umts and wlan interoperability

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UMTS and WLAN interoperability

by

Anja Louise Schmidt

SupervisorsHenrik Christiansen and Lars Dittmann

Technical University of DenmarkResearch Center COM

31 July 2004


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Abstract

This research studies different approaches to how interworking between the two network

technologies Universal Mobile Telecommunications System (UMTS) and Wireless LocalArea Network (WLAN) best can be achieved. The different approaches being discussedare the network layer mobility protocols Mobile IPv4 and Mobile IPv6, the transport layermobility protocol mSCTP and the application layer mobility protocol SIP.Conceptual and practical comparisons showed that in a here and now situation, mSCTP isconsidered the best approach for achieving interworking between UMTS and WLANfollowed by SIP, Mobile IPv4 and Mobile IPv6 in that order. In an ideal situation whereIPv6 has been implemented on a large scale and SCTP is commonly supported, MobileIPv6 is considered the best approach followed by mSCTP, SIP and Mobile IPv4,respectively.


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Acknowledgments

This masters thesis project was developed at Research Center COM at Technical

University of Denmark in the period from January 5 to July 31, 2004, with assistance frommy two supervisors Henrik Christiansen and Lars Dittmann.

A number of people have helped me during the work process. Especially, I would like tothank Henrik Christiansen for his indispensable help with my numerous questions as wellas with OPNET. It is difficult to imagine, what I would have done without the help. Also, thevarious people at COM, employees as well as fellow students, deserve my thanks forhelping me with answers to my questions as well as technical and moral support wheneverneeded. The help was greatly appreciated.

Anja Louise Schmidt, s020271July 31, 2004


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Table of contents

1 INTRODUCTION.......................................................................................................... 12 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM ...................................... 3

2.1NETWORK ARCHITECTURE .....................................................................................................................3UE domain ..........................................................................................................................................4UTRAN domain...................................................................................................................................4CN domain ..........................................................................................................................................4

2.2USAGE SCENARIOS ...............................................................................................................................5Network attachment ............................................................................................................................5Circuit-switched connections .............................................................................................................. 5Circuit-switched idle............................................................................................................................6Packet-switched connections ............................................................................................................. 6Packet-switched idle ...........................................................................................................................8Network detachment ...........................................................................................................................8

2.3MOBILITY MANAGEMENT........................................................................................................................8Location management ........................................................................................................................8Handover management ......................................................................................................................9

3 WIRELESS LOCAL AREA NETWORK..................................................................... 113.1NETWORK ARCHITECTURE ...................................................................................................................11

Basic service set...............................................................................................................................11Distribution system............................................................................................................................ 11

3.2USAGE SCENARIOS .............................................................................................................................12Network attachment ..........................................................................................................................12Packet-switched connections ........................................................................................................... 14Packet-switched idle .........................................................................................................................16Network detachment .........................................................................................................................17

3.3MOBILITY MANAGEMENT......................................................................................................................17

Location management ......................................................................................................................17Handover management ....................................................................................................................17

4 UMTS AND WLAN COMPARISON...........................................................................195 HANDOVER............................................................................................................... 23

5.1HANDOVER REQUIREMENTS .................................................................................................................23Terminal requirements ......................................................................................................................23Network requirements.......................................................................................................................23

5.2HANDOVER PROCEDURE......................................................................................................................24Measurements ..................................................................................................................................24Decision ............................................................................................................................................25

Execution ..........................................................................................................................................27

6 MOBILITY PROTOCOLS ..........................................................................................286.1MOBILE IP ..........................................................................................................................................286.2DYNAMIC DOMAIN NAME SYSTEM (DDNS) .......................................................................................... 326.3MOBILE STREAM CONTROL TRANSMISSION PROTOCOL (MSCTP).........................................................336.4SESSION INITIATION PROTOCOL (SIP)..................................................................................................346.5MOBILITY PROTOCOL COMPARISON...................................................................................................... 37

7 NETWORK MODELLING ..........................................................................................417.1PROJECT MODEL.................................................................................................................................417.2NODE MODEL ......................................................................................................................................427.3PROCESS MODEL ................................................................................................................................43Context definition ..............................................................................................................................43


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Process level decomposition ............................................................................................................ 44Enumeration of events...................................................................................................................... 44Event response table development .................................................................................................. 44Specification of process actions ....................................................................................................... 45

7.4 INPUT VALUES.....................................................................................................................................49Code input values .............................................................................................................................49Attribute input values ........................................................................................................................537.5NETWORK MODEL OVERVIEW ............................................................................................................... 54

7.6VALIDATION ........................................................................................................................................568 NETWORK SIMULATION .........................................................................................61

8.1AVERAGE APPLICATION RESPONSE TIME PER LOCATION UPDATE INTERVAL ...........................................618.2AVERAGE APPLICATION RESPONSE TIME PER LOCATION UPDATE DELAY ................................................65

No location update ............................................................................................................................65One location update..........................................................................................................................68Two location updates ........................................................................................................................73

8.3NETWORK SIMULATION OVERVIEW ....................................................................................................... 779 DISCUSSION............................................................................................................. 7810 CONCLUSION ........................................................................................................... 80

FUTURE WORK ..........................................................................................................................................8011 REFERENCES...........................................................................................................8112 APPENDICES............................................................................................................ 84

APPENDIX A:OPNET SOURCE CODE ......................................................................................................... 84APPENDIX B:SEED VALUE AND RUN TIME TEST RESULTS ............................................................................ 84APPENDIX C:ADDITIONAL AVERAGE APPLICATION RESPONSE TIMES PER LOCATION UPDATE INTERVAL......... 84APPENDIX D:ADDITIONAL FUNCTIONS FOR PROBABILITY DENSITY OF APPLICATION RESPONSE TIME ............. 84APPENDIX E:MODIFIED PROCESS MODEL FOR SIMULATION WITH NO LOCATION UPDATE ............................... 84APPENDIX F:MODIFIED PROCESS MODEL FOR SIMULATION WITH ONE LOCATION UPDATE ............................. 84APPENDIX G:MODIFIED PROCESS MODEL FOR SIMULATION WITH TWO LOCATION UPDATES........................... 84


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Anja Louise Schmidt (s020271) Page 1

1 Introduction

The telecommunication landscape has been dramatically changed during the last twodecades by powerful forces; among them the emergence of wireless mobilecommunication and the growth of wireless networking. There has been an explosivegrowth in the use of different communication technologies, as mobile telephony hasoffered mobile communication between people, and wireless networking has providedflexible communication between computers. This change of the technological landscapealso means that the radio systems of today are challenged by the increasing amount ofcapacity-demanding services. The services span from traditional conversational audio toconversational video, voice messaging, streamed audio and voice, fax, telnet, interactivegames, web browsing, file transfer, paging and e-mailing. No single radio system caneffectively cover all these services from a multi-service point of view, if QoS requirementsare to be met. Consequently, the development moves towards interworking between

different but complementary radio systems that together can provide this unparalleled levelof services.There are many alternatives to an interworking solution but research [1] has shown thatthe complementary characteristics of Universal Mobile Telecommunications System(UMTS) and Wireless Local Area Network (WLAN) make them ideal for interworking.UMTS provides a low-bandwidth circuit- and packet-switched service to users withrelatively high mobility in large areas whereas WLAN provides a high-bandwidth packet-switched service to users with low mobility in smaller areas. The WLAN thereforecomplements UMTS on the packet-switched services.The natural trend today is to utilise the high-bandwidth WLANs in hot spots and switch toUMTS networks when the coverage of WLAN is not available or the network condition in

the WLAN is not good enough. This, however, also implies that some sort of handovermechanism must be in place to ensure that any ongoing connections are handed over tothe new network without breaking or deteriorating the connection. At present, no suchmechanism exists by nature. The only way to switch between the two networks is to signoff the first network and then sign on to the next network and start a new connection. Theinterworking between UMTS and WLAN, where packet-switched services can be usedinterchangeably across network borders, is therefore far from realised.

