04_UMTS Traffic Management

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CTXX5323enIssue 4.0

© Nokia Networks Oy 1 (63)

SYSTEM TRAINING

UMTS Traffic Management

Training Document




2 (63) © Nokia Networks Oy CTXX5323enIssue 4.0

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Contents

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Contents

1 Module objectives ..................................................................................5

2 Introduction to UMTS traffic management...........................................6

3 Subscriber information and databases................................................8 3.1 Network databases...................................................................................8 3.2 Subscriber addressing and identities .....................................................10

4 Network traffic and radio connection.................................................13 4.1 Characteristic of a network bearer .........................................................14 4.1.1 Types and configuration of bearers........................................................15 4.2 Bearer transmission in the network........................................................16 4.3 Bearers and the different levels of QoS .................................................17 4.3.1 The end-to-end service and UMTS bearer service ................................18 4.3.2 The radio access bearer service and the core network bearer

service....................................................................................................19 4.3.3 The radio bearer service and the Iu-bearer service ...............................19 4.3.4 The backbone network bearer service...................................................19 4.4 Managing the bearer through the network .............................................20 4.5 Managing the bearer over UTRAN.........................................................21 4.5.1 Example: Simplified bearer establishment for a call...............................22 4.5.2 Managing the bearer when the subscriber is moving.............................24

5 Mobility management...........................................................................25 5.1 Cellular architecture ...............................................................................26 5.1.1 Network location areas...........................................................................27 5.1.2 Network routing areas ............................................................................28

5.1.3 UTRAN registration areas ......................................................................28 5.1.4 Location based information services......................................................29 5.2 Mobility procedure - Location updating ..................................................30 5.2.1 Location area based procedures............................................................31 5.2.2 Routing area update (packet switched)..................................................33 5.2.3 Location info retrieval (circuit and packet switched)...............................35 5.2.4 Management of the UTRAN registration areas ......................................35 5.3 Mobility management - Paging the subscriber .......................................36 5.4 Mobility management - Roaming in another network.............................37 5.5 Mobility management procedures..........................................................38

6 Session management ..........................................................................39 6.1 Initially accessing the network................................................................40

6.1.1 IMSI attach for an existing subscriber ....................................................41 6.1.2 IMSI attach when roaming......................................................................42 6.1.3 Requesting for a dedicated bearer.........................................................42 6.1.4 Access security in UMTS .......................................................................43 6.1.4.1 Mutual Authentication.............................................................................44





6.2 Managing a real time (circuit switched) bearer...................................... 48 6.3 Managing a non-real time (packet) bearer ............................................ 51

7 Communication management ............................................................ 56 7.1 Call control for circuit switched (real time) calls..................................... 57

7.2 Generation and collection of charging data ........................................... 58 7.3 Note on handling emergency calls ........................................................ 59

8 Review questions ................................................................................ 60



Module objectives

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1 Module objectives

The aim of this module is to give the student the conceptual knowledge needed

for explaining how traffic management is visualised in a UMTS network.

Topics to be covered in this module include understanding the network databases and the information stored within them. At an overview level, we willlook at the different management layers in the network.

After completing the module, the participant should be able to:

• List and identify the databases used within the UMTS network

• Identify the subscriber addressing information

• Name the characteristics of a bearer

• Describe how the connection moves with the subscriber when a bearer is

in use

• Explain what is meant by the term URA

• List the procedures used to maintain mobility management in the network

• List the procedures done when the mobile gains access to the network.

Also, identify how the network selection is made

• With the help of the material, describe how the session management of

real time and non-real time bearers are handled through the network

without using any references (if not otherwise stated).





2 Introduction to UMTS trafficmanagement

When visualising a UMTS network, there are three ways to approach this. Thefirst view is from the point of view of the architecture and the functions of the

elements within the RAS and the core network subsystem. The second approach

is through the different interfaces between the mobile, RAS, and the core

network. The third approach is to look at how the data and signalling are carriedthrough the network (management layers).

Fiber Fiber

AXC

ATM Access

Internet

RAN (UTRAN & GERAN) Core Network

Control Plane

Gateway Plane

PSTN2G

SGSN

3G

SGSN GGSNRNC

BSC

Node B

Node B

Node B

HLR

&AuC

3GMSC

SCEP

SMSC

BTS

Figure 1. 3G/UMTS network architecture

The above figure illustrates the UMTS Release 99 architecture, which is divided

into two planes. The control plane is responsible for the control of theinformation through the network, whereas the gateway plane manages the user

data or bearer through the network.

When thinking of managing the subscriber within a network, there are many

procedures used for locating and paging, as well as for control activities such as

moving and charging. We can think of all these activities as management

functions that the network is performing. The functions and procedures are

clearly defined in the specifications.

The functions can be divided into management layers. Each management layer

is responsible for certain procedures. The following figure illustrates the four

management layers in the network. The higher layers require the functions and



Introduction to UMTS traffic management

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procedures that are used on the lower layers. For example, you must have a

connection to the mobile before you can send or receive signalling messages.

In UMTS networks, we can identify three network-wide layers of functionality.

Mobility Management (MM)

Communication Management (CM)

Radio Resource Management (RRM)

UE RAN CN

• CS (circuit switched): Call control (CC), supplementary services (SS) and short message service (SMS).

• PS (packet switched): Session management

Figure 2. 3G-network management layers

The radio resource management (RRM) is completely covered between the

radio access network (RAN) and the user equipment (UE), and it involves

managing how the channels are allocated. The mobility management, sessionmanagement and call control are maintained by the core network (CN) domains.

There the function depends on whether the domain is CS (circuit switched) or

PS (packet switched). The higher-layer functions performed between the UE

and CN are often called as communication management (CM). The CM entity

covers the topics like call control (CC), supplementary services (SS) and shortmessage service (SMS).

The radio resource management is the lowest level and it is responsible for the

network communication with the mobile over the air interface. We will discuss

RRM only briefly in this module.

In this module we will first look at how the subscriber's information is stored.Also the structure of the cellular network and the functions of the management

layers will be explained.





3 Subscriber information and databases

Information about the subscriber is stored in several parts of the network. This

information is used to identify the location of the subscriber when transmitting

the paging signal. The network uses unique information to identify a subscriber,and there are different types of databases throughout the network. Most of the procedures are similar compared with GSM and GPRS. Hence, this chapter is

mainly of a repetitive nature.

3.1 Network databases

The databases are used all the time to control activities such as paging, channel

set-up and authentication. Other information about the subscriber may include,

for example, rights to services, security data, and identification numbers. The

figure below summarises the databases that are found within the network.

Figure 3. Network registers

Fiber Fiber

AXC

ATM access

Internet

Mobility CoreRAN

Control Plane

Gateway Plane

PSTN2G

SGSN

3G

SGSN GGSNRNC

BSC

HLR

AuC

EIR

3G

MSC

SMSC

Used as anintermediate store

for SMS

Contains subscriber IDs, serviceinformation and location

attributes. In AuC subscribersecurity information and insome implementations EIR

Used to supportsubscriber services

Contains temporaryinformation on

subscriber

Contains temporaryinformation on the

subscriber

SCEP

ModeBTS

TripleMode BTS

ModeBTS

WCDMABTS

ModeBTS

GSM BTS



Subscriber information and databases

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Since the core network will not change dramatically in the first release of

UMTS, the registers are similar to those in GSM and GPRS.

The Visitor Location Register (VLR) is considered to be an integral part of the

Serving MSC. The VLR maintains mobility management related procedures

like location update, location registration, paging, and security activities. TheVLR database contains temporary copies of the active subscribers, who have

performed a location update in its area.

The Home Location Register (HLR) contains permanent data of the

subscribers. One subscriber can always be in only one HLR. The HLR is

responsible for mobility management related procedures in both the circuitswitched and packet switched domains.

The Authentication Centre (AC/AuC) is a database handling the

Authentication Vectors. These contain the parameters that the VLR uses for

security activities performed over the Iu interface. The Equipment Identity

Register (EIR) maintains the security information related to the user equipment(UE) hardware.

The Short Message Service Centre (SMSC) is an intermediate store for thereceived/sent short messages. Thus, it has signalling connections with the VLR,

GPRS Support Nodes, and Gateway/Interworking MSC.

The IN Service Control Point (SCP) nowadays has INAP (Intelligent Network

Application Part) and/or CAP (Camel Application Part) connections towards thecore network circuit switched (CN-CS) domain elements. The CN-CS domain

elements having the IN connection is called Service Switching Points (SSPs).

In the packet switched domain, the HLR is still a centralised source of

information. However, two service nodes are used to supply the required IPaccess information: the Domain Name Server (DNS) and Firewalls. The DNS

is used for APN name to GGSN IP address translation. The Serving GPRSSupport Node (SGSN) needs to find out which Gateway GPRS Support Node

(GGSN) that supports access to this a specific access point. The role of the DNSis therefore to give the SGSN the IP address to the GGSN. After this, the GGSN

is able to route the user's request further. The border between the corporatenetworks, public IP, and 3G CN-PS domain is maintained by the GGSN which

may use the RADIUS database for user authentication. Firewalls are used for

security control of external network connections.

