A case study: IEEE 802.21 enabled mobile terminals foroptimized WLAN/3G handovers

Antonio de la Olivaa,b Carlos J. Bernardosb

Telemaco Meliaa Ignacio Sotob

Albert Vidala Albert Banchsb

delaoliva|melia|vidal@netlab.nec.de aoliva|cjbc|isoto|banchs@it.uc3m.esaNEC Network Laboratories, Heidelberg, Germany

bDepartamento de Ingenieria Telematica, Universidad Carlos III de Madrid, Madrid, Spain

Wireless LAN hotspots are becoming widely spread. This, combined with the availabilityof new multi-mode terminals integrating heterogeneous technologies, opens new businessopportunities for Mobile Operators. Scenarios in which 3G coverage is complementedby Wireless LAN deployments are becoming available. Therefore all IP based networksare ready to offer a new variety of services across heterogeneous access. However, toachieve this, some aspects still need to be analyzed. In particular, how and when to executehandovers in order to minimize service interruptions and maximize the use of the mostappropriate technologies according to user’s preferences(for example, a user may preferto use a lower cost technology if available). This paper presents a simulation study ofhandover performance between 3G and Wireless LAN access networks. The mobile devicesare based on the IEEE 802.21 cross layer architecture and useWireless LAN signal levelthresholds as handover criteria.

I. Introduction

In the recent past wireless broadband internet con-nectivity has been massively deployed. WirelessLAN (WLAN) Hotspots are nowadays available inmost public environments either for business or forentertainment. With wireless broadband access, basedon WLAN technologies and ADSL wired connections,it is becoming less expensive to provide mobile userswith a wide variety of services every where and everytime. There is also a trend towards more sophisticatedmobile devices incorporating different technologies,such as WLAN, UMTS or DVB-H, each being usefulunder different circumstances. However, real integra-tion of the different technologies requires providing”seamless end to end services” with ubiquitous useof the heterogeneous technologies. This is especiallyimportant when considering real time services such asvideo or voice real time streams (e.g. VoIP, MobileTV). In this environment, mobile devices have acombination of macro and micro cells of differenttechnologies available, and a critical issue is how tochoose the most suitable network, taking into accountnetwork availability, user profiles and applicationrequirements among other criteria.

An important effort to deal with these challenges isundergoing at the IEEE 802.21 [1] proposed standard.The IEEE 802.21 Working Group is specifying a

method to provide enhanced vertical handover mecha-nisms across the 802.x and the 3GPP/3GPP2 family ofnetworks and defining internetworking functionalities.The 802.21 solution is based on a 2.5 middleware(the Media Independent Handover Function, MIHF)that abstracts layer-2 specific characteristics to upperlayers (namely layer-3). Thus, IP based protocolsare provided with a method to control the underlyingtechnologies by means of a set of appropriate SAPs(Service Access Points). For example, a variety ofparameters useful for selecting a handover targetare defined in an abstract way, and the levels abovelayer-2 only need to deal with these abstract parameters.

The IEEE 802.21 work provides a useful frameworkto efficiently implement solutions for inter-technologyseamless handovers and internetworking, but concretesolutions are out of its scope: 802.21 does neitherspecify which parameters should be taken into accountnor how those parameters should be used.

The mixed WLAN-3G environment is going to bean important and widespread case in the near futureand it is the focus of this paper. Existing solutionsfor handover decision in WLAN environments havebeen based mainly on signal level. In this mixedWLAN-3G environment we argue that WLAN signallevel will be the most critical parameter in networkdetection and selection. A typical scenario will be

3G universal coverage and a preference for WLANaccess when available. Therefore, it is essentialto characterize how the use of WLAN signal levelthresholds influences the performance of handoversbetween WLAN and UMTS cells. This is the objectiveof this paper. Through an extensive simulation study,and using an IEEE 802.21 architecture for the mobiledevice, we provide insightful guidelines on how theuse of signal level information and how the chosencriteria impacts the performance of handover betweenWLAN and UMTS. Performance is analyzed in termsof packet loss and achieved utilization of the WLANnetworks. This has a direct impact on business modelsand users’ requirements such as mobile phone bill costs.