The objective of this research is to study the two network technologies UMTS and WLANand how interworking best can be achieved between the two. This involves describing thetwo technologies to the extent that is necessary for understanding the basic network

dynamics and the mobility management capabilities. It also involves comparing the twonetwork technologies as to how they relate and differ not only to substantiate why they area perfect match for interworking but also to uncover the complications of such match.Moreover, it involves a discussion and comparison of different approaches to achieveinterworking as well as a more practical evaluation to determine what approach is the mostsuited for interworking between UMTS and WLAN.


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The research is organised into 10 chapters.

Chapter 1 provides an introduction to the research as regards the background for theresearch area as well as the objective of the research.

Chapter 2 presents an overview of UMTS in terms of the basic network architecture,different usage scenarios and mobility management.

Chapter 3 presents an overview of WLAN similar to UMTS in terms of the basic networkarchitecture, different usage scenarios and mobility management.

Chapter 4 compares UMTS and WLAN with regards to similarities and differences in orderto substantiate why they are ideal for interworking and to uncover the complications ofsuch interworking solution.

Chapter 5 goes more into details with the aspects of interworking between the two

network technologies in terms of handover, more specifically what is required to perform ahandover, what is the procedure of handovers and what will be the specific area of interestin the handover procedure for this research.

Chapter 6 discusses and compares different approaches to interworking in terms ofdifferent mobility protocols.

Chapter 7 takes a more practical approach and focuses on developing a network modelthat can evaluate the different approaches to interworking.

Chapter 8 continues the efforts from chapter 7 and performs a range of networksimulations based on the network model to evaluate and compare the practicalperformances of the different approaches to interworking.

Chapter 9 discusses the conceptual and practical findings with the purpose of addressingthe objective of this research.

Chapter 10 finally sums up on the findings and presents a conclusion as well assuggestions for future work.


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2 Universal Mobile Telecommunications System

Universal Mobile Telecommunications System (UMTS), also referred to as WidebandCode Division Multiple Access (WCDMA), is one of the most significant advances in theevolution of telecommunications into third-generation (3G) networks. It represents anevolution in terms of services and data speeds from today's second generation (2G)mobile networks such as Global System for Mobile Communications (GSM) and theenhanced 2G (2G or 2+) mobile networks such as General Packet Radio Services(GPRS). The following three sections go much more into details with interesting aspects ofUMTS in terms of the UMTS network architecture, different UMTS usage scenarios andUMTS mobility management.

2.1 Network architectureThe UMTS network architecture (Release 99) consists of three domains: The User

Equipment (UE) domain, the UMTS Terrestrial Radio Access Network (UTRAN) domainand the Core Network (CN) domain. See Figure 1.

Figure 1. UMTS network architecture

The UE domain represents the equipment used by the user to access UMTS serviceswhile the UTRAN domain and the CN domain, together known as the infrastructuredomain, consist of the physical nodes which perform the various functions required toterminate the radio interface and to support the telecommunication services requirementsof the user. The three domains are further described in the following sections.


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UE DOMAIN

The UE domain encompasses a variety of equipment types with different levels offunctionality such as cellular phones, PDAs, laptops etc. These equipment types aretypically referred to as user equipment. The UE domain consists of two parts: The UMTSSubscriber Identity Module (USIM) and the Mobile Equipment (ME).

The USIM is a smartcard that contains user-specific information and the authenticationkeys that authenticates a users access to a network. The USIM is physically incorporatedinto a SIM card and linked to the ME over an electrical interface at reference point Cu.The ME is a radio terminal used for radio communication with the UTRAN domain over theUu radio interface. [2][3][4].

UTRAN DOMAIN

The UTRAN domain handles all radio-related functionality. It consists of one or moreRadio Network Sub-systems (RNS) where each RNS consists of one or more Node Bsand one Radio Network Controller (RNC).

The Node B, also known as a Base Station and equivalent to the Base Transceiver Station(BTS) from GSM, converts the signals of the radio interface into a data stream andforwards it to the RNC over the Iub interface. In the opposite direction, it preparesincoming data from the RNC for transport over the radio interface. The area covered by aNode B is called a cell.The RNC is the central node in the UTRAN and equivalent to the Base Station Controller(BSC) from GSM. It controls one or more Node Bs over the Iub interface and isresponsible for the management of all the radio resources in the UTRAN. The RNCinterfaces the CN domain over the Iu interface. If there are more than one RNC, they canbe interconnected via an Iur interface. [2][3][4].

CN DOMAINThe CN domain is responsible for switching and routing calls and data connectionsbetween the UTRAN domain and external packet and circuit switched networks. It isdivided into a Packet Switched network (PS), a Circuit Switched network (CS) and a HomeLocation Register (HLR).

The PS network consists of a Serving GPRS Support Node (SGSN) and a Gateway GPRSSupport Node (GGSN). The SGSN is responsible for routing packets inside the PS as wellas handling authentication and encryption for the users.The GGSN serves as the gateway towards external packet switched networks like theInternet, Local Area Networks (LANs), Wide Area Networks (WANs), General Packet

Radio Service (GPRS) networks, Asynchronous Transfer Mode (ATM) networks, FrameRelay networks, X.25 networks etc., and thus completes the routing function of the SGSN.

The CS network consists of a Mobile Services Switching Centre (MSC)/Visitor LocationRegister (VLR) and a Gateway MSC (GMSC). The MSC/VLR serves as a switch anddatabase. The MSC part is responsible for all signalling required for setting up,terminating, and maintaining connections, and mobile radio functions such as callrerouting, as well as the allocation/deallocation of radio channels, i.e. the switchingfunction. The VLR part is controlled by the MSC part and is used to manage users that areroaming in the area of the associated MSC. It stores information transmitted by the

responsible HLR for mobile users operating in the area under its control, i.e. the databasefunction.


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The GMSC serves similar to the GGSN as the gateway towards external circuit switchednetworks like other Public Land Mobile Networks (PLMNs), Public Switched TelephoneNetworks (PSTNs) and Integrated Service Digital Networks (ISDNs) etc.

The HLR is a database located in the users home system that stores all important

information relevant to the user, e.g. telephone number, subscription basis, authenticationkey, forbidden roaming areas, supplementary service information etc. The HLR also storesthe UE location for the purpose of routing incoming transactions to the UE. [2][3][4].

2.2 Usage scenariosTo understand the dynamics of UMTS, it is useful to consider the different usagescenarios. The usage scenarios describe how the different parts of the UMTS networkinteract before, during and after communication. The usage scenarios for UMTS are foundbased on the UE service states.The UE exists in three service states: detached, connected and idle. The UE is in the

detached state when it is switched off and there is no communication between the UE andthe network. The UE cannot send or receive anything. In order for the user to make use ofthe network the UE needs to attach to the network by switching on, selecting a cell towhich it can attach, and attaching to that cell. When the UE is attached to the network, itmoves to the connected state and starts communication, or moves on to the idle state andbecomes inactive. [5].The states differ depending on whether the UE is in CS or PS mode. Thus there are siximportant usage scenarios to consider: network attachment, CS connection, CS idle, PSconnection, PS idle and network detachment. These are all further explained in thefollowing.

NETWORK ATTACHMENTThe network attachment process starts when the user turns on the UE. The user mustthen enter a personal PIN code to authenticate to the USIM. If the USIM authenticationgoes through, the UE starts searching for a cell (Node B) to attach to. The attachprocedure is always initiated by the UE. When the UE finds a Node B to attach to, itsynchronises with it, and then attempts to attach to it by sending an attach request to thenetwork (RNC). The network responds by sending the UEs 15-digit USIM identificationnumber to the HLR to inform the HLR of the UEs network attachment request. The HLRand the USIM share a 128-bit secret key, which the HLR applies to a random number. Theresult and the random number are then sent to the network. The network subsequentlychallenges the USIM with the random number. Similarly, the USIM applies the secret key

to the random number and returns the result to the network. If the USIM replies with thesame result as the one sent by the HLR the network accepts the UE and attaches it to thenetwork. Finally, the network downloads any user data there must exist from the HLR tothe VLR to prepare for upcoming network connections. [6].

CIRCUIT-SWITCHED CONNECTIONS

After the network attachment process the UE can proceed with the CS connectionprocess. The CS connection process covers both the setting up a call and the receiving ofa call. Common for both is, however, that a signalling connection must be establishedbetween the UE and the CN.