Other nodes (such as voice mail systems and application servers) can also

contain subscriber and network information.





3.2 Subscriber addressing and identities

Each subscriber has to be uniquely identified. As in 2G networks, unique

addressing codes are used to identify the subscriber. The figure below

highlights the identities used and where the information is stored.

Fiber Fiber

AXC

ATM Access

Internet


Control Plane

Gateway Plane

PSTN2G

SGSN

3G

SGSN GGSNRNC

BSC

Node B

Node B

Node B

HLR

&AuC

3G

MSC

SCEP

SMSC

BTS

Used as anintermediate store

for SMS

Contains subscriber IDs, serviceinformation and location

attributes. In AuC subscribersecurity information and insome implementations EIR

Used to supportsubscriber services

Contains temporaryinformation on

subscriber

Contains temporaryinformation on the

subscriber

Figure 4. IMSI and MSISDN addresses in the network

The unique identity for the mobile subscriber is called IMSI (International

Mobile Subscriber Identity), which is the same as the GSM:

IMSI = MCC + MNC + MSIN

Where:

• MCC = Mobile Country Code (3 digits)

• MNC = Mobile Network Code (2 digits)

• MSN = Mobile Subscriber Identity Number (normally 10 digits).

This number is stored in the SIM card (USIM).

The MSISDN (Mobile Subscriber international ISDN Number) is used for service separation. One subscriber may have several services provisioned and

activated, with only one IMSI. For instance, the mobile user may have one

MSISDN number for speech service, another MSISDN number for facsimileand so on.



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The MSISDN consists of three parts:

MSISDN = CC + NDC + SN

Where:

•

CC = Country Code (1 to 3 digits)• NDC = National Destination Code (1 to 3 digits)

• SN = Subscriber Number.

This number format follows the E.164 numbering specification. Very often this

number is called ‘directory number’ or just simply ‘subscriber number’.

Due to security reasons it is very important that the unique identity(IMSI/IMUI) is transferred in non-ciphered mode as less as possible. For this

purpose, the UMTS system uses TMSI (Temporary Mobile Subscriber Identity)

number, which is also called TMUI (Temporary Mobile User Identity). The

packet switched domain of the core network allocates similar temporaryidentities for the same purpose. In order to separate this type from the

TMSI/TMUI, it is named P-TMSI (Packet Temporary Mobile Subscriber Identity).

Fiber Fiber

AXC

ATM Access

Internet


Control Plane

Gateway Plane

PSTN2G

SGSN

3G

SGSN GGSNRNC

BSC

Node B

Node B

Node B

HLR

&AuC

3G

MSC

SCEP

SMSC

BTS

P-TMSI is allocatedfor packet

transactions by theSGSN.

The TMSI/TMUI isgenerated by the VLR

and used for CStransactions

Figure 5. Temporary information stored in the network

TMSI/TMUI and P-TMSI are random-format numbers, which have limitedvalidity time and validity area. The TMSI/TMUI numbers are allocated by theVLR and they are valid until the UE performs the next location update

procedure. The TMSI/TMUI may also change earlier, and the network controls

this pace of change. The P-TMSI is allocated by the SGSN and it is valid over





the SGSN area. The P-TMSI is changed when the UE performs routing area

update.

IMEI (International Mobile Equipment Identity) is a number uniquely

identifying the user equipment's hardware. There is a separate register called

EIR (Equipment Identity Register) handling these identities. The network mayor may not ask the UE to identify itself with IMEI number either in context of

every transaction or occasionally in the cases defined by the network operator.

Fiber Fiber

AXC

ATM Access

Internet


Control Plane

Gateway Plane

PSTN2G

SGSN

3G

SGSN GGSNRNC

BSC

Node B

Node B

Node B

HLR

&AuC

EIR

3G

MSC

SCEP

SMSC

BTS

RANThe IMEI is used to tracklegal mobile equipment

(optional)

Figure 6. Ensuring terminal equipment security

All the IMEI numbers are handled in three categories within the core network.These categories are called lists, that is, White List, Grey List and Black List.White listed IMEI numbers are normal identities, which do not have any

troubles. The grey listed IMEI numbers are under observation, and every time a

UE having grey listed IMEI used, the network produces an observation report

about the transaction. If the accessing UE is on the black list, the network rejects the transaction, except in case of an emergency call.

There are several other addresses that are used. One is the MSRN (Mobile

Subscriber Roaming Number), which is used for call routing purposes. Theformat of the MSRN is the same than MSISDN, that is, it consists of three parts

(CC, NDC, and SN) and it follows E.164 numbering specification. The MSRN

is used during a call set-up between the network and a subscriber on another

MSC. The implementation of this explanation is beyond the scope of thismodule.



Network traffic and radio connection

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4 Network traffic and radio connection

In the previous chapter we looked at the type and location of information that is

stored about a subscriber within the network. In this chapter, the focus is on

how the user traffic (also known as the user plane) is visualised in the network and how the connection is managed in the air interface.

The first concept to clarify is the bearer. The figure below illustrates that a

bearer is like a tunnel that goes through the different network elements and is

carried on the different network interfaces.

UE Node B RNC

Uu Iub/Iur Iu

Core Network

Figure 7. Thinking in terms of a network bearer

The application (such as video) in the mobile has a point-to-point connection to

a remote application (such as video on another terminal). From the physical network's point of view, the UMTS radio access network (UTRAN) must

ensure that the bearer is maintained over the air interface and is correctly routed

to the core network.

The core network ensures that the bearer is either connected into the service platform, Internet, external network or, in the case of a voice/video call, onto

the PSTN (see the figure below). In the case of the PSTN, the information in the

bearer pipe must be converted to a form that is understood by the outside world.

UE Node B RNC

Uu Iub/Iur Iu

Core Network

Data/Wireless Protocols are transparent to surrounding network

MGW

The MGW for 3G-MSC performs speech transcoding

Figure 8. Data and speech through the bearer





4.1 Characteristic of a network bearer

If you think in terms of GSM, you probably consider the traffic channel to be

the same as a bearer in the air interface. A traffic channel does share some same

characteristics; for example, it can carry different types information (such asspeech and circuit switched data). The fundamental difference between GSM

and UMTS is that in UMTS, the bearer is flexible. The type of the bearer

reserved and the way it is routed through the network depends on thesubscriber's service need. To better understand this concept, let us take two

examples.

Example 1: Voice traffic

Voice requires a data speed of, for instance, 12.2 kb/s. (The bit rate depends of

course on which speech coding method we use.) If we add error correction

information (to ensure quality), the total amount of data needed in the air interface is approximately 24 kb/s. For the interfaces within the radio access

network (Iub, Iur) and towards the circuit switched core network (Iu), the bitrate required is around 16-19 kb/s, including overhead. Therefore, we need aconnection from the mobile to the Media Gateway that can support these bit

rates. Also, we have to take the delay factor into account. As subscribers we

are not tolerant of delays in our speech or video conversations.

• Conversational class

• Voice and video

• Streaming class

• Streaming video

• Interactive class• Web browsing

• Background class• Mail downloading

Figure 9. The different air interface classifications

Example 2: Internet connection

The first characteristic to remember is that Internet traffic is often bursty andasymmetric (there is usually more to download than to upload). Also, the delay

factor is not as significant as for conversation, which means that we can toleratemore variable bit rates. On the other hand, the data may be very sensitive to




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errors, compared with, for instance, voice transmission. It means that we may

need to apply more ambitious error correction.

As a conclusion from these two cases, the network will allocate the bearer based

upon the request of the subscriber's need. To be more precise, it is the radio

network controller (RNC) that makes the decision about the bearer allocation.

4.1.1 Types and configuration of bearers

As with all mobile systems, the largest bottlenecks in allocating resources to amobile subscriber is in the air interface. This is the reason why the RNC is

responsible for the bearer allocation. The air interface is limited in terms of the

maximum amount of subscribers, the maximum data rates, the coverage area,

and quality. In UMTS, all of these factors are linked together. If you introducemore people to a cell, then the size and bit rate reduces.

The UMTS specification defines four classifications of bearers. These were

summarised in the previous figure. The below figure illustrates typical servicesand their required data rates. Of course the transmission and core networks must

be capable to support the different needs; one of the important tasks for the

network planners is to dimension the accurate capacity in the network beyondthe air interface.

VoiceVoice Messages

MessagingTransactionalInfoservices

WWW browsingIntranet accessDownloading

Audio Streaming

0 8 16 32 48 64 80 96 112 128 144 160 kbit/s

Video StreamingVideotelephony

UE Node B RNC

Uu Iub/Iur Iu

Core Network

Figure 10. Typical data speeds needed for common 3G services





Let us assume that a video call is to be made through the network. A dedicated

traffic channel for the air interface must then be requested. The UE must also

inform the network about the needed classification and data speed. It is then theRNC's responsibility to allocate an air interface channel and to establish the

connections through to the core network.