References [7], [6] and [13] analyze performanceissues of handovers based on Mobile IP betweencellular networks. However these works only study theproblems related with upper layers (mainly TCP) due tothe differences between the two involved technologies.Some previous works (e.g. [5]) analyze the integrationof WLAN hotspots into 3G networks. These workshowever, in contrast with our own, are not based onthe 802.21 framework. The first paper, to the bestof our knowledge, that treats the specific problemsof intertechnology handovers based on IEEE 802.21is [9], but in contrast with our work, in this paperSIP (Session Initiation Protocol) is used as mobilityhandler. The WLAN signal level model used in ourpaper is based on [15] and [12].

The remainder of the paper is organized as follows.Section II gives an overview of the upcoming IEEE802.21 draft standard. Section III briefly summarizesMobile IPv6 operations. Section IV describes an archi-tecture for mobile devices based on the IEEE 802.21draft standard, and an algorithm for handover executiondecisions. Section V depicts the simulation setup andsection VI illustrates the obtained results. Finally weconclude with section VII.

II. IEEE 802.21 overview

The goal of the IEEE 802.21 (Media IndependentHandover) group is to develop a standard that supportshandovers between heterogeneous networks. Its aim isto provide link layer intelligence and related networkinformation to upper layers to facilitate vertical han-dover operations. This includes links within cellularnetworks specified by 3GPP1, 3GPP22 as well as bothwired and wireless networks in the IEEE 802.x family.

1http://www.3gpp.org2http://www.3gpp2.org

The standard may play an important role in future 4Gnetworks as well, supporting the integration of WLANnetworks in 3G cellular technologies like UMTS.

The 802.21 standard supports handovers for bothmobile and stationary users. For mobile users, han-dovers may occur either due to changes in the wirelesslink conditions or due to a signal level degradation as aresult of terminal mobility. For stationary users, han-dovers may be required when the network environmentor the users’ requirements change. For instance, a usermay require a higher data rate channel when startingthe download of a large data file.

The aim of the handover procedure is to maximizethe service continuity by providing seamless mainte-nance of active communications when the user changesits point of attachment to the network, either wired orwireless. To avoid unacceptable disruptions to ongoingcommunications, the network should establish the linkto the new point of attachment prior to releasing theprevious link. Such soft handover would prevent anyperceptible interruption, for example during a voicecall.

The IEEE 802.21 standard envisions the cooperativeuse of both mobile terminals and network infrastructurefor making vertical handovers. The mobile terminalis assumed to be multi-mode, i.e. to support differentradio standards. The network is composed by bothmicro cells (IEEE 802.11 or IEEE 802.15 coverage)and macro cells (3GPP, 3GPP2 or IEEE 802.16), whichtypically overlap. In scenarios where there is notenough overlap between different cells, disruption inthe communication is unavoidable.The cooperation between terminal and network in802.21 includes the following functions. The MobileNode detects available networks. The network infras-tructure stores overall network information, includingneighborhood cell lists and the location of mobiledevices. Then, the handover process is (typically)sustained by the information supplied from networkto the terminal, in addition to the information that theterminal collects from the link layer. This informationincludes signal quality measurements, synchronizationtime differences, maximum data rates, higher layercapabilities. Supporting handover decision with thesedata contributes to the optimization of handover algo-rithms.Note that the 802.21 standard does neither specifyrules or policies for handover decisions nor determineswhether the handover has to be terminal or networkinitiated. Instead its aim is to specify an architecture to

allow and facilitate such decisions. Concrete rules andpolicies are out of the scope of the standard and theirdefinition and specification is up to the user (WirelessService Provider).

Figure 1 depicts the role of the 802.21 standardwithin the framework of IEEE and it shows where thenew functions reside. Note that standardization work iscurrently ongoing and the presented 802.21 architecturemight be affected by updated functionalities introducedin future documents.