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To set up a call, a CS connection must first be established. The UE therefore signals to theMSC that it requires a CS connection to a particular number. The MSC looks up thedownloaded user data in the VLR to see if the user has permission to call the number. Ifthe call is permitted, the MSC checks whether circuits are available at the MSC, andwhether the UTRAN has the resources to support the call. If this is the case, it sets up the

circuit connection from the UE, over the air interface, across the UTRAN, to the MSC inthe CN. The MSC then switches the call to the GMSC, which switches it into the externalCS network. The external CS network then performs the necessary switching functions todirect the call to the destination.When the call terminates, both the MSC and the GMSC produce a Call Detail Record(CDR). The CDR contains information about the called and calling party identity, resourcesused, time stamps etc., and is forwarded to the billing server to make an appropriate entryon the users billing record.

To receive a call the procedure is much different. First, the call is routed through theexternal CS network to the GMSC. The GMSC then determines the HLR in which the user

data is store in based on the telephone number. The HLR knows the location area of theUE, i.e. a group of cells throughout which the UE will be paged, and is therefore able tosend a query for a roaming number indicating the destination MSC to the VLR responsiblefor this area. The VLR responds with the number of the MSC, and the HLR then forwardsthe number to the GMSC. The GMSC is now able to route the call to the MSC. Throughthe VLR, the MSC knows the RNC responsible for the UE location area and can thereforerequest that this RNC sets up a channel to the UE. The RNC then pages the UE in the lastknown location area and sets up a connection to the UE over the Node B when the UEresponds to the page. Once the transmission link is established, the UE starts to ring.When the user picks up the phone the connection is switched through.

When the signalling connection for CS services is released, e.g. at call release or radio linkfailure, the UE can be triggered to move to the CS idle state. Alternatively, the UE canmove to the network detachment state either triggered by the user or the network. To beexplained further on. [4][5][6].

CIRCUIT-SWITCHED IDLE

If the signalling connection for CS services is released, the UE moves from the CSconnected state to the CS idle state. The network stops tracking the UE and the UE simplylistens to the broadcast channel of the cells. As long as the UE remains inside the samelocation area the situation remains unchanged. Only if the UE moves into a new location

area, it informs the MSC of the change of location. The new location update is stored inthe HLR and copied to the VLR.If the user wants to make a call, the UE reverts to the CS connection state and performsthe call set up procedure. If there is an incoming call for the UE, the RNC pages the UE.When the UE responds to the page, the RNC sets up the connection and the phone startsto ring. When the user picks up the phone, the connection is switched through. Commonfor both is that the UE must move from the CS idle state to the CS connection state andestablish a connection. Alternatively, the UE can move to the network detachment state.[4][5][6].

PACKET-SWITCHED CONNECTIONS

An alternative to the CS connections is the PS connections. The PS connection processcovers similar to the CS connection process both the setting up a call and the receiving of


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a call. Common for both is that, similar to before, a signalling connection has beenestablished between the UE and the CN.

To set up a call a PS connection must be established. The UE first activates a Packet DataProtocol (PDP) context in the GGSN. A PDP context is a range of settings that defines

which packet data networks a user may use for exchanging data. The list of permitted PDPcontexts is stored in the HLR. To activate the PDP context, the UE establishes aconnection over the RNC to the SGSN and sends a message that the user would like toestablish an external PS connection. The SGSN forwards the query to the GGSN, whichthen sends a query to the HLR to check if the user is authorised to access external PSconnections. If the user is authorised, the GGSN activates the context and informs the UEincluding an IP address. The activation of the context creates a fixed IP tunnel to whichoutgoing data packets are sent to the RNC over the SGSN to the GGSN. The GGSN thenswitches the call into the external PS network, which performs the necessary switchingfunctions to direct the call to the destination. The tunnel is active until the UE deactivatesthe context either by closing the application or disconnecting from the SGSN.

The SSGN is continuously informed about the UEs current routing area, i.e. PS equivalentto the CS location area. If the user changes routing area to an area with a new responsibleSSGN, the route in the GGSN is adapted to this.From the HLR query the SGSN and the GGSN are aware of the Quality of Service (QoS)requested for the packet transfer and are able to set up parts of the packet transfer path inadvance. The QoS categories for PS connections are conversational (voice), streaming(streaming video), interactive (web browsing) and background (file transfer, emails).When the call terminates, the SGSN generates a billing record from the PDP context(based on e.g. the duration of call or the amount of data) and sends it to the billing serverthat makes an appropriate entry on the users billing record.

To receive a call another process is required. First, the incoming call is routed through theexternal PS network to the GGSN. The GGSN then determines the HLR in which the userdata is stored based on the telephone number. The GGSN can next look in the HLR anddetermine whether the UE is attached to the network and has an active PDP context. If theUE is not attached the call is rejected. If the UE is attached but does not have an activePDP context, the UE needs to be located and paged to set up an active PDP context.The HLR knows the location of the UE within the accuracy of the routing area. It thereforealso knows the destination switching node (SGSN). The GGSN obtains this information atthe same time it checks the HLR for UE network attachment and PDP context status. TheGGSN is now able to route the call to the SGSN. The SGSN knows the RNC responsible

for the UE routing area and requests that this RNC sets up a channel to the UE. The RNCpages the UE in the last known routing area and sets up a connection the UE over theNode B used by the UE when it responds to the page. Once the transmission link isestablished the UE receives the call and the PS connection is switched through.If the UE already has an active PDP context the packet transfer can be transmitted directlyto the UE.

When the signalling connection for PS services is released e.g. at release of PS service,because of very low level of activity, or at radio link failure, the UE can be triggered tomove to the PS idle state. Alternatively, the UE can move to the network detachment state.[4][5][6].


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PACKET-SWITCHED IDLE

If the UE has been idle for PS traffic for a while it goes from PS connected state to PS idlestate as the network timer expires. The network stops tracking the UE, and the UE simplylistens to the broadcast channel of the cells. Only if the UE moves into a new routing areait informs the SGSN of the change of location. The new location update is stored in the

HLR and copied to the VLR.The logical connection between the GGSN and the UE is maintained also in PS idle stateif the PDP context has not been de-activated. If the user wants to make a call and the PDPcontext is still active, the UE simply reverts to PS connected state and starts the call. If thePDP context is inactive, the UE first needs to revert to the PS connected state and activatethe PDP context before proceeding with the call. If there is an incoming call for the UE andthe PDP context is still active, the UE automatically reverts to PS connected state whenreceiving the call. If the PDP context is inactive at an incoming call, the UE is paged by theRNC in the last known routing area. When the UE responds to the page, the RNC sets upa connection to the UE and the incoming call is directed to the UE. Alternatively, the UEcan move to the network detachment state. [4][5][6].

NETWORK DETACHMENT

When the UE no longer requires the services of the UMTS network, it can explicitly moveto the network detachment state and detach from the network by sending a detachrequest. Network detachment can also be initiated by the network either explicitly byrequesting detachment or implicitly by the network detaching the UE without notifying theUE a configuration-dependent time after the mobile reachable timer expires or after anirrecoverable radio error causes disconnection of the logical link. When networkdetachment is invoked all buffered data is removed. [5].

2.3 Mobility managementMobile communication systems like UMTS are by definition meant to handle mobilitymanagement. Mobility management involves two mechanisms: location management andhandover management. Location management is the mechanism of keeping track of ausers location outside an active connection, while handover management is themechanism of handing over an active connection from one cell to another. Bothmechanisms are further studied in the following two sections.

LOCATION MANAGEMENT

To transfer an incoming connection to an inactive user, the network must continuously be

up-to-date with the users location. The location update process is defined for both CS andPS services.

In terms of CS services, the network is divided into Location Areas (LA). A LA consists of anumber of cells between which the user can move without updating his location. All NodeBs in such a LA beam a specific number, a Location Area Index (LAI), which is interceptedby the UE. The UE becomes aware that it has changed LA, when this parameter changes.It consequently executes a Location Area Update (LAU) with the MSC, which thenforwards the information to the HLR.