4.2 Bearer transmission in the network

On its journey throughout the network, the bearer 'sits' in a physical channel. On

the connection between the BTS and the RNC and towards the MSC/SGSN, a

frame-structure protocol (typically ATM) is used.

3G

SGSN

GGSN

RNC

HLR

AuCEIR 3G

MSC

Channels from the air interface alongthe Iub interface

Physical channels in the

air interface are separated by codes

ATM link to the MGW at the MSC

Packets tunnelled to theSGSN

Packets tunnelled from theSGSN to the GGSN

Packet Data

Dedicated circuit

PSTN

Internet

Figure 11. Transmission through the network

The air interface also has physical channels, which are used to carry signalling

messages and data between the terminal and the network.

The above figure shows that between elements we have pipes. The network

elements ensure that the right information is moved from one pipe to another. In

the circuit switched core network (CS-CN) domain, there is always a dedicated

circuit for the connection and it is only released at the end of the call.

In the packet switched core network (PS-CN), we use tunnelling to make avirtual connection between IP network elements. Although tunnelling ensures a

semi-dedicated channel in an IP network, it is still not the same as having a

dedicated circuit in the network. Basically, the tunnel enables a virtual circuit




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between the RNC via the Serving GPRS Support Node (SGSN), and towards

the Gateway GPRS Support Node (GGSN).

Student Exercise:

Why do you think the RNC makes the decision on the type of bearer that isallocated to a subscriber?

4.3 Bearers and the different levels of QoS

Network services are considered end-to-end, this means from terminal

equipment (TE) to another TE. An end-to-end service may have a certain QoS,

which is provided for the user of a network service. It is the user that decides

whether he/she is satisfied with the provided QoS or not.

To realise a certain network QoS, a bearer service with clearly defined

characteristics and functionality is to be set up from the source to the destinationof a service.

A bearer service includes all aspects to enable the provision of a contracted

QoS. These aspects are among others the control signalling, user planetransport, and QoS management functionality. UMTS bearer service layered

architecture is depicted in the below figure (taken from the specifications). Each bearer service on a specific layer offers its individual services and uses services

provided by the layers below.





TE MT UTRAN CN IuEDGENODE

CNGateway

TE

End-to-End Service

TE/MT LocalBearer Service

UMTS Bearer Service External Bearer Service

UMTS Bearer Service

Radio Access

Bearer Service

CN Bearer Service

BackboneBearer Service

Iu Bearer Service

Radio Bearer Service

UTRAFDD/TDD

Service

PhysicalBearer Service

1

2

34

Figure 12. Layered architecture of the bearer services in UMTS

4.3.1 The end-to-end service and UMTS bearer service

1

On its way from the terminal equipment (TE) to another, the traffic has to pass

different bearer services of the network(s). A TE is connected to the UMTSnetwork by use of a mobile terminal (MT). The end-to-end service on the

application level uses the bearer services of the underlying network(s). As the

end-to-end service is conveyed over several networks (not only UMTS), it is notsubject for further elaboration in the present document.

The end-to-end-service used by the TE will be realised using a TE/MT local

bearer service, a UMTS bearer service, and an external bearer service.

TE/MT local bearer service is not further elaborated here as this bearer service is

outside the scope of the UMTS network.

It is the various services offered by the UMTS bearer service that the UMTS

operator offers. In other words, it provides the UMTS QoS.

The external bearer service is not further elaborated here as this bearer may beusing several network services, such as another UMTS bearer service.




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4.3.2 The radio access bearer service and the core network bearer service

2

The UMTS bearer service consists of two parts: the radio access bearer serviceand the core network bearer service. Both services reflects the optimised way to

realise the UMTS bearer service over the respective cellular network topologytaking into account such aspects as, for example, mobility and mobile subscriber profiles.

The radio access bearer service provides confidential transport of signalling and

user data between MT and CN Iu Edge Node with the QoS adequate to the

negotiated UMTS bearer service or with the default QoS for signalling. This

service is based on the characteristics of the radio interface and is maintained for a moving MT.

4.3.3 The radio bearer service and the Iu-bearer service

3

The radio access bearer service is realised by a radio bearer service and an

Iu-bearer service.

The role of the radio bearer service is to cover all the aspects of the radio

interface transport. This bearer service uses the UTRA FDD/TDD. UMTSTerrestrial Radio Access/Frequency Division Duplex will be forming the

physical layer in the first phase of UMTS. Later also Time Division Duplex isexpected to be implemented.

To support unequal error protection, UTRAN and MT shall have the ability tosegment and reassemble the user flows into the different subflows requested by

the radio access bearer service. The segmentation/reassemble is given by the

SDU payload format signalled at radio access bearer establishment. The radio

bearer service handles the part of the user flow belonging to one subflow,according to the reliability requirements for that subflow.

The Iu-bearer service together with the physical bearer service provides the

transport between UTRAN and CN. Iu-bearer services for packet traffic shall provide different bearer services for variety of QoS.

4.3.4 The backbone network bearer service

4

The core network bearer service uses a generic backbone network service. The backbone network service covers the Layer 1/Layer 2 functionality and is

selected according to operator's choice in order to fulfil the QoS requirements of

the core network bearer service. The backbone network service is not specific toUMTS but may reuse an existing standard.





4.4 Managing the bearer through the network

The UMTS network is responsible to establish a flexible bearer for user data

transport between the Mobile Terminal (MT) and the external networks. In the

bearer set-up phase, the QoS parameters must be known, so that the individualnetwork elements within the UMTS network “know”, how to set-up the bearer.

TEexternalnetwork3G-(G)MSC/GGSNMT UTRAN 3G-MSC/SGSN

UMTS BSManager

UMTS BSManager

UMTS BSManager

RABManager

CN BSMana-

ger

IuBSMana-

ger

Ext. BSMana-

ger

CN BSMana-

ger

Iu BSMana-

ger

RadioBS

Mana-ger

RadioBS

Mana-ger

LocalBS

Mana-ger

BB NSMana-

ger

Iu NSMana-

ger

BB NSMana-

ger

Iu NSMana-

ger

UTRAph. BSMana-

ger

UTRAph. BSMana-

ger

SubscrControl

Adm/ Cap.

Control

Adm/ Cap.

Control

Trans-lation

Adm/ Cap.

Control

Adm/ Cap.

Control

Trans-lation

Figure 13. QoS management in the control plane

As can be seen, a hierarchical approach is used for bearer establishment: In

order to establish a bearer in accordance to the QoS requirements of the user’s

circuit switched application, a peer-to-peer bearer service (BS) signalling between the MT, MSC, and (G)MSC takes place. In case of a packet orientated

service request, bearer related signalling and control information must be

exchanged between the MT, SGSN, and GGSN. (In the next lines, we refer to

MT, SGSN, and GGSN. Please note, that there is no significant difference for the circuit switched case.) The peer-to-peer signalling is necessary, so that the

affected network elements can determine the required QoS parameters for the

end-to-end bearer. If one network element is not capable to establish the bearer,

a re-negotiation can be initiated to find an alternative bearer – if the user’sapplication permits it – or the UMTS PLMN is not capable to offer the

requested service.

If the UMTS bearer service (BS) manager use the GPRS Tunnelling Protocol

(GTP) for QoS negotiation between each other. If they have agreed on the QoS parameters for the bearer, the UMTS BS manager of the CN inform the CN BS

manager about the QoS parameters for the bearer between SGSN and GGSN. It




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lies then in the responsibility of the CN BS manager to negotiate on how to

make the bearer available, which route to take between the SGSN and GGSN. If

they have agreed on the QoS parameter on their level, they inform then theBackbone network service (BB NS) manager about the set QoS parameter.

Within the backbone, IP over ATM may be applied, IP over Frame Relay, etc.

Depending on the underlying transmission technology and signalling protocolsused, the network elements must conduct signalling to step by step establish the

bearer between SGSN and GGSN.

A bearer also must be established between the MT and the SGSN. The RNC is

responsible for the resource management within UTRAN. The RNC ismanaging so-called Radio Access Bearer (RAB). A RAB stand for one bearer/

connection between a MT and a core network edge element (SGSN, MSC).

The RNC must establish the bearer on Uu, Iub, Iu, and – if required – on Iur.

After determining the QoS parameter internally used from the QoS parametersset by the bearer service manager in the SGSN, it informs its Iu bearer service

manager to negotiate and establish the bearer between itself and the SGSN. The

RAB manager also informs the Radio BS manager about the required QoS

parameter; the Radio BS manager then determines the radio QoS parameters.The physical parameters for the transmission via the radio interface are then

determined in the underlying UTRA physical BS manager, parameters such asspreading codes, spreading factor, type of convolutionary coding.

The whole process is conducted to establish on every physical link within theUMTS operator’s network a bearer in accordance to the QoS required for the

subscriber’s application.

Bearers for signalling can be negotiated, too. But often, they are made availableduring operation and maintenance.