Figure 1: IEEE 802.21 MIH reference model

III. Mobile IPv6

Mobile IPv6 [10] is the standard the facto for IPmobility, adopted by the main standardization bodiessuch as IETF and 3GPP2, and it is one of the protocolsproposed in 802.21. It allows nodes to remain reachableacross heterogeneous access technologies.

In Mobile IPv6, Mobile Node (MN) is identified by aHome Address (HoA), regardless of its current locationand point of attachment to the Internet. When theMN visits a Foreign Network, it configures a Care-ofAddress (CoA), which is the address used to reach theMN while visiting the Foreign network.

IPv6 [8] packets directed to the Mobile Node’sHome Address are transparently routed to the newlocation by the use of an IPv6 over IPv6 tunnel. Theentity in charge of forwarding the packets to the MobileNode is called Home Agent (HA). This entity capturesthe packets directed to the Mobile Node’s HomeAddress and forwards them to the Care-of Address. Inorder to inform the Home Agent of the new Care-ofAddress of the MN, a control handshake is defined.The Binding Update (BU) carries the information about

the Home Address/Care-of Address association. TheHome Agent completes the handshake with a BindingAcknowledgment (BACK).

Mobile IPv6 defines a Route Optimization mecha-nism by which the Correspondent Node (CN) can com-municate directly with the Mobile Node. In this workwe have not taken into account the possibility of routeoptimization because operators have some security con-cerns with this procedure. In any case, we do not expectthat the use of the route optimization would affect theresults presented in the paper.

IV. Model overview

In this section we present the model we have used in thispaper. The section contains the following three parts:

• 802.21 Model

• Modification to the Mobile IPv6 stack

• Handover algorithm

IV.A. 802.21 Model

The Media Independent Handover (MIH) functionalityhas been implemented in the OMNET++3 simulationtool. It consists of three elements: the MIH Function,the Service Access Points (SAPs) with their correspon-dent primitives and the MIH Function Services. This isshown in Figure 2.

Figure 2: IEEE 802.21 MIH OMNET++ model

The MIH Function (MIHF) is defined in the currentspecification [1] as a logical entity and the specific MIHimplementation of the Mobile Node and the networkare not included. In fact, it is important to notice thatin order to facilitate the overall handover procedure,the MIH Function should be implemented following

3http://www.omnetpp.org

a crosslayer design, allowing the communicationwith the management plane of every layer within theprotocol stack. Hence, the intelligence of the handover,(described in section IV.C), has been realized as part ofthe MIH Function.

The Service Access Points (SAPs) are used to enablethe communication between the MIH Function andother layers. In the presented implementation there isone technology independent MIHSAP which allowsthe communication between the MIH function andupper layers, namely IP, transport, and application.Two technology dependent SAPs are also implemented:WLAN SAP and 3GSAP, which communicate theMIH Function with the management plane of the802.11 link layer and the 3GPP link layer, respectively.Note that every SAP defines certain number of primi-tives that describe the communication with the servicesin the MIH Function. Since the implemented scenariodoes not cover all possible use cases, we have definedhere only the primitives needed for our scenario.

The MIH Function is supported by three basicservices: events (Media Independent Event Service,MIES), commands (Media Independent Command Ser-vice, MICS) and information (Media Independent In-formation Service, MIIS). These services can either belocal or remote. We say that a service is local when theorigin and the destination of the service are the sameMIH entity, while we say that it is remote when the ori-gin and destination are different entities (e.g., the originis the mobile terminal and the destination is an Informa-tion Server located at the operator network). The focusof this paper is on a specific scenario where the terminalneither needs to discover nearby MIH remote Informa-tion Services nor to receive remote events/commands.Accordingly, hereafter we only take into account localservices.

The Media Independent Event Service (MIES) (2)has been implemented to process the following eventsfrom the link layers:

• MIH WLAN BEACON IND: This message issent by the WLANSAP to the MIH Functionwhen no beacon frame has been received within aperiod of 3 seconds while being connected throughthe WLAN (Msg 1 in Figure 2). This prevents thecase of a sudden disconnection from the AccessPoint (AP) where no disconnection message hasbeen received by the WLAN interface.