In terms of PS connections, the UE will receive short data packets more frequently than is

the case with CS connections. This means an increased and often unnecessary amount ofpaging. Consequently, the location update for PS connections divides the network into


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even smaller areas called Routing Areas (RA) to limit the amount of paging. A RA is simplya smaller area of cells completely surrounded by a LA. The principle is the same as withLA. When the UE becomes aware of a change in RA is executes a Routing Area Update(RAU) with the SGSN, which similar to the MSC forwards the information to the HLR. [4].

HANDOVER MANAGEMENTTo forward an active connection from one cell to another, the network must perform ahandover. Similar to location management, the handover process is defined for both CSand PS services.

In terms of CS services, handovers can be implemented as soft handovers, softerhandovers and hard handovers.Soft handover can take place in the following scenario. The UE initially only communicateswith the Node B in its current cell, cell one. Then the UE starts to move in the direction ofanother Node B in another cell, cell two, and starts to receive the same information as onits own physical channel. Physically, the UE is in two overlapping sectors from separateNode Bs. The UE can communicate simultaneously with up to three Node Bs. As the UEmoves, the UE continuously monitors the signal quality from the other cell. If the receivedsignal strength from the Node Bs in cells one and two differ by a maximum of an amountcalled the handover margin during a certain period of time, a connection is alsoestablished to the Node B in cell two. When the received signal strength from Node B incell one is smaller by a certain amount than that of the Node B in cell two, the connectionto the first Node B in cell one is cleared. A soft handover has then taken place.A softer handover functions in principle the same way as a soft handover in thattransmission also run in parallel over different sectors but of the sameNode B. The UEinitially communicates with Node B in sector one of cell one. As the UE starts to move it

also starts to receive a signal from the same Node B in the same cell one but from anothersector, sector two. The signal from sector two is a reflected signal of the direct signal. Thiscan happen if for example a large building is in the line of the direct signal and thusunintentionally relays the signal in another angle. If the received signal strength fromsector one and two differ by a maximum of the handover margin, a connection isestablished to sector two. When the received signal strength from sector one is smallerthan that of sector two, the connection from sector one is cleared and a softer handoverhas taken place.Hard handover takes place when the connection to the current cell is broken before aconnection to a new cell is made, i.e. from one frame to the next one. There are differentsub-types of hard handover: inter-frequency, intra-frequency and inter-system. An inter-

frequency hard handover is made between two different frequencies within the same cellor adjacent cells. An intra-frequency hard handover is performed in situations where theIur interface between two RNCs is not available for a soft handover. A hard handover isthen performed from one cell belonging to one RNC to the next cell belonging to anotherRNC using the same frequency. Finally, an inter-system hard handover is performed whenit is required to change the radio access technology from UMTS to GSM.

In terms of PS services, there is one type of handover defined for a UMTS network: cell re-selection. Cell re-selection takes place in the following situation.The UE continuously monitors the signal quality from other cells as the user relocates.Typically, the UE is instructed to send a measurement report to the serving RNC, when the

quality of a neighbouring cell exceeds a given threshold and the quality from the currentcell is unsatisfactory. When the RNC receives the measurement report, it initiates a


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handover, given that all the criteria for handover have been fulfilled. It then asks the driftRNC to reserve resources. The drift RNC returns a handover command message includingthe details of the allocated resources via the core network and current air interface to theUE. When the UE receives the handover command, it moves to the new cell andestablishes the radio connection in accordance with the parameters included in the

handover command message. The UE indicates successful completion of the handover bysending a handover complete message to the drift RNC, after which the drift RNC initiatesthe release of the old radio connection. Finally, when the cell re-selection has beencompleted, the UE initiates the routing area update procedure as described before.

Even though the network solely communicates with the UE using one access technologyat a time, the UE needs to perform measurements on the new cell while communicating inthe current cell. Since UMTS uses continuous transmission and reception in the PSconnected state, a regular UE cannot measure other cells while communicating in UMTS,if the UE has a single radio receiver. To overcome this obstacle, compressed mode isintroduced.

Compressed mode is a method that creates short gaps or idle spaces in transmission andreception. To maintain a perceived constant bit rate, the actual transmission bit rate isincreased or compressed just before and after the gap. A constant bit rate is required forservices such as voice, but for data services, a constant bit rate is not necessary. Thetransmission can therefore just be delayed to create a gap.The UE makes use of compressed mode to measure other cells, if the UE has a singleradio receiver. If the UE, however, contains separate network radio receivers, it can useeach receiver in parallel, performing measurements on one network while communicatingon the other without compressed mode in the downlink.If the UE, however, contains separate network radio receivers, it can use each receiver inparallel, performing measurements on one network while communicating on the otherwithout compressed mode in the downlink.

The downside of both hard handover and cell-reselection is that so far they only work forhandovers between UMTS and GSM. For handovers between UMTS and WLAN supportfrom other protocols is required. [3][4].


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3 Wireless Local Area Network

Wireless networking targeted at computer networks, especially the 802.11b Wireless LocalArea Network (WLAN) specification, has encountered increasing recognition over wirednetworking the last few years. This is a result of the world becoming progressively moremobile. The following three sections go much more into details with interesting aspects ofWLAN in terms of the WLAN network architecture, different WLAN usage scenarios andWLAN mobility management.

3.1 Network architectureThe 802.11b WLAN network architecture basically consists of one or more Basic ServiceSets (BSSs) and a Distribution System (DS). See Figure 2 below. Both are furtherdescribed in the following.

Figure 2. WLAN network architecture [7]

BASIC SERVICE SET

The basic part of the network architecture is the BSS. It consists of a group of Stations

(STAs) that are under direct control of a single coordination function. The STAs arecomputing devices (laptops, handheld computers etc.) with wireless network interfaces.The STAs communicates through a wireless medium. The geographical area covered bythe BSS is known as the Basic Service Area (BSA). The BSA is analogous to a cell in theUMTS network.In an infrastructure network all STAs communicate by channelling all traffic through acentralised Access Point (AP). The AP controls the communication in the BSS as well asproviding network connectivity between other BSSs and thus has a bridging function. TheAP is analogous to the Node B in the UMTS network.Opposite in an independent BSS (IBSS), also known as an ad hoc network, any STA cancommunicate with any other STA without channelling the traffic through an AP.When interconnecting a wireless to other networks an AP is required. Therefore only theinfrastructure wireless network is of interest and to be considered in this research. [7][8][9].

DISTRIBUTION SYSTEM

A common distribution system (DS) integrates multiple BSSs. The DS does not specifyany particular backbone technology and can be wired to a wide range of mediums.The integration of multiple BSSs using a DS is called an Extended Service Set (ESS). TheESS provides not only access for multiple wireless users but also gateway access forwireless users into a wired network such as the Internet. This is done through a portaldevice that incorporates functions analogous to a bridge. [7][8][9].


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3.2 Usage scenariosTo understand the dynamics of WLAN, it is useful to consider the different usagescenarios, like in the UMTS case. The usage scenarios for WLAN are found based on theservices states, the STA can exist in.The STA can exist in the three states detached, connected and idle. The detached state iswhen the STA is switched off and there is no communication between the STA and anetwork. To make use of a network the STA must first switch on and detach to a networkthrough a process of scanning for networks, deciding on a network to join andauthenticating and associating with the chosen network. When the STA is connected to anetwork it can start sending/receiving packet-switched data frames, or move to the power-saving mode, the idle state. Finally, the STA can choose not to make use of the networkresources anymore and detach from the network or the network can choose to detach theSTA from the network for various reasons. [9].This gives the four usage scenarios network attachment, packet-switched connections,packet-switched idle and network detachment that are all further explained in the following.

NETWORK ATTACHMENT

In order to attach to a network, the STA must first be switched on by the user. In mostcases this also includes entering a password to authenticate to the STA. If theauthentication goes through, the STA can proceed with the network attachment.