4.5 Managing the bearer over UTRAN

In UMTS there may be a number of connections between the core network and

the mobile. As an example, a subscriber may have a video, voice, and Internet

connection bearer open. This means that the subscriber will be using multiple

bearers to support each service. Each of this connection is known as a RAB,

(radio access bearer).

The RABs for an individual subscriber are grouped together into a RRC (radio

resource control). RRC is a stack structure, in which the RABs are located.Therefore, if we need to move the RRC (in the case of a handover from one

BTS to another), then we need to move the whole RRC.





CNRAN

RANAP Connection

Radio Access Bearer

RANAP Connection

R a d i o

A c c e s

s B e a r e r

RRC Connection

MS

PacketNetwork

Circuit SwitchedNetwork

Packet Data Se rvice

Speech Service

Video Service

Radio Access Bearer

Figure 14. The relationship between the RAB and RRC in the UTRAN

As the above figure illustrates, the different RABs are received by the RNC andcombined together to form a single RRC connection. The 3G Specifications

make provision for procedures that allow for the RAB to be added, modified,

and removed. This would happen if the subscriber needed an additional service(for example, downloading email).

To control the connection between the network and the mobile, a signalling protocol called radio resource control is used. By use of the protocol, the

network can carry messages that are required to set up, modify and release radio

resource connection.

Student Exercise:Can you think of an example where it may be necessary to modify the radioresource connection?

4.5.1 Example: Simplified bearer establishment for a call

The UMTS bearer service manager in the SGSN requests a bearer set-up

between the MT and itself. It send a RAB Assignment request to the radioresource control unit RNC. The bearer control messages are exchanged between

SGSN and RNC with RANAP messages. RANAP stands for Radio Access

Network Application Protocol.

Following the UMTS specific RAB Assignment Request message, the Iu-PS

bearer between SGSN and RNC can be set up in accordance to the required

quality of service parameter. Between SGSN an RNC the Iu-PS bearer iscurrently an AAL5 virtual channel.

Then, an Iub bearer between Node B and RNC can be established. This bearer

will be later on used for user data transport and is an AAL2 virtual channel.




CTXX5323enIssue 4.0


A signalling connection already exists between the UE and the RNC. This

connection is used to send the Radio Set-up Bearer message to the UE. The UE

is informed about the physical layer characteristics, MAC layer characteristics(e.g. puncturing, data rate), RNC modus (e.g. acknowledged/unacknowledged

mode), etc.

The Radio Link Reconfiguration message informs the Node B among others

about the physical and MAC layer characteristics of the Uu interfacetransmission.

The Node B confirms this message by returning a Radio Link Configuration

Complete message.

The UE confirms the Radio Bearer Set-up message with the Radio Bearer Set-

up Complete message.

Now the bearer between the UE and the RNC exists. The RNC returns the RAB

Assignment Complete message to the SGSN, with which the UMTS bearer between UE and SGSN is established.

Please note, that this example is highly simplified. The bearer establishmentwithin UTRAN is very complex and allows a wide range of different options.

Uu Iub Iu

RAB Assignment Request

RAB Assignment Complete

SGSN RNC

Node B

Iub bearer set-up(AAL 5 virtual channel)

Iub bearer set-up(AAL 2 virtual channel)

Radio Bearer Set-up Complete

Radio Bearer Set-up

Radio Link Reconfiguration

Radio Link Reconfiguration Complete

Figure 15. RAB establishment (simplified)





4.5.2 Managing the bearer when the subscriber is moving

As a subscriber moves through the network, the radio resource connection must

follow. The below figure illustrates a network with the path of a bearer beingconnected from end to end (UE to MSC and UE to SGSN). As the mobile

moves, the signal it receives from the serving BTS will change (possiblydecrease as in the below example) and the signal received from a neighbouring

BTS will increase.

It makes sense that the mobile should receive the signal from the BTS with the best signal quality and strength. One characteristic of a CDMA network is that

there could be simultaneous connections between different base stations and themobile. It is the same information, just being transmitted and received by

different sources.

3GSGSN

RNC

3GMSC

RNC

RNC

Mobile has a singleRRC

Figure 16. The initial situation of an end-to-end bearer connection

The base stations that have simultaneous radio resource connections to the sameterminal are known to belong to an active set. As the mobile moves, the base

stations are constantly being added and removed from the active set.

One common question that students ask is why use the extra resources, surely it

would be better to just use the one connection? In principle this is true, but by

transmitting the signal from different sources there are advantages in gain thatcan be achieved. In theory, we are able to decrease the interference and power

in the radio network, therefore increasing capacity.



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5 Mobility management

Note for self-studying

You will notice that this chapter quite much repeats the concepts from GSM andGPRS. But, please be aware that some new concepts are introduced in thischapter too.

As the user terminals are not fixed to certain positions, the network must keep

track on where the mobile is located. The system must at least be capable of

knowing the geographical area in which the subscriber is located. As in GSMnetworks, UMTS has a cellular architecture that allows the network to identify

the subscriber. As discussed in the previous section, the network maintainsinformation about the location of a subscriber, and the procedures are specified

to allow a constant updating of these databases as the subscriber moves around

the network, and also from one network to another.

3G

SGSN GGSN

RNC

3G

MSC

3G

SGSN

GGSN

RNC

3GMSC

3G

MSC

Core Network

Foreign Network

Radio AccessHLR

AuC

EIR Supplementary Services

Location Updating

Locating the Subscriber

Service Information

Figure 17. The role of the HLR as the centralised database

The Home Location Register (HLR) is the central database that storesinformation on the subscriber, such as the IMSI and MSISDN. The HLR also

stores information on which serving MSC and SGSN the subscriber can be

found.





Also in the HLR we store information on the subscriber's service profile. In

other words, we have a record of the different services (teleservices,

supplementary and packet services) that the subscriber can/cannot use.

Therefore, if the network needs to locate the subscriber in the case of a mobile

terminated call, or if the network needs to check if the subscriber is valid, thenall requests are sent to the HLR.

5.1 Cellular architecture

The smallest entity within the radio network is known as a cell, which is beingserved by a base station. The operating size of the cell (CI) can change

geographically depending on the parameters used. The cells are groupedtogether geographically into location areas (LA), routing areas (RA), and

UTRAN registration areas (URA).

LA = Location Area (MSC)RA = Routing Area (SGSN)URA = UTRAN Registration Area (RNC)Cell = Cell (RNC)

CELL URA CELL

URA

ROUTING

AREA (RA)

CELL

Position = PositioningService (UTRAN)

CELL

CELL

CELL

CELL

CELL

CELL

CELL

CELL

ROUTING

AREA (RA)

Location Area (LA)

Figure 18. UMTS cellular architecture

The above figure illustrates the structure of the network. This may look

confusing and overly complex. The reasoning behind the structure is to make

UMTS backward compatible with GSM and GPRS. The location areas are usedin the circuit switched domain as the routing areas are used in the packet

switched domain. A single cell can belong to both a LA and RA and thisinformation is used by the core network for routing information to the radio

access network (RAN).



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In GSM, two separate connections are made for circuit and packet switched

data. In UMTS, there is a single connection that can carry multiple bearers.

Therefore, to reduce the excessive amount of signalling that may occur, anUTRAN registration area (URA) is introduced to more intelligently monitor the

location of a subscriber in the RAN.

Student Review:One MSC can have many LAs, but an LA cannot cross MSCs. A RA can crossBSCs, but not MSCs. A cell cannot belong to different LA or RA; they must beunique. A cell can belong to more than one URA.

5.1.1 Network location areas

The location area (LA) is used in the circuit switched domain. The LA consistsof cells; minimum is one cell and the maximum is all the cells under one VLR.

Thus, the maximum size of one LA could be the same than VLR area.

In the location update procedure the location of the UE is updated in the VLR

with LA accuracy. This information is needed in case of mobile terminated call;

the VLR pages the desired UE from the location area it has performed the latestlocation update.

It should be noted that in other respects than the VLR, the LA does not have any

other hardware bindings. For instance, one RNC may have several location

areas or one location area may cover several RNCs.

Code LA MNC MCC LAI ++=

Where the MCC and MNC ( see previous section) is the same as in IMSI

number. The LA code is just a number identifying LA. LAI is unique number throughout the world.

To globally separate cells from each other, the identity must be expanded and in

this case it is called cell global identity (CGI):

The CGI value covers the country of the network (MCC), the network within a

country (MNC), location area in the network, and finally the cell number within

the network.

CI LACode MNC MCC CGI +++=





5.1.2 Network routing areas

The packet switched domain has its own location definitions based on a routing

area (RA). A routing area is definition-wise very similar than the LA, that is, itis the area where the UE may move without performing the routing area update.

