined in the RNC.

Micro diversity means the situation where the propagating multi path components are combined in the MS

Micro diversity means the situation where the propagating multi path components are combined in the MSC

8. In UTRAN Authentication, what is the length of the Master Key?

16 bits

32 bits

64 bits

128 bits


TM51104EN03GLA3


04 tm51104en03gla3 procedures

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