A STA must then identify a compatible network. This process of identifying existingnetworks in the area is called scanning. The scanning procedure is based on severalparameters that can be either default values or user specified. The parameters include:BSSType, BSSID, SSID, ScanType, ChannelList, ProbeDelay, and MinChannelTime andMaxChannelTime. BSSType scans for ad hoc networks, infrastructure networks, or all

networks. BSSID scans for a specific network to join (individual) or for any network(broadcast) that is willing to allow the STA to join. SSID assigns a string of bits to an ESS,most often the network name, allowing the STA to scan for a specific network. ScanTypeallows for both passive and active scanning, more about this later. ChannelList specifies alist of channels the STA can scan through. ProbeDelay is a specified delay interval beforethe procedure for an active scanning of a channel begins. And finally MinChannelTime andMaxChannelTime are time values of the minimum and maximum amount of time the scanworks with any particular channel.The scanning can be either passive or active. In passive scanning the STA saves batterypower because it does not transmit. It simply moves from channel to channel on thechannel list and waits for beacon frames from nearby APs. The beacons contain thenecessary information needed for the STA to match with a BSS and begin communication.In active scanning the STA takes a more active role and attempts to find the networkinstead of listening for the network to announce itself. Probe request frames are used tosolicit responses from a networks AP that in return sends back a probe response frame.The probe request frame is targeted at all the networks belonging to the STAs own ESSbut can also be targeted at all networks in the area by using the broadcast BSSID.

When the scanning procedure has been completed a scan report is generated. It containsall the BSSs the scanning discovered and their parameters. The parameters include inaddition to the BSSID, SSID and BSSType, beacon interval, DTIM period, timing

parameters, PHY and CF parameters, and BSSBasicRateSet. The beacon intervalspecifies each BSS interval in which it can transmit beacon frames. The DTIM frames are


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used as part of a power-saving mechanism. The timing parameters contain some timinginformation used to synchronize the STAs timer to the BSS timer. The PHY and CFparameters contain channel information and contention-free operation information. Finally,the BSSBasicRateSet contains a list of data rates the STA must support in order to join thenetwork.

When the scan report has been generated, the STA can choose to join one of the BSSs.Choosing to join a BSS does however not enable network access. The STA must also gothrough authentication and association.The process of choosing a BSS to join is an implementation-specific decision that can betriggered by some network default values or even by user intervention. The most commondecision criteria are power level and signal strength.

When the STA has decided to join a BSS, the next step is authentication. There are twomajor authentication approaches: open-system authentication and shared-keyauthentication.

Open-system authentication is the only required authentication method in wirelessnetworks. It involves the AP accepting the STA without actually verifying the STAsidentity. The STA sends an authentication request frame to the AP with its MAC addressas unique identifier/source address. The AP then processes the authentication requestand returns an authentication response to the STA using the source address.Shared-key authentication is an optional authentication method. If this authenticationmethod is used the entities involved must implement the Wired Equivalent Privacy (WEP)protocol, which is an 802.11 security protocol that encrypts data. Shared-keyauthentication involves a shared key be distributed to the STA before attemptingauthentication. The STA first sends an authentication request frame to the AP similar tothe open-system authentication approach. The AP then returns either an authenticationdenied response frame or an authentication challenge response frame. The challengeframe contains a 128-bytes challenge text. The STA responds to the challenge frame byencrypting the frame body of the challenge text with its shared-key and returning it to theAP. The AP then decrypts the challenge text frame with its shared-key and verifies theintegrity of the frame. A positive authentication message is returned to the STA, if theintegrity of the frame is intact.

A STA must authenticate with an AP before associating with it. The authentication is,however, not required to take place immediately before the association. A STA canauthenticate with several APs during the scanning process so that the STA is already

authenticated when association is required. This is called pre-authentication. Pre-authentication means both time savings and smoother roaming operation relative toauthentication.

When a STA has authenticated or pre-authenticated itself to an AP, it can associate or re-associate with the AP to gain full access to the network.Association is a recordkeeping procedure of the STAs location, which is used by the DS toforward frames destined for the STA to the correct AP. A STA can only associate with oneAP at a time. The association procedure is initiated by the STA, which sends anassociation request frame to the AP. The AP then processes the request based on someimplementation-specific parameters. There are no specifications on how to determine

whether an association should be granted. Most often the amount of space required forframe buffering is being considered. If the AP grants the association request, it responds


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with a positive association message containing an Association ID (AID). The AID is usedto logically identify the STA when buffered frames need to be delivered. Finally, the APcan begin to process frames for the STA, and the association is completed.Re-association is the process of moving an association from an old AP to a new AP. It isdifferent from association in the sense that the APs interact with each other. Re-

association is initiated by the STA. The STA monitors the signal quality from its current APas well as from other APs in the same ESS. If the STA decides that another AP is a betterchoice, the STA sets off re-association. The decision to make a re-association is based onproduct-dependent factors. The STA sends a re-association request to the new APcontaining the address of the old AP. The new AP then communicates with the old AP todetermine whether a previous association existed. If the old AP does not verify that itauthenticated the STA, the new AP returns a de-authentication frame to the STA and endthe procedure. Otherwise, the new AP responds with a positive re-association messagecontaining an AID. The new AP then contacts the old AP and finish off the re-associationprocedure. The old AP sends any buffered data frames for the STA to the new AP andterminates its association with the STA. The new AP begins to process frames for the STA

and the re-association is completed. Notice that the STA during the re-association processis only associated with one AP at a time during the whole process. [8][9].

PACKET-SWITCHED CONNECTIONS

Once the STA has associated/re-associated with an AP it can begin to send and receivedata frames also known as Mac Protocol Data Units (MPDUs).

To send data frames in an infrastructure WLAN, all frames must go through the AP,including frames to other STAs in the same service area. The STAs make use of the MACService Data Unit (MSDU) delivery service to send the data frames. The MSDU delivery

service defines two coordination functions: the distributed coordination function (DCF) andthe point coordination function (PCF). The DCF is a compulsory coordination function forboth infrastructure as well as ad hoc WLANs while the PCF is an optional function forinfrastructure WLANs only.

The DCF is a fundamental access method all STAs must support. It supportsasynchronous time-insensitive data transfer (e.g. e-mail, web browsing, file transfers etc.)on best effort basis. The DCF is based on the contention principle, which means that allSTAs with data queued for transmission must contend for access to the medium. Once aSTA has transmitted its data frame it must recontend for access to the medium for theremaining frames. This ensures fair access to the medium for all STAs.

The DCF is based on the Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA) access protocol. Carrier sensing involves monitoring the medium to determinewhether it is idle or busy. Two types of carrier sensing have been specified: the physicalcarrier-sensing and the virtual carrier-sensing. Physical carrier-sensing is provided by thephysical layer, which detects the presence of other users by analyzing all the detectedpackets and by measuring the relative signal strength from other sources. Virtual carrier-sensing is provided by the Network Allocation Vector (NAV).The NAV is a timer set by theSTAs specifying the amount of time the medium will be reserved. When the NAV equalszero the virtual carrier-sensing function indicates that the medium is idle. The medium ismarked busy if either of the two carrier-sensing mechanisms indicates the medium is busy.If the medium is idle the source STA initiates transmission preparations. The access to the

medium is controlled through the use of interframe space (IFS) time intervals. The IFSintervals are specified in three different priority classes: short IFS (SIFS), PCF-IFS (PIFS)