On the other hand, the RA is kind of a 'subset' of LA: one LA may have severalRAs within it, but not vice versa. In addition, one RA cannot belong to two

LAs. The reason why these two definitions co-exist is the possibility to have a

UE supporting either circuit or packet traffic, but not both. At the core network side the VLR and the SGSN can have a common optional interface, Gs, through

which these nodes may change location information. For example, if the UE performs a location update, the VLR may inform SGSN through the Gs

interface that the UE should also perform routing area update in order to

guarantee packet traffic.

5.1.3 UTRAN registration areas

As mentioned before, the reasoning for having the CIs, LAs and RAs is toensure compatibility to GSM and GPRS networks. In 3G/UMTS, an additional

grouping of cells is introduced, the UTRAN registration area (URA).

As the RNC has greater mobility management functions, and it controls

handovers between RNCs, it must identify which cells belong to which RNC.As a subscriber moves into the geographical range of the RNCs serving area,

the subscriber is allocated into the serving URA. Only when the subscriber

moves from the control or supervision of one RNC to another, the information

has to be updated.

When transmitting the paging signal, the RNC can limit the paging to the URAarea, thus reducing the amount of signalling in the network. SGSN uses the

RNC address when routing packets for a designated user. With URA, it is also

possible to create more accurate demographic areas within the network. It

means that we can define an URA more flexibly than LA/RA with respect to

where people are located (and what patterns of movement they have).



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3G

SGSN

HLR

&AuC

RNC RNC RNC

URA URA URAURAURAURA

3G

MSC

If the LA and/or RAchanges, RNC will ensurethatupdated informationis sentto the core network

As the subscriber moves from oneURA to another, a

URA update is performed

Figure 19. RNC and URA architecture in the network

Notes on GSM evolution of UMTS

The MSC and the VLR still use the LA-based method for mobility management

functions for circuit switched operations such as CS call set-up. As for GPRS,the 3G-SGSN still works on the basis of RAs. Therefore, the only new entities

are the URA and UTRAN positioning services. Unlike in GSM, the RNC can

handle inter-RNC handovers via the Iur interface. In GSM, the MSC is always

responsible for inter-BSC handovers. As UMTS networks are designed to work with different types of core network, the only way that the network can identify

which cells belong to which RNC is based upon the use of URA.

5.1.4 Location based information services

Another characteristic of a 3G/UMTS network is that it is possible to determine

a more accurate position of the subscriber by using the UTRAN positioning

service. Unlike the URA, LA and RA, which are used for controlling mobility

management (that is, subscriber location for call set-up), the future for UTRAN

positioning service is for the provisioning of services that are based upon thesubscriber's exact location. Examples of such services could be emergency

calls, viewing maps, and locating the nearest doctor.

The aim is to be capable of locating the subscriber within a 50 - 70 m range.There are different techniques that can be used, such as GPS (GlobalPositioning System). However, this may have limitations due to line of sight,

indoor coverage, and even political reasons. Other techniques exist, which usethe triangulation between base stations to measure the delay in signals.





Figure 20. Service possibilities

5.2 Mobility procedure - Location updating

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

subscriber's movements. In practice, there are three basic types of locationupdate procedures:

• Location registration (power on / cell attach)

• Movement between area

• Periodic update

These are explained in more detail in the forthcoming pages. In a GSM network

the BSC took no responsibility for mobility management; instead the mobile

would contact the core networks directly to inform about a change in location.

In UMTS, the situation is different as the RNC not only keeps information on

which subscribers are in which URA, but is also responsible for the location

updating to the core network.

When location updating is

active (option that thesubscriber can enable with the

exception of law enforcement),

then the mobile is constantlyinforming the network of itscurrent location.

This information can then beaccesses by different types of

service applications. For

example, if an emergency call ismade, the mobile's location can

be given to the police /

ambulance services.



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3G

SGSN

3GMSC

RNC

RNC

HLR

When switching on/off

IMSI Attach/Detach

When moving from area to area -

On a cell/LA/RA/URA level

If the VLR is not informed

about the user then the

data is removed

VLR

The RNC keeps track of the

mobiles within the URAs. It also

informs the core network when

the subscriber changes LA and/or

RA

The SGSN keeps a track

of the mobiles

Periodically over time if mobile

does perform an update

Figure 21. Location update generic procedures and information in thenetwork

As the RNC receives a location updating message, it takes responsibility for

informing the core network. The RNC will update its own information about thesubscriber within the URA and inform the SGSN and VLR, respectively, if the

routing area / location area also change.

Why bother to keep updating a location? The reason is that the network's VLR

and SGSN databases are only temporary. Depending on the parameters that the

operator use, the information is only stored for a certain time. If there has been

no updating, it is assumed that the information is old. Therefore, to stop havinga huge amount of useless data in the network, the information is removed.

5.2.1 Location area based procedures

Location registration (IMSI attach) takes place when a user equipment (UE) is

turned on and it informs the VLR that it is now back in service and is able to

receive calls. Related to this process, the network sends the UE two numbers

that are stored in the USIM (Subscriber Identity Module) card of the UE.

These two numbers are the current Location Area Identity (LAI) and the

Temporary Mobile Subscriber Identity (TMSI). The network, via the controlchannels of the air interface, sends the LAI. The TMSI is used for security





purposes, so that the IMSI of a subscriber does not have to be transmitted over the

air interface. The TMSI is a temporary identity, which regularly changes.

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 a location update process by accessing the MSC/VLR that sent the location data.

A channel request message is sent that contains the subscriber identity (that is,

IMSI/TMSI) and the LAI stored in the USIM card. When the target MSC/VLR

receives the request, it reads the old LAI, which identifies the MSC/VLR that

has served the mobile up to this point. A signalling connection is established between the two MSC/VLRs and the subscriber's IMSI is transferred from the

old MSC to the new MSC. Using this IMSI, the new MSC requests thesubscriber 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 anylocation update request from the mobile in a specified time. Such a situation is

created when a mobile is switched on but no traffic is carried, in which case themobile is only reading and measuring the information sent by the network. If the subscriber is moving within a single location area, there is no need to send a

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 circuit and packet

switched domains. In case of packet switched domain the MSC/VLRs arereplaced with SGSNs.

When the VLR/SGSN is changed, the new VLR/SGSN sends information aboutthis change to the HLR. The HLR responds by sending the subscriber

information to the VLR/SGSN. If the subscriber had earlier locationinformation present in the HLR, the HLR cancels the previous location.

IMSI attach/detach (circuit switched)

In the circuit switched domain the UE may have two states, attached anddetached. In the attached state the UE is able to handle transactions and it is

active in the network. The UE continuously analyses its radio environment,

LAC and cell identities being 'visible'. When the UE is switched off, detached,it stores the latest radio environment information into its memory and informs

the network that it is now being switched off. The VLR stores this state change

and the UE is not tried to be reached in case of mobile terminated transaction.

When the UE is switched on again, it first checks whether the radioenvironment matches to the one it has in its memory. If it matches, the UE just

informs the VLR that it is now attached again and able to handle transactions. If not, the UE performs a location area update.



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5.2.2 Routing area update (packet switched)

As a procedure, the routing area update is very similar to the location update

and it is performed for the same purpose. Periodic routing area update is usedfor checking that a UE that has not performed any routing area updates for some

period of time is still reachable.

The UE performs a cell update (also cell reselection) when it changes cell

within a routing area in Ready mode. This could be compared to a handover in

UMTS/GSM for circuit switched connections. Cell update and routing areaupdates halt possible reception or sending of data. The possibility of buffering

data in the Serving GPRS Support Node (SGSN) can be in such cases.

SGSN-1

RNC-1

RA-1

Old cell New cell

Figure 22. The routing area update

When the UE changes cells between the different routing areas, it performs arouting area update. There are two types of routing area updates: the intra-

SGSN routing area update and the inter-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.

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 newSGSN.





Cell attach/detach

In the core network packet domain the MM-state changes during the packet

switched connection, and it can be said that the MM-state mostly depends on

the activity of the connection. That is, when there are packets to send or receive,

the MM-state of the connection is MM-connected. When there is nothing totransfer, the MM-state of the connection is MM-idle. The MM-detached state

has the same meaning in both of the CN domains.

Detach DetachPacket Attach

Packet Attach

Packet Received Packet Received

Radio Link Release Iu Connection Release

MM States in UE: MM States in SGSN:

Figure 23. Mobility management state diagram in packet domain

In order to utilise the 3G network resources (such as radio bandwidth) as

effectively as possible, the MM-state management is as such not enough for the

packet switched (PS) traffic. In PS traffic, the traffic delivered can be presented

as occasional packet bursts. Between these bursts the connection is not used.This leads to the situation where it is reasonable to 'cut' the connection through

the network in order to make the network resources available for other activeconnections. The way to suppress the packet connection, but still remain the

necessary information in both ends of the connection, is called cell attach /detach.

Notes on GPRS evolution to UMTS

If you are familiar with GPRS, the above figure about the different states may

seem confusing. In UMTS, the RNC has different RRC states depending on thetraffic situation. Therefore, the two figures are purely from the point of view of

the UE and the SGSN. From the point of view of SGSN, it is in MM-connected

state when there is a packet attach/received message. Signalling may be opened

to the RNC, but the MM-connected state is only used when there is actualtraffic.