• MIH WLAN LINK ON: This message is sent tothe MIH Function by the WLAN interface as soon

as the WLAN interface receives an associationconfirmation from the AP (Msg 2 in Figure 2).

• MIH WLAN LINK OFF: This message is sent bythe WLAN SAP when the WLAN signal qualityis below a certain threshold (Msg 3 in Figure 2).This events can trigger an active or passive scan.

• MIH 3G LINK ON: This message is sent to theMIH Function when the 3G interface is informedthat the PDP (Packet Data Protocol) context isstarted after the activation procedure (Msg 4 inFigure 2). At this point data through the 3G in-terface can be sent.

• MIH 3G LINK OFF: This message is sent to theMIH Function when the 3G interface is informedthat the PDP context has been released. (Msg 5 inFigure 2).

• MIH 3G LINK GOING ON: This message issent to the MIH Function after the 3G interface hassent a connection request to the 3G network (Msg6 in Figure 2). Communication is still not possible.

• MIH IP HO SUCCESS: After a Binding Ac-knowledge (BACK) is received by the Mobile IPentity, this message is sent by the MIHSAP to in-form the MIH Function of the handover success(Msg 7 in Figure 2).

• MIH IP HO FAILURE: After the expiration ofthe timeout defined to receive a BACK in the Mo-bile IP entity, this message is sent by the MIHSAPto inform the MIH Function of the handover fail-ure (Msg 8 in Figure 2).

The Media Independent Command Service (MICS)provides the means to upper layers to configure, controland obtain information from lower layers. These are thecommands defined in our model:

• MIH CONNECT3G: This message is sent by theMIH Function to the 3GSAP to notify the 3G in-terface that a connection procedure must be initi-ated (Msg 9 in Figure 2).

• MIH DISCONNECT3G: This message is sent bythe MIH Function to the 3GSAP to inform to the3G interface that a disconnection procedure mustbe initiated (Msg 10 in Figure 2).

• MIH INITIATE L3HO: This message is sent bythe MIH function to inform the Mobile IP entitythat a layer 3 handover has to be initiated. The

interface towards which the handover has to be ex-ecuted (i.e., WLAN or 3G) is specified as a param-eter of this command (Msg 11 in Figure 2).

The goal of the Media Independent Information Ser-vice (MIIS) is to create a schema of available neighbor-ing networks with accurate and up to date characteris-tics values of both upper and lower layers (e.g. Qualityof Service (QoS) on the link, Mobility protocols avail-able in a specific network). Our scenario, as mentionedbefore, does not require remote communication (we as-sume network information available at the terminal) butlocal, and the information implemented is the follow-ing:

• MIH WLAN RSSI: This message is sent by theWLAN interface to the WLANSAP, in order toreport the signal strength of the current link. TheWLAN SAP forwards this information to the MIHFunction (Msg 12 in Figure 2).

The MIH Function has always up to date informationof the state of both higher and lower layers. Therefore,it will be able to decide when and how a handover pro-cedure should be carried out.

IV.B. Modification to the Mobile IPv6stack

In order to have a reasonable control over the handoverperformance, some modifications to the Mobile IPstack have been implemented.

Mobile IPv6 signalling (Binding Update BU andBinding Acknowledge) sent by a node for WLAN-3Ginterworking, could be lost in the network before reach-ing the destination or could be lost in the wirelessmedium when the Mobile Node suffers from poor sig-nal conditions. Taking into account that the signallingis always sent through the new link in our implementa-tion, a signalling loss may occur due to varying WLANsignal conditions when moving from a 3G to WLAN.Notice that, as detailed later, we assume in our modelno packet loss in the 3G channel. When a BU or BACKis lost the handover at layer 3 is supposed to fail. Whenthe handover fails, the state of the signalling flow canbe:

• The BU has not arrived at the Home Agent: thepacket flow is reaching the Mobile Node throughthe old link so no packet loss happens (no han-dover).