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and DCF-IFS (DIFS). SIFS is the shortest time interval and thus has highest priorityaccess to the medium followed by PIFS and DIFS. For basic access, the STA waits aDIFS time interval and then senses the medium again. If the medium is still idle, the STAsets the duration field in the data frame and then transmits it. The duration field is used tolet all STAs in the BSS know how long the medium will be busy and adjust their NAV. The

duration field includes the expected time of transmission for the data frame, the IFS timeinterval for the acknowledgment frame and the acknowledgment frame. When thereceiving STA receives the data frame it calculates the checksum to see whether theframe was received correctly. For positive acknowledgment (ACK) the receiving STA waitsa SIFS time interval and then transmits the ACK to the source STA. Since the higherpriority SIFS time interval is shorter than the DIFS interval the ACK frame will not collidewith data frames. If the source STA does not receive an ACK, the transmission isconsidered lost and the STA must retransmit the data frame by contending for the mediumagain.If a source STA wants to transmit a data frame but senses the medium to be busy the STAwaits until the medium becomes idle for a DIFS time interval and computes a random

backoff time to schedule a reattempt. The STA then decrements the backoff timer until themedium becomes busy again or the timer reaches zero. If the medium becomes busy andthe timer has not reached zero the STA freezes its timer. The timer is reactivated when themedium has been idle for a DIFS time interval again. When the timer reaches zero theSTA transmits its data frame.If there are multiple STAs that want to transmit when the medium is busy they all wait aDIFS time interval and computes a random backoff time. The STA with the lowest backofftime gets to transmit first and the other STA freezes their timers.In case the timers of two STAs decrement to zero at the same time a collision will occurwhen they start to transmit at the same time. Both STAs will then have to generate a newbackoff time.WLANs cannot handle long transmissions due to the relatively large error rates. Largedata frames exceeding a certain fragmentation threshold are therefore broken into multiplefragments to increase the transmission reliability. A STA transmitting fragmented dataframes is required only to wait a SIFS time interval. This STA therefore has a higherpriority compared to other STAs that are required to wait a DIFS time interval and can beassured a continuously transmission of the data frame fragments.The DCF can be improved by implementing the Request to Send (RTS)/Clear to Send(CTS) control frames. A STA cannot hear if a collision occur and therefore continues totransmit the complete data frame. Especially in the case of large data frames a largeamount of bandwidth can be wasted. If the source STA instead sends an RTS control

frame after a DIFS time interval to the destination STA before it sends any data frameother STAs can adjust their NAV and less collisions will occur. The destination STAresponds with a CTS control frame after a SIFS time interval. The source STA the waitsanother SIFS time interval and proceeds with the transmission. As for any data frametransmission, the destination STA responds with an ACK frame after a SIFS interval. Theadvantage of the RTS/CTS implementation is that should a collision occur with an RTS orCTS control frame, far less bandwidth is wasted in comparison with a large data frame.

The PCF is an optional access method. Since the point coordinator (PC) resides in the AP,the PCF is restricted to infrastructure WLANs. In contrast to the DCF, the PCF supportsdelay-sensitive data transfer (e.g. packetized voice and video, streamed packetized audio

and video etc.). The PCF is based on the contention-free principle. The PC controls all


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traffic by polling the STAs in turns. Only after being polled a STA is allowed to transmit aframe. This way there is no contending for the medium.When the PCF and DCF coexist the capacity is shared between contention-free traffic andcontention traffic in a contention-free period (CFP) and a contention period (CP)respectively. Alternating intervals of contention-free services and contention-based

services repeat at regular intervals called the repetition intervals.The CFP starts when the medium has been idle for a PIFS time interval. The PC, i.e. theAP, transmits a beacon frame including the maximum duration of the CFP to help theSTAs synchronise and update their NAV to the maximum length of the CFP. The PC thenstarts polling the STAs on its polling list by sending a polling frame. The STAs may onlytransmit if they have received a polling frame and then only after a SIFS time interval. TheSTAs may piggyback an ACK of the received polling frame together with the data frame.The CFP ends when all STAs have been polled. The PC announces this by issuing a CFEnd frame. The STAs reset their NAV and the CP begins according to the DCF. The CFPcan be shortened in case it is lightly loaded and provide the remaining bandwidth to theCP.

When the AP receives the data frames, it transfers the frames to the destination STA if it islocated in the same BSS. If the data frames are destined for a STA in another BSS but inthe same DS, it forwards the frames across the DS to the appropriate AP, which thentransfers the frames to the destination STA. Finally, if the data frames are destined for aSTA in an integrated network outside the DS, the frames are transferred to the AP, acrossthe DS on to the portal and through the Internet, where it is routed to the destination STAusing standard routing mechanisms.

When the DS receives frames for a STA, either through the portal from an integratednetwork or from an AP in the DS, it delivers the frames to the right AP. The AP then relaysthe frames to the intended associated destination STA. If the destination STA is in powersaving mode (explained in the next part), the AP buffers the frames for it. [7][8][9].

PACKET-SWITCHED IDLE

The main advantage of wireless networks is mobility. Mobility, however, implies that theSTAs run on batteries, which is a scarce resource. To deal with this problem wirelessnetworks operate with powering down the transceiver causing great power savings. Whenthe transceiver is off it is sleeping, dozing, in power-saving or idle mode. When it is on it isawake, active or simply on. The optimal power saving in wireless networks is obtained byspending the maximum time in power-saving mode and the minimum time in the opposite.

All this without sacrificing connectivity.The AP plays the key role in power management. First of all, it is assumed to have accessto continuous power as it must remain active at all times. Second of all, it is per definitionaware of the location of the STAs and has access to the power management state of theSTAs through the STAs themselves. This key role has lead to two power-management-related tasks for the AP. Since the AP is aware of the power management state of everySTA associated with it, it can also determine whether a frame should be delivered to thenetwork because the STA is awake or should be buffered because the STA is in power-saving mode. Also, the AP has to periodically announce which STAs have frames waitingfor them. This announcement of buffer status helps contributing to the power savings. Itrequires much less power to powering up the receiver and listening to the buffer status

than periodically transmitting polling frames. During the association process the STA andAP agree on a listen interval. The listen interval is the number of beacon periods for which


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the STA may choose to sleep. When the listen interval runs out the STA must power up toactive mode and listen to the buffer status of waiting frames. Failing to do so, may result inthe AP discarding the frames without notification.When a STA powers up, listens to the buffer status and finds out that there are waitingframes it uses PS-poll control frames to retrieve the buffered frames. One PS-poll frame

retrieves one buffered frame. The retrieved buffered frame must be positive acknowledgedbefore it is removed from the buffer and before the STA can retrieve the next waiting framein line. The STA must remain awake until the polling transaction has concluded, i.e. untilall the waiting frames have been delivered or discarded. [8][9].

NETWORK DETACHMENT

When a STA no longer requires the services of the AP, it can terminate the existingassociation by using the disassociation service. Disassociation is a polite task to do,however, the network is designed to cope with STAs leaving the network without properdisassociation. When the STA invoke the disassociation service, any buffered data in theAP is removed.The disassociation service can also be used by the AP to inform the STA that the AP nolonger provide the link either because of resource restraints or because the AP is shuttingdown or being removed from the network for a variety of reasons.Disassociation is a one-frame notification and can be invoked by either associated party,i.e. the STA or the AP. Neither party can refuse termination of the association.

If a STA wishes to be removed from a BSS, it can send a de-authentication frame to theAP to notify the AP of the removal from the network. Once a STA is de-authenticated, ithas no longer access to the network since de-authentication terminates any currentassociation. To make use of the network resources, it must therefore perform the

authentication function again.The de-authentication service can also be used by the AP to eliminate a previouslyauthorised user from any further use of the network.De-authentication is a one-frame notification that can be invoked by either associatedparty. Neither party can refuse the de-authentication. [8][9].

3.3 Mobility managementThe WLAN specification handles mobility management in a very simple way. There are nodistinct definitions of location management and handover management but to illustrate thedifferences to UMTS mobility management this section looks more into how WLANactually deals with location management and handover management.

LOCATION MANAGEMENT

The WLAN location management is quite different from UMTS location management in thesense that when a STA has first associated with an AP, the AP continually knows thelocation of the STA, as the STA is required to be within the reach of the AP, and no furthermechanisms are required in order for the AP to be able to forward frames destined for theSTA.

HANDOVER MANAGEMENT

WLAN also handles handover management but in terms of transitions. There are three

different types of transitions defined: no transition, BSS transition and ESS transition.


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No transition occurs when the STA does not move out of the APs service area. There aretwo sub-states of this state: no movement and local movement. When the STA is staticthere is no movement, and when the STA only moves within the APs BSS there is localmovement.BSS transition occurs if the STA moves from one BSS in one ESS to another BSS within

the same ESS. What basically happens is that the STA moves its current association fromone AP to another within the same ESS without loosing the connection. This is also knownas re-association, described in an earlier section.Finally, ESS transition occurs when the STA moves from a BSS in one ESS to a BSS inanother ESS. The WLAN specification supports this transition in the sense that the STAmay move but no guarantees are made as to whether the connection is maintained. Infact, the connection must be expected to break. This means that higher-layer connectionsare almost guaranteed to be interrupted. In order to maintain higher-level connections andprovide seamless ESS transition, support from other protocols is required. [8][9].