5.3 Mobility management - Paging the subscriber

From the HLR, the network is able to determine at the very least in which

location area/routing area the subscriber is located. The network (e.g. MSC) will

contact the MSC/SGSN serving that area and request contact to the mobile. TheVLR/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.

BTS BTS

Paging

BTS

Mobile responds

to paging

Routing Area

Paging

Paging

Figure 24. Paging in the network

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 signalling channel to determinewhat the network is asking (such as set up a call, or receive the SMS).

In GSM, the VLR/SGSN would ask 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 (UTRAN registration area), the RNC can intelligently page for

the subscriber in the URA, therefore reducing the signalling in the network.



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5.4 Mobility management - Roaming in another network

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 homenetwork (remember that 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 theinformation on the subscriber is stored in the HLR.

Every VLR in the world has a unique address. As a subscriber moves from onenetwork to another, the location updating proceeds as normal. The HLR is

always informed of the unique VLR, in which the subscriber was last seen.

3GSGSN

HLR

3G

SGSN

3G

MSC

RNC

HLR

3G

MSC

RNC

BSC

BSC

Home Network

Visitor Network

International Network

3. Network Confirm access

1. User requests access (IMSI Attach)

2. If operators have a roaming agreement, information iscopied to visitor network (from VLR/HLR)

Figure 25. Roaming in another network

Let us take one example: A subscriber is roaming in another network. When the

network needs to contact him/her (for example to receive a video call), thesubscriber's location is always checked from the HLR . The HLR will then

contact the serving MSC to check that the subscriber is still located in the VLR (HLR request). This information is returned to the MSC and a call is routed to

the foreign MSC. So, the paging process can begin.

Even if the calling subscriber is located in the foreign network, the call still hasto be initially placed back to the home MSC.





5.5 Mobility management procedures

There are several different mobility management procedures, some of which

have been mentioned in this module. The following is a short list of UMTS

specified procedures. In the following chapter we will look at some of the procedures in their context of the session management layer:

• Location registration (CS and PS)

• Location update (CS)

• IMSI attach/detach (CS)

• Routing area update (PS)

• Cell attach/detach (PS)

• Location info retrieval (CS and PS)

• Paging (CS)

• Paging (PS)• Authentication procedure (CS/PS)

• Ciphering procedure (CS/PS)

• UE identity checking (CS/PS)

• UE hardware (IMEI) checking (CS/PS)

This chapter has given the initial first look at how mobility management isachieved in a 3G network. If you require more information and a detailed look

at different scenarios, please check the specifications.

The specification contains the so-called SDL figures. The figures illustrate how

signalling messages are passed from one element to another, in the sequence of time and condition (that is, what happens in case of a failure). Also, thespecifications give detailed information on the content of such messages.







6.1 Initially accessing the network

When the mobile is switched on, it starts the network selection procedure.

Without going into too much detail about the air interface, the mobile is aware

of the possible frequencies that are available in UMTS and all the possiblecodes (512) that are used by the cells.

The mobile will first check the last frequency and code (used to identify thecell), to check if it is still valid. If the cell cannot be found, the mobile will start

applying each code to each possible frequency in an attempt to detect a signal

that will indicate that there is a cell present.

Once the scanning process is over, the mobile will select as first choice its home

network (information of this is on the SIM). If the home network is not present,

then it can choose a preferred network (usually set by the home network operator). If that is not available, the mobile will randomly select another

network providing the signal level is adequate.

The procedure of network selection can be made manually, but it is usually

performed automatically.

3G

SGSN

3G

MSC

RNC

HLR

Previous cell is tried and if not therethe terminal scans frequencies and

applies codes. If possible homenetwork is selected.

Networks transmitinformation about

themselves (MCC+MNC)

Operator A (frequency 1)

Operator B (frequency 2)

Operator C (frequency 3)

Figure 27. Initial network access (switch on)

On the selection of the network, the mobile will request a location update (IMSI

attach) of its position. The RNC will then request for the location update, and if it is the home network and it is the first time, the update is made. Theinformation on the subscriber will be copied to the serving VLR for the MSC

area and the current information on the subscriber will be updated to the HLR.

The subscriber will also be registered into the current URA.



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6.1.1 IMSI attach for an existing subscriber

If the subscriber is already registered in the network and is still registered in the

same VLR, the information is updated. Also the HLR is informed of the newinformation.

3G

SGSN

RNC

HLR

AC

EIR 3G

MSC

3GMSC

RNC also keeps information

about subscribers within

the URA.

1. Request Location Update (IMSI Attach)

2. New VLR requests and receives authentication

information and IMSI from previous VLR

4. New MSC/VLR updates HLR with new location. HLR

sends acknowledgement and subscriber information.

3. Authentication request and response

6. TMSI/TMUI allocation/acknowledgement

and location update acknowledgement

5. HLR cancels the old location,

old VLR sends acknowledgement

1,

2,

3,

4,

6,

5,

Figure 28. Moving the subscriber's information between VLRs

Let us now assume that the mobile has moved between to VLR areas while being switched off. The figure above shows such a case. When the user

switches on the mobile again, a location update request will be transmitted to

the new VLR (1). Then the authentication and IMSI information is copied

between the old and the new VLR (2). (Similar steps would take place betweenthe UE and the SGSN in case of a routing area update.) After a successful

authentication (3), the HLR is updated with the new location information, after which the HLR sends the subscriber information to the new VLR (4), and

cancels the old VLR (5). Finally, an acknowledge message is sent to the mobile,

together with the TMSI/TMUI number (6). The packet core domain is also

updated with the new location information.

Moreover, the RNC is constantly keeping track of all the connected subscribers'

current URAs.





6.1.2 IMSI attach when roaming

The procedure for updating the VLR/SGSN when the subscriber is in a visiting

network is exactly the same as described above. As two operators have adedicated signalling link, then information is copied into the visiting network.

The HLR is updated with information on the unique VLR/SGSN address wherethe subscriber is located.

6.1.3 Requesting for a dedicated bearer

When the mobile does not have a RRC (radio resource connection) to thenetwork, it is known to be in idle mode. If we wish to have a service, we require

a bearer. Therefore, when the subscriber requests a service (such as video call or

Internet connection), the mobile must make a request to the network.

In the air interface there is a special physical channel that is used to receive

request messages from the mobile, namely the RACH (Random AccessChannel). Depending on the type of channel that the subscriber is interested in,

the network will attempt to secure a bearer.

3G

SGSN

3G

MSC

RNC

HLR 1. Mobile sends a request for a bearer (onRACH and listens for reply)

2. RNC will allocate a bearer for thesubscriber

Mobile in idle mode andcamping on a cell.

Figure 29. Requesting a bearer from the network



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How to gain access without interfering with other mobiles too much?

Technically, when the mobile attempts to gain access to the network, it is not

aware of the power level to use. Hence, it estimates an appropriate level. Then

the mobile sends a short burst of information, which includes a random

sequence to the random access channel. When the network receives the request,it will re-transmit the random part of the initial burst on a separate channel. If

the mobile detects this signal, it assumes that the network has heard it. If not,

the mobile must re-transmit again, but using more power. This processcontinues until either the power level set by the network is reached or the

network responds. Then, the network will transmit on a different channelinformation about the channel that the mobile can use, given that one is

available.

6.1.4 Access security in UMTS

In UMTS requirements for access security are not changed. It is required that

end users of the system are authenticated , i.e. identity of each subscriber isverified; nobody wants to pay for calls that are made by a cheating impostor.

The confidentiality of voice calls is protected in radio access network, as well asthe confidentiality of transmitted user data. This means that the user has control

of choosing the parties with whom he/she wants to communicate. Users also

want to know that the confidentiality protection is really applied: visibility of applied security mechanisms is needed. Privacy of the user's whereabouts is

generally appreciated. Most of the time an average citizen does not care whether

anybody can trace where he/she is. But if a persistent tracking of users would

occur, he/she would be quite irritated. Similarly, exact information aboutlocation of people would be useful, e.g., for burglars. Also, privacy of the user

data is a critical issue when data is transferred through the network. Privacy and

confidentiality are largely synonymous in this presentation. Availability of the UMTS access is clearly important for a subscriber who is paying for it. Network operators consider reliability of the network functionality

to be important: they want control inside network to function effectively. This is

guaranteed by integrity of all radio network signalling; it is checked that allcontrol messages have been created by authorised elements of the network. In

general, integrity checking protects against any manipulation of a message, e.g.,

insertion, deletion or substitution.

The most important ingredient in providing security for network operators andsubscribers is cryptography. That consists of various techniques which all have

roots in the science and art of secret writing . It is sometimes useful to makecommunication deliberately incomprehensive, i.e. to use ciphering (or,

synonymously, encryption). This is the most effective way to protectcommunications against malicious purposes.





