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SYSTEM TRAINING
UMTS Traffic Management
Training Document
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The information in this document is subject to change without notice and describes only theproduct defined in the introduction of this documentation. This document is intended for theuse of Nokia Networks' customers only for the purposes of the agreement under which thedocument is submitted, and no part of it may be reproduced or transmitted in any form or means without the prior written permission of Nokia Networks. The document has beenprepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia Networks welcomes customer comments aspart of the process of continuous development and improvement of the documentation.
The information or statements given in this document concerning the suitability, capacity, or performance of the mentioned hardware or software products cannot be considered bindingbut shall be defined in the agreement made between Nokia Networks and the customer.However, Nokia Networks has made all reasonable efforts to ensure that the instructionscontained in the document are adequate and free of material errors and omissions. NokiaNetworks will, if necessary, explain issues which may not be covered by the document.
Nokia Networks' liability for any errors in the document is limited to the documentarycorrection of errors. Nokia Networks WILL NOT BE RESPONSIBLE IN ANY EVENT FORERRORS IN THIS DOCUMENT OR FOR ANY DAMAGES, INCIDENTAL ORCONSEQUENTIAL (INCLUDING MONETARY LOSSES), that might arise from the use of thisdocument or the information in it.
This document and the product it describes are considered protected by copyright accordingto the applicable laws.
NOKIA logo is a registered trademark of Nokia Corporation.
Other product names mentioned in this document may be trademarks of their respectivecompanies, and they are mentioned for identification purposes only.
Copyright © Nokia Networks Oy 2004. All rights reserved.
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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
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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
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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).
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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
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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.
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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
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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.
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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
RAN (UTRAN & GERAN) Core Network
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
RAN (UTRAN & GERAN) Core Network
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
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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
RAN (UTRAN & GERAN) Core Network
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.
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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
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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
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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.
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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.
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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.
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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.
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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)
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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.
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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 +++=
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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)
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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.
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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.
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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.
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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).
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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
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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
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Review questions
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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?
__________________
__________________
__________________
__________________
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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 ❏