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4 UMTS and WLAN comparison

The two previous chapters described some fundamental characteristics of UMTS andWLAN networks in terms of network architecture, usage scenarios and mobilitymanagement. This chapter will be more specific as to how the two network technologiesrelate and differ by comparing them. Table 1 presents an overview of comparativedifferences followed by explanatory remarks.

Characteristics UMTS WLAN

Services Circuit- and packet-switched services Packet-switched servicesData rates 144 kbps

384 kbps

2 Mbps

satellite and rural areasmin 120 km/h, max 500 km/hurban outdoor environmentsmax 120 km/hindoor and low range outdoormin 0 km/h, max 10 km/h

1 Mbps

2 Mbps

5.5 Mbps

11 Mbps

max 100 m (indoor)max 450 m (outdoor)max 90 m (indoor)max 300 m (outdoor)max 70 m (indoor)max 150 m (outdoor)max 30 m (indoor)max 100 m (outdoor)

Coverage Cellular, national/international coverage Non-cellular, local coveragePower control Flexible power control Max. effect of 100 mW requiredMobility High, global (UMTS, UMTS-GSM) Low, local (WLAN)Deployment costs Expensive CheapStandardisationbodies

Closed standardisation body Open standardisation body

Technologicalorigin

Telecommunication Data communication

Air interface WCDMA HR-DSChannel

bandwidth

5 MHz 5 MHz

Frame length 10 ms VariableChip rate 3.84 Mcps 11 McpsFrequencyregulations

Regulated frequency spectrum Unregulated frequency spectrum

Frequency band WCDMAFDD

WCDMATDD

New band

1920-1980 MHz (up link)2110-2170 MHz (down link)12 channels1900-1920 MHz2010-2025 MHz7 channels2500-2690 MHz

2.412-2472 GHz13 channels

Table 1. UMTS and WLAN comparison

The first difference between the two network technologies especially from the users pointof views is the range of supported services. The UMTS standard supports a variety ofcircuit- and packet-switched services such as voice, video telephony, video games, videoconferencing, streamed voice and video, SMS, MMS, email, fax, telnet, interactive games,web browsing, ftp etc., while the WLAN specification only supports the correspondingpacket-switched services. [1][8].

Another and more significant difference is the data rates. UMTS supports data ratesranging from 144 kbps up to 2 Mbps according to the specific environment and speed oftravelling. High mobility users, classed as users travelling over 120 km/h and max 500

km/h in satellite and rural areas can expect data rates of 144 kbps. Full mobility user,those travelling at less than 120 km/h and in urban outdoor environments, can expect 384


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kbps. Finally, low mobility users, those based indoor or at low range outdoor travelling atless than 10 km/h or stationary, can expect data rates of up to 2 Mbps. [1][10].In comparison, WLAN supports data rates ranging from 1 Mbps up to 11 Mbps alsoaccording to the specific environment. For 1 Mbps data transmission the estimatedmaximum range indoor is 100 meters while the estimated maximum range outdoor in line-

of-sight is 450 meters. For 2 Mbps the estimated maximum range for indoor and outdoor is90 and 300 meters, respectively, while the estimated maximum indoor and outdoor rangesfor 5.5 Mbps are 70 and 150 meters, correspondingly. Finally, for 11 Mbps the estimatedmaximum ranges for indoor and outdoor are 30 and 100 meters, respectively. [8][11].

A fundamental difference between the two network technologies is the coverage. UMTSbuilds on the cellular concept, which means that instead of covering a large area with oneNode B, the large area is divided into a number of smaller areas or cellseach covered bya separate Node B. By splitting the area up into a number of smaller cells, the samefrequency can be reused over relatively small distances and thus enabling coverage for agreater number of users. Also, by reducing the area to be covered by a Node B, the

transmitter power can be lowered. By the use of handover mechanisms and roamingagreements, the network can moreover provide national and even international coverage.The advantages of cellular networking are therefore increased capacity, reducedtransmitter power and better coverage. [4].In comparison, WLAN incorporates a non-cellular concept in terms of much smaller locallysituated network islands, the so-called hot spots or WLAN cells, not tailored to largecoverage. Usually these WLAN cells cover homes, small enterprises, campuses, hotels,hospitals, airports, restaurants etc. Since the WLAN only covers smaller limited areas, thecoverage is local. [7].

Another aspect close connected with coverage is power control. UMTS communicationhas the flexibility to optimise the range of communication with suitable effect, while WLANrequires a maximum effect of 100 mW. This means that while a typical WLAN cell has therange of approximately 50 meters, a UMTS cell can reach up to 35 kilometres. [12].

Mobility is also a related aspect. UMTS handles mobility by performing handover/cell re-selection. The handover/cell re-selection mechanisms work well as long as within thesame network technology. For mobility between network technologies, inter-system hardhandover and cell re-selection also work as long as between UMTS and GSM networks.There are, however, no mechanisms defined for handover between UMTS and othernetworks e.g. between UMTS and WLAN. UMTS is therefore defined to provide high and

global mobility within UMTS networks and between UMTS and GSM networks. [3][4].In comparison, WLAN handles mobility quite differently. WLAN supports user relocationwithin the BSS and even between BSSs in the same ESS by means of the transitionmechanism also known as re-association. There are, however, no mechanisms defined fortransition between one BSS in one ESS to a BSS in another ESS why transitions betweenWLAN and other networks or even between independently operated WLANs simply arenot possible. This means that WLAN only provides a low and local mobility. [8][9].

The costs of deploying the two network technologies also differ. The UMTS deploymentrequires an expensive Node B in every network cell as well as an expensive license andfrequency use fee. The license is additionally provided with obligations. In contrast, the

WLAN deployment requires inexpensive access points and no license or frequency usefee or obligations. [12].


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The standardisation bodies that handle development of the two network technologies arealso very different. UMTS is defined by a closed standardisation body consisting ofInternational Telecommunication Union (ITU), European Telecommunications StandardsInstitute (ETSI), and national radio frequency administrations and national licenseproviders, while WLAN is defined by an open standardisation body consisting of Institute of

Electrical and Electronics Engineers (IEEE) and national radio frequency administrators.[12].

A fundamental difference is the dissimilar technological origin. UMTS origins from thetraditional large, monopoly-encumbered telecommunication sector where implementationstend to follow a traditional top-down oriented development approach corresponding to e.g.its predecessor GSM.In comparison, WLAN has its roots in the more dynamic, younger and fragmented datacommunication sector, where the innovation and diffusion patterns may be characterisedas much more un-coordinated, like e.g. development of the Internet. [12].

Finally, there are also some more technical differences. UMTS utilises the air interfacestandard WCDMA, which is based on Code Division Multiple Access (CDMA). UMTSimplements the special CDMA technique called Direct Sequence (DS-CDMA) that spreadsthe user data bit stream over a wide bandwidth by multiplying each user data bit with a(chipping) sequence of 8 code bits called chips derived from CDMA spreading codes. Achip is mathematically a bit, which means that the chipping sequence is basically a bitsequence, but to distinguish the original user data bit stream from the spread signal theterm chip is used. The result of multiplying each bit of the user data bit stream with achipping sequence is a chip stream with flattened amplitude across a relatively widefrequency band. This flattening of the amplitude over a wide band means that fairly largechannels are required. While other systems use a smaller channel bandwidth of about 1MHz, the UMTS system consequently uses a channel bandwidth of 5 MHz, hence thename WidebandCDMA. The chip stream is transmitted simultaneously with other chipstreams in the same frequency range in frame lengths of 10 ms and at a transmission rateof 3.84 Mcps. This translates into the actual data rates ranging from 144 kbps up to 2Mbps. In the receiver, the bits of the user data stream are recovered from the chip streamwith a correlator, which simply inverts the spreading process.UMTS operates within a regulated frequency spectrum around 2 GHz. Since UMTSsupports two basic duplex modes of operation, Frequency Division Duplex (FDD) andTime Division Duplex (TDD), the European spectrum allocation has been reserved forWCDMA FDD in the bands 1920-1980 MHz (up link) and 2110-2170 (down link) and for