3G

SGSN

3G

MSC

RNC

HLR The IMSI and IMEI are checked (optionaland at a changeable interval)

The HLR contains information on thesubscriber, terminal, location and

services. Authentication codes andkeys are stored and generated in the

AuC

The SIM and terminal

contain authenticationkeys

Information is checked from the HLR

Figure 30. Network authentication

UMTS security features

The most important security features in the access security of UMTS are the

following:

• Mutual authentication of the user and the network

• Use of temporary identities

• Radio access network encryption

• Protection of signalling integrity inside UTRAN

Note that publicly available cryptographic algorithms are used for encryption

and integrity protection. Algorithms for mutual authentication are operator-

specific.

Each of these features are described in the following subsections.

6.1.4.1 Mutual Authentication

There are three entities involved in the authentication mechanism of the UMTS

system being:

• Home network

• Serving network (SN)

• Terminal, more specifically USIM (typically in a smart card)







VLR AuC

RAND K SQN

XRES AUTN CK IK

IMSI

RAND, AUTN, XRES, CK, IK

SGSN

Figure 6.31 Authentication Data Request and Authentication DataResponse

In the serving network, one authentication vector is needed for each

authentication instance, i.e. for each run of the authentication procedure. Thismeans the (potentially long distance) signalling between SN and the AuC is not

needed for every authentication event and it can in principle be done

independently of the user actions after the initial registration. Indeed, theVLR/SGSN may fetch new authentication vectors from AuC well 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, i.e. in UMTS IC card (UICC). TheUSIM contains the master key K , and using it with the parameters RAND and

AUTN as inputs, USIM carries out a computation that resembles the generation

of authentication vectors in AuC. This process also contains executions of

several algorithms, as is the case in the corresponding AuC computation. As theresult of the computation USIM is able to verify whether the parameter AUTN

was indeed generated in AuC and, in the positive case, the computed parameter RES is sent back to VLR/SGSN in the user authentication response. Now the

VLR/SGSN is able to compare user response RES with the expected response

XRES which is part of the authentication vector. In the case of match,

authentication ends positively. This part of the process is depicted below.



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VLR

RAND, AUTN

RES

RAND K AUTN

RES SQN CK IK

UE SGSN

VLR/SGSN checkswhether RES = XRES

UE checks whether theSQN is big enough

Figure 6.32 User Authentication Request and User AuthenticationResponse

The keys for radio access network encryption and integrity protection, namely

CK and IK, are created as a by-product in the authentication process. Thesetemporary keys are included in the authentication vector and, thus, are

transferred to the VLR/SGSN. These keys are later transferred further into the

RNC in the radio access network when the encryption and integrity protection

are started. On the other side, the USIM is able to compute CK and IK as wellafter it has obtained RAND (and verified it through AUTN). Temporary keys

are subsequently transferred from USIM to the mobile equipment where the

encryption and integrity protection algorithms are implemented. The SQN is acounter. 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.





3G

SGSN

3G

MSC

RNC

HLR

Authentication codes are used tocreate a signal which is

scrambled with the data. It isalmost impossible to decode thesignal without knowing the keys

Ciphering is performed inthe RNC and the mobile

In UMTS, multiple air-interfacechannels are used in theconnection to the mobile

Figure 33. Ciphering in UMTS/UTRAN

6.2 Managing a real time (circuit switched) bearer

The following section describes how a real time bearer is allocated. The whole process is summarised in a figure at the end of this chapter, with brief

descriptions of the steps. The next two pages discuss the process in more detail.At the first stage of any mobile originated action, a signalling channel needs to

set up between the mobile and the RNC. This channel is used to verify theUSIM, to identify the subscriber, to find out what the subscriber needs, and to

perform the authentication procedures.

The mobile will request a connection (1), the RNC will then instruct a BTS to

reserve a signalling channel (2), and through a common channel (that is, allmobiles in the area can share), inform the mobile which channel to use (3). The

mobile can then use the signalling channel to communicate with the RNC.

The mobile will now inform the RNC what are its service or bearer

requirements. If the mobile just wishes to perform signalling, the already

dedicated channel will be sufficient (that is, location update).

A subscriber wishing to access the Internet will be discussed in the next section.

Should the subscriber wish a QoS assured service (such as voice), the RNC

forwards the call set-up message to the CS-CN (5). Depending how the network is configured, the subscriber's identity is checked before any bearer set-up



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proceeds. These transactions are usually performed not by using the subscribers

IMSI, but by the TMSI. If this is not available, then the IMSI is used.

The network will check if the subscriber is allowed to use the service. Also, it is

possible that the user equipment can be crosschecked to ensure that it is valid.

In the case of a call, the RNC informs the CS-CN (MSC) that a traffic channelis needed.

The MSC will respond to the RNC with information about the bearer it should

provide (6). In return, the RNC will allocate the correct bearer service to mobile

in the radio network (7). Once the connection is made, the RNC informs the

MSC that the connection is complete and the transaction can start (8).

The system knows that a voice/video call is required, so the MSC or Media

Gateway (MGW) understands where the end point should be.

In this case, the subscriber wants to call to another mobile. The procedure is the

same as in GSM, where the HLR enquiry is sent from the MSC/VLR (9). TheHLR will now request the MSRN (Mobile Station Roaming Number) from the

target VLR (10). The HLR will also inform the requesting MSC of the MSRN

of the target subscriber (11). The serving MSC will now contact the target MSCin order to make the final connection to the subscriber (12).

The target MSC will now page the called party. As the VLR only knows thelocation area of the subscriber, then all the cells in the target LA are requested

to send a paging message (13). The mobile will then answer by requesting asignalling channel.

If the call terminates in a UMTS network, a similar bearer assignment procedure will happen as described in steps 1 - 8 above. The set-up procedure

for the target subscriber starts with the allocation of a bearer for a signalling

channel. The subscriber identity is checked and a bearer for the traffic channel

is allocated. Once the radio access bearer is in place, the RNC will respond witha confirmation of the set-up. Now, the two parties can start the conversation.

This process provides a basis for UMTS to add easily into an existing GSM

network. In case of services such as video, the core network will either have adirect connection via an ATM network, or through a server that supports video

streaming.

In the next figure, a simplified UMTS originated - GSM terminated call set-up

case is shown. This case clearly shows that specifications have as much as possible been based on the GSM procedures.





UE BS BSCRNC BTS MSMSC/VLR MSC/VLR HLR

9. HLR Enquiry

10. Send Routing Information (MSRN)

11. Call Setup Continues

13. Paging Response

14. Setup

15. Call Confirmed/Alert16. Conversation

5. Call Setup

7. RAB Setup / Radio Link Modification

6. Bearer Assignment Request (Create) - Bearer Parameters and Binding ID

8. Bearer Assignment Complete

4. RRC Setup Complete

2. Radio Link Setup

3. Radio Resource Connection Setup

1. Radio Resource Connection Request

12. Paging

Figure 34. Simplified UMTS originated – GSM terminated call set-up

Summary of the steps in the figure:

1. A radio resource connection request for a signalling channel is requested.

2. The RNC sets up the radio link between the base station and itself.

3-4. RRC set-up (downlink) and RRC set-up complete (uplink) messages.

5. Call set-up message to the MSC/VLR. Security procedures are also performed (not shown in the figure above).

6. Bearer assignment request. In this step, the bearer parametersare defined; also a binding identification number is allocated. Binding ID is

used to tie together control information with user data for a certain

connection.

7. Radio access bearer set-up and radio link modification. Given the inputsfrom the MSC/VLR concerning the bearer, the RNC allocates an

appropriate radio access bearer (RAB) in the air interface. Also the radio

link between the BTS and the RNC is modified in accordance with the

bearer need.

8. The RNC informs the MSC/VLR that the bearer has been assigned.

9-17. Since the circuit switched core network (CS-CN) is common for UMTS

and GSM, the call set-up procedures within the CN are the same, including

HLR enquiry, MSRN allocation, etc. Note also that for calls terminating in

a UMTS network, a bearer and a radio link in the terminating side should be allocated in a similar way as in the originating side.







The next step of context activation is to find the requested GGSN and send the

request for context creation to it. The SGSN gets the GGSN IP address from the

Domain Name Server (DNS) (6-7). The DNS finds the correct GGSN IPaddress based on the access point name (APN).

The SGSN now sends a 'Create PDP Context Request' message to the GGSN(8). The request includes the APN and the proposed tunnel identification (TID).

TID consists of the IMSI number and the network service access point identifier (NSAPI). NSAPI is used as a reference number of the PDP context.

The GGSN now selects the access point it will use (not shown in the figure).

The APN is associated with the external network the subscriber wants to use. Itis a physical or logical interface in the GGSN. One could say that the access

point is similar to the default gateway defined for a normal IP-subnetwork − it isa point out from the subnetwork. For the UE, the access point is its ‘default

gateway’. In the case of a dynamic address, the GGSN or an external network element can issue the IP address. The external element may be a Dynamic Host

Configuration Protocol (DHCP) server, which issues dynamic addressing

information. The external element might alternatively be a Remote Access Dial

In User Service (RADIUS) server, whose primary function is user authentication.