• The BU reaches the Home Agent but the BACKis lost: in this case the packet flow starts arriving

to the Mobile Node through the new link. Therecould be packet loss if the Mobile Node experi-ences sudden signal condition variation.

Binding Updates are usually retransmitted upontimeout. If a Binding Ack is not received after atimeout expiration, a retransmission is scheduled andthe next timeout is set to the double of the originalone. This policy is kept until the timeout reaches amaximum (MAX BINDACK TIMEOUT is 32 secondsas specified in the Mobile IP RFC [10]).

As the Mobile Node has no way of knowing if theBinding Update has reached the Home Agent or not af-ter a handover failure, the handover algorithm must pro-ceed with an action to stabilize its state. This action isto perform a handover to the 3G leg.The major modification introduced into the Mobile IPstack is in the way the retransmission of the BindingUpdates is handled.As in the 802.21 model the decisionof the handover must be handled by the MIH layer, theretransmission algorithm of Mobile IP has been omit-ted. When a Binding Update is sent, triggered by theMICS module, the Mobile IP module sets a timeoutof 1.5 seconds (timeout of the first BU transmission asspecified in [10]). After this timeout expires, a messageindicating the failure of the handover is sent to the MIHlayer, which takes the required actions (namely rollingback to the 3G channel).

IV.C. Handover Algorithm

Our handover algorithm is based on signal thresholds.It relies on the information provided by the MediaDependent layers and the Mobile IP Layer. A completeflow diagram of the handover algorithm is presented inFigure 3.

The handover algorithm reacts upon the receptionof three possible signals,i) an RSSI (Received SignalStrength Indicator) sample,ii) a notification about thestatus of the handover andiii) a wireless LAN link Offmessage.

The handover algorithm is based on two thresholds.The first one,3G → WLAN threshold, defines theminimum wireless LAN signal level that must bereceived in the Mobile Node to trigger a handoverfrom the 3G to the wireless LAN. The second one,WLAN → 3G threshold, defines the wireless LANsignal level below which a handover to the 3G legis triggered. A handover to the wireless LAN isperformed when the signal level reaches the valuespecified by the3G → WLAN threshold. The mean

Figure 3: Handover Algorithm Flow Diagram

value of the two last samples of signal level is takenin order to measure the signal level; with this thesignal level variability is decreased. A handover to3G is performed when the signal level goes below theWLAN → 3G threshold, or when a wireless Link Offmessage is received.

After the MICS triggers a handover to the MobileIP Layer, the handover algorithm is not allowed toperform another handover until the reception of ahandover status message informing of the last handoverresult. If a handover is not successful, the algorithmperforms a handover to the 3G part to fix the state ofthe algorithm. There are different causes for the failureof a handover. The BU may not reach the Home Agentor the BU reaches the Home Agent but the BindingAck is lost.

Before performing a handover some conditions mustbe satisfied. The interface should be completely con-figured, with a global routable IPv6 address (DuplicateAddress Detection (DAD) procedure completed) anddefault router associated. Also, all previous handoversshould have been completed. If these conditions are notfulfilled the handover is delayed. In the case of han-dover to WLAN, if the conditions are not met, the han-dover is skipped until another signal sample arrives. Inthe case of handover to 3G, the handover is delayed bya timer, waiting for the conditions to be satisfied. Thetimer has been fixed to 100 ms. The value of 100 ms isthe default period of the beaconing in WLAN. The re-

trial of a handover to the 3G is performed each time thesignal level goes down theWLAN → 3G threshold. Ifany of these trials is successful before this timer expires,the timer is cancelled. The main purpose of this timer isto assure the retrial even if the MN is out of the wirelessLAN cell, in this case the Mobile Node will not receivea frame in the 100 ms period and the timer will retry thehandover even if no power indication arrives.

V. Simulation Setup

The handover study is conducted by simulating a mo-bile node attached to the 3G network and performingseveral handovers between 3G and wireless LAN.