WCDMA TDD in the bands 1900-1920 MHz and 2010-2025 MHz. A new frequencyspectrum in the frequency band 2500-2690 MHz has additionally been identified but hasnot yet been taken in use. For the paired channels in the frequency bands 1920-1980 and2110-2170 MHz a total of 12 channels are available. This implies that for each 5 MHZchannel in the up link band, another channel in the down link band exists. For the unpairedchannels in the frequency bands 1900-1920 and 2010-2025 MHz a total of 7 channels areavailable. [3][4].In contrast, the 802.11b WLAN specification utilises the High Rate Direct Sequence (HR-DS) air interface standard for data transmission. HR-DS is developed from the 802.11 DSencoding method, where the user data bit stream is spread by applying an 11-bit Barkerword, a special-defined bit sequence, by a modulo-2 adder to each bit or two bit in the user

data bit stream. Similar to with UMTS, this flattening of the amplitude calls for fairly largechannels, why WLAN also operates with 5 MHz channel bandwidth. The outcome of the


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multiplication procedure is a chip stream, which is transmitted in variable frame lengths ata transmission rate of 11 Mcps. This translates into actual data rates of 1 or 2 Mbpsdepending on whether the 11-bit Barker word is applied to each bit or two bits.To provide for higher data rates, HR-DS was released as an extension to the 802.11specification, the 802.11b specification. Achieving higher data rates requires that each

code symbol carry more information than a bit or two. The 802.11 DS encoding processdid not prove suitable for carrying more bits, which led to the use of an alternate encodingmethod Complementary Code Keying (CCK) for the 802.11b HR-DS. CCK uses 8-bitsequences to encode 4 or even 8 bits per code word. This translates into actual data ratesof 5.5 Mbps or 11 Mbps according to the specific environment.Furthermore, WLAN operates within an unregulated frequency spectrum around 2.4 GHz.A total of 13 channels, each 5 MHz wide, are located in the frequency range 2412-2472GHz. [8][9][13][14].

Based on the listed differences there are some characteristics that are striking. First of all,the degree of offered services at first seems unequal. However, since it has never been

the intention that WLAN should complement UMTS on the circuit-switched services,WLAN in fact matches up with UMTS on the packet-switched service level. Also, in thelong run the development is expected to proceed towards all-IP networks where allservices are delivered via packet-switched networks which eventually will equalise anydifferences of supported services.Next, the difference in data rates is also significant. UMTS supports data rates from 144kbps to 2 Mbps of which the average user is expected to receive data rates of 384 kbps.WLAN oppositely supports much higher data rates in the range from 1 Mbps to 11 Mbpsand thus unquestionably outdistances UMTS.Also important is the level of coverage and mobility. UMTS offers wide coverage and highmobility whereas WLAN only offers very local and therefore low coverage and has limitedmobility. Common for both is, however, that they have limited mobility in terms ofrelocating beyond the network boundaries. UMTS handles mobility only within UMTSnetworks and between UMTS and GSM networks, and WLAN handles mobility only withinthe BSS and between BSSs in the same ESS. To extend mobility to other networks, e.g.between UMTS and WLAN, the use of additional mobility protocols are therefore required.

These complementary characteristics have meant that UMTS and WLAN are consideredjust the right candidates for interworking to provide wide-spread multi-service wirelessaccess by utilising high-bandwidth WLANs in hot spots and switching to UMTS networkswhen the coverage of WLAN is not available or the network condition in the WLAN is not

good enough. One complication is, however, the shortcomings of both UMTS and WLANwith regards to mobility between the two network technologies.


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5 Handover

The shortcomings of mobility between the two network technologies must first be resolvedin order to realise any interworking. Since UMTS handles mobility in terms of handoversand cell re-selections and WLAN handles mobility in terms of transitions the mobilityhandling between the two networks is referred to simply as handover henceforth forconsistency. The following two sections will go much more into details with therequirements for handling handovers as well as the actual handover procedure.

5.1 Handover requirementsIn order to perform handovers there are a few basic requirements that must be in place.The requirements can largely be divided into two: terminal and network requirements.These are further discussed in the following.

TERMINAL REQUIREMENTSThe mobile terminal, in the following referred to as the client instead of UMTS UE orWLAN STA, must be a dual-mode terminal set up for mobile radio communication andwireless networking in terms of incorporated USIM and 802.11b wireless LAN card,necessary software, and adjusted configurations, and signed up with one or more networkoperators. There are no requirements for it to be able to operate on both networks at thesame time, but it must support basic one-at-a-time UMTS and WLAN operation as well assupport handover between the two networks.This evidently leads to modifications of existing mobile terminals in the direction of somehybrid device incorporating features from both cellular phones and laptops. Going into

details with the design and functionality of this hybrid device is however out of scope withthis research.

NETWORK REQUIREMENTS

The network interworking involves two possible types of ownership/managementconfigurations: the cellular operator configuration and the wireless Internet service provider(WISP) configuration.The first configuration is the case in which the cellular (UMTS) operator owns andmanages the WLAN. The cellular operator can enhance its data service capabilities withhigh-speed data connectivity in strategic locations such as airports and hotels byaugmenting its cellular data system with WLAN. This allows the users to roam, i.e. to

access another operators network (nationally as internationally) at the price of a local callor at a charge considerable less than the regular long-distance charges, outside the UMTSnetwork. The cellular operator has the advantage (over the WISP) of an establishedcustomer base to which it can market such capabilities. Additionally, the cellular operatorhas authentication and billing mechanisms in place for its users, which can be reused inthe WLAN.The second configuration is the case in which a WISP or enterprise owns and managesthe WLAN. Although the WISP or enterprise can not offer the same service set, the sameuser experience may be achieved by multilateral roaming agreements between theWISP/enterprise and cellular operators. The authentication and billing mechanisms can beprovided by the cellular operator in case of a WISP-owned configuration whereas the

authentication mechanism would be limited and the billing mechanism non-existing in caseof an enterprise-owned configuration.


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Irrespective of network ownership/management configuration roaming between thenetworks is assumed possible and in effect.

Billing is close related to roaming in the sense that all operators involved in establishingcommunication expect to receive a share of the earnings. How and how much to bill all

depends on what the operators have agreed on beforehand. Billing models can becomprehensive and very complex and is out of scope with this research. As a result, billingissues are not considered any further but assumed to be in place and function.

5.2 Handover procedureThere are many parameters to take into account when dealing with handovers as theprocedure of handovers is both complex and comprehensive. This section will be muchmore specific as to how a handover is performed and what aspects of the handoverprocedure are being focused on in this research.The procedure of handover is threefold. First some measurements need to be performed

according to some parameters and the results gathered in a measurement report. Then ahandover decision is taken based on the measurement report. Finally the handover isexecuted if the handover decision is positive. The following considers each of the threesteps in details.

MEASUREMENTS

The first step of the handover procedure is the measurements of several parameters thatare required to assess the current status of the existing connection between the client andthe serving cell and of the quality of other available cells.The measurements can in theory be performed by one of the two entities: the client or thenetwork. In practice is it, however, only the client that performs the measurements

because it is the only entity that has the full and exact access to and overview of themeasurements. The measurements are usually carried out continuously. [15].

The measurements include both some static user preferences and profiles and somedynamic measurements. [16] suggests an approach, which organizes the static anddynamic parameters into four classifications: application, user preference and usercontext, terminal, and network.The static input parameters of the application classification contain a list of the services theuser has subscribed to, and a preference list indicating the priority of the services in casethe resources are scarce. The dynamic input parameters contain a list of services that aresupported/unsupported in the network, a list of services that are active/suspended, and an

indication of the delivered service quality.The static input parameters of the user preference and user context classification containuser preferences for specified contexts in terms of role-dependence as on business or inleisure time, time-dependence as in time-of-day or day-of week, situation-dependence asin driving a car or waiting for a plan

umts and wlan interoperability

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