In (9) the GGSN sends a 'Create PDP Context Response' back to the 3G-SGSN,

which includes given IP address, TID confirmation, and a charging ID.

This is followed by (10) the 3G-SGSN sending a bearer assignment request

(Create) to the RNC.

The RNC will then modify the radio link and set up the bearer over the air

interface (11).

When this is done, the RNC will send a message to 3G-SGSN to notify that the bearer assignment is completed (12).

Finally, the 3G-SGSN can send an 'Activate PDP Context Accept' message tothe UE (13). The 3G-SGSN is now ready to route user traffic between the user

equipment and the GGSN.



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9. Create PDP Context Response; Dyn. IP-address, Charging Id

8. Create PDP Context Request; APN, Dynamic IP Address

UE BS RNC 3G-SGSN DNS GGSN-1

12. Bearer Assignment Complete

1. Radio Resource Connection request

2. Radio Link Setup3. Radio Resource Connection Setup / Complete

4. PDP-Context Activation Request - (Access Point Name, QoS, IP address if available)

11. RAB Setup / Radio Link Modification

13. PDP Context Activation Accept: NSAPI, given dynamic IP-address, offered QoS

10. Bearer Assignment Request (Create): NSAPI, offered QoS

5. Authentication

6. GGSN address resolution Request (APN)

7. GGSN address resolution Response

GTP PDUPDP PDU GTP PDU

PDP: Packet Data ProtocolAPN: Access Point NameGTP: GPRS Tunneling Protocol

Figure 35. PDP context activation

A very important concept of the packet switched session management is PDP

context (PDP = Packet Data Protocol). The PDP context is used for two purposes: for PDP address allocation to the user and to make a logical

connection with the required/desired QoS (Quality of Service) level through the

3G network. The PDP context is an entity defining all required information for the UE – network connection establishment. From the session management

(SM) point of view, the PDP context has two states, active and inactive.

The inactive state means that the packet data services related to a certain PDP

address are not active; the network does not have any routing information

available for that PDP address and thus it is not possible to transfer any data. If

the location of the UE changes, the PDP context information is not updated.

In SM active state the network has routing information available and it is possible to transfer data between the UE and the network. Also the UE location

information is updated in the PDP context.





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The user data packets are carried from RNC to GGSN via 3G-SGSN in

‘containers’. When a packet from an external packet network arrives at the

GGSN, it is inserted into a container and sent to the SGSN. The container isthen opened and packed into a new container towards the RNC. The stream of

containers from RNC all the way to the GGSN is totally transparent to the user.

It seems as if the user is connected directly via a router to an external network,or to an application. In data communications this type of virtual stream of

containers is called a tunnel – the GSNs and RNC perform tunnelling of user

packets.

UE BS RNC 3G-SGSN

1. PDP Context Deactivation Request

2. PDP Context Deactivation Accept

3. Bearer Assignment Request(Release)

4. Radio Link Release

5. Bearer Assignment Complete(Release)

Figure 38. The user quits the connection

Although the connection may remain active for some time, the mobile maydeactivate the PDP context. This is shown in the figure above. A request is sent

from the mobile, through the RNC to the 3G-SGSN to release the resources(1-2). Once the mobile is deactivated, the radio resources are then released

(3-4). Finally, the RNC sends an acknowledgement to the 3G-SGSN (5).





7 Communication management

A UMTS network is a platform to give the operator's the best solution to

provide a varied amount of services. The subscriber's applications and control

components sit upon the bearer. Therefore, communication management inUMTS is all about managing mobility, security and charging of a bearer.

The below figure illustrates the services and control of the services that sit upon

the physical connection. It is the role of communication management to route

the bearer to the high application layers, manage the connection through

mobility management, and handle the bearer security and charging for the

session.

Network ApplicationIntelligence

Connection

Service and customer

application control

• Subscriber's service profile

- location information

- billing of service

- push control

- personalisation- etc...

• Security

• Mobility

• Charging information collection

End user application • Email

• MO-MT SMS

• etc...

Figure 39. Functions of communication management

The communication management needs the services of the lower layers, as thesemaintain the bearer.

This chapter exemplifies the high level functions in terms of the call control

process for circuit switched calls. Also, there are a few notes about emergencycalls and charging.



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7.1 Call control for circuit switched (real time) calls

Call control is a high-level name describing the functions required for incoming

and outgoing call handling within a switch. Generally speaking, the switch

should perform three activities before a call can be connected through. Thoseactivities are number analysis, routing, and charging. Call control can

functionally be divided into three phases, which the call attempt must pass in

order to perform through connection.

Number analysis is a collection of rules how the incoming call should be

handled. Number analysis investigates both the calling and called numbers and

makes decisions based on the rules defined. Number analysis is performed both

in call control Phase I and Phase II. In Phase I the switch checks whether thecalled number is reasonable at all and if any restriction such as call barring is to

be applied with the calling number.

I II III

Fail:- Unsuccessful Setup- Restrictions- Unreasonable Numbers

Fail:- Wrong Dialling- InterWorking Failure

Fail:- Circuits Not Available- Service Not Supported

Number Analysis Routing

Charging

A call coming in A call going out

Figure 40. Call control principles

In call control Phase II the system concentrates on the called number. Thenature of the call is investigated: Is it an international or national call and is

there any routing rule defined for the called number at all? In addition, the

system checks if the call requires any inter-working equipment (like a modem)

to be connected and if the call is chargeable or not. Also statistics for this call isinitiated in this phase.

As a successful result of call control Phase II, the system knows where the callattempt should be routed. When the correct destination for the call is known, the

system starts to set up channel(s)/bandwidth towards the desired destination by





using, for instance, ISUP signalling protocol. During the call, the switch stores

statistical information about the call and its connection and collects charging

information (if the call was judged to be chargeable). When the call is finished,call control Phase III takes care of releasing all the resources related to the call.

7.2 Generation and collection of charging data

The 3GPP Specifications give a detailed list of requirements for the type of

CDR (charging data record) to collect. The Charging Gateway, MSC, HLR and

many elements within the service platform generate CDRs, which describe

different events in the network. An event could be a call, SMS, data usage,

location update, and in fact almost any different type of network activity.

Operators select what causes a CDR to be generated. The CDRs are transferred

to the Billing Centre where the information is collected and priced.

RNC

Billing Centre

Charging Gateway

HLR

3G

SGSN

3G

MSC

GGSN

CDR (Charging DataRecords) are sent to the

Billing Centre

Figure 41. Collection of charging data

As part of the evolution from GSM/GPRS towards UMTS, the amount of

information contained in the CDR has been increased to include details of the

service quality and the network elements used.

Also, the UMTS Specifications describe features that allow the subscriber to see

more information on the cost of a service. This network feature is called AoC(Advice of Charge) and is a supplementary service, which give the subscriber

details of the service cost almost immediately.



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7.3 Note on handling emergency calls

In UMTS (and in the evolved GSM), when location based servers are in place, it

will be possible to actually specifically locate the subscriber within 50-70 m.

When an emergency call is received, the operator can check in which locationthe subscriber is based and hence direct the emergency services to the scene

more quickly than today.

RNC

HLR

3G

MSC

The MSC defines if calls areallowed and where they

should be routed to, basedupon location

3G

SGSNGGSN

To differentregional call

centres

No checking is done (alsoSIM does not need to be

active) to make emergencycall

Location Based Server

If location based services areused, then the emergencyservices can gain accurate

information on thesubscriber's location

Figure 42. Handling emergency calls





Review questions

CTXX5323enIssue 4.0


4. Which of the following is/are characteristic(s) of a UMTS bearer?

a. Variable data rate

b. Transparent through the RAN

c. Different lengths of delay

d. Asymmetric connection

e. None of the above.

5. In the following figure, fill in the missing names of the cellular network architecture.

6. Which of the following sentences about location update is not true?

a. IMSI attach is always made when the terminal is switched on.

b. Location/routing area update takes place when a subscriber moves

between LAs and/or RAs.

c. Periodic location updates are not used in UMTS.

d. None of the above.

7. The URA (UTRAN registration area) is used by the core network to keep

track of subscribers in the network.

True ❏ False ❏

Used by the CN CS Domain

______________________

Used by the CN PS Domain

______________________

Used in the RNC

______________________

What other information is kept?

__________________

__________________

__________________

__________________

Used by the CN CS Domain

______________________

Used by the CN PS Domain

______________________

Used in the RNC

______________________

What other information is kept?

__________________

__________________

__________________

__________________





Review questions

13. When a subscriber triggers an event in the network (for example a call),

a CDR is generated. What is a CDR?

a. Charging data record which is sent to a Billing/Charging Centre

b. Caller digital reset to the MSC

c. Customer data receipt, which is a bill that is sent to the customer

d. All of the above.

14. SMS messages can be sent via the SGSN.

True ❏ False ❏

04_UMTS Traffic Management

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