The specific scenario analyzed is based on an indoorenvironment with several non-overlapping wirelessLAN cells and a full coverage of 3G technology. Weargue that this represents a scenario that will be atypical deployment in the future. Please note that thispaper does not cover the WLAN to WLAN handovercase. The reader is referred to [4] for an extensive studyof WLAN to WLAN handover which complements thework presented here. This work considers a wide spacewith indoor characteristics in which the user couldmove faster than walking speed, such as in an airportusing, for example, an electric vehicle.

The Mobile Node speed is fixed to 10 m/s. This valuerepresents an upper limit of the speed expected in thebig size indoor scenario. Indeed, all pedestrian speeds

are well below this threshold.The movement pattern selected is the Random Way-Point Model. With this model each node moves alonga zigzag line from one waypoint to the next one, all thewaypoints being uniformly distributed over the move-ment area.The traffic studied is a downstream audio, with a packetsize of 160 bytes at application layer and interarrivalpacket time of 20 ms (83 kbps). Notice that usual VoIPcodecs generate bit rates around 80 kbps. 60 simulationruns were performed for each experiment. This num-ber was chosen as a tradeoff between simulation timeand confidence interval. In Figure 4 a schematic view

Figure 4: Simulated Scenario

of the simulation scenario is shown. The audio serveris the node C2, the Home Agent is the Router R1 andthe Mobile Node is the phone MN. The line connectingMN and R1 is the emulated 3G channel. The RoundTrip Time between the node C2 and the Mobile Nodehas been taken equal to 350ms for the 3G leg of thethe MN and equal to 150ms for the Wireless LAN. TheRTT of 150ms using the WLAN leg is higher than mostreal scenarios, and is chosen to test our algorithm in aworst case situation.

V.A. WLAN Model

The standard wireless LAN propagation model definedin OMNET++ is based on free space losses with shad-owing and a variable exponential coefficient. The orig-inal model implemented in OMNET++ is suitable forstudies that do not analyze in depth the effect of the sig-nal variation. However, the objective of this paper is tohave a realistic wireless LAN model, suitable for indoor

scenarios based on empirical results. For this purpose,we used the empirical model in [11], which includesvariation in the signal due to shadowing and differentabsorption rates in the materials of the building. Thepath loss model is the following:

Losses = 47.3 + 29.4 ∗ log(d) + 2.4 ∗ Ys

+ 6.1 ∗ Xa ∗ log(d) + 1.3 ∗ Ys ∗ Xs

Xa = normal(0, 1)

Ys = normal(−1, 1)

Xs = normal(−1.5, 1.5)

(1)

being d the distance between the Access Point andthe Mobile Node.

The power transmitted by the AP and Mobile Nodeare defined in the UMA specification [3], [2]. TheAP transmission power is 15dBm while the MobileNode transmission power is 10 dBm. Following thesespecifications, the AP antenna gain is set to 0 dBi whilethe Mobile Node antenna gain is set to -10dBi. Thetransmission rate of the wireless LAN is fixed to 11Mbps.

The OMNET++ wireless model defines two thres-holds, the Sensitivity threshold and the Active Scanningthreshold. The Sensitivity threshold is the minimumlevel of signal that the receiver can detect. Real prod-ucts specifications set this level of signal to -90 dBm4.This is the value that we have used in our simulations.The Active Scanning threshold defines when the wire-less card starts scanning for other APs in order to per-form a WLAN to WLAN handover. When this levelof signal is reached the Mobile Node detaches from thecurrent AP. The IEEE 802.11b specification does notspecify the value for this threshold, its value being de-sign dependant. In the model presented, this value isset to -80 dBm. This value was selected after analyzingvia simulations the maximum variability of the wirelessLAN signal model. With this threshold we gain that theMobile Node disconnects from the AP before reachingthe sensitivity threshold.

V.B. 3G channel Model

The 3G channel has been modelled as a Point to PointProtocol (PPP) channel with a connection time of 3.5seconds, disconnection time of 100 ms, bandwidth of384 kbps (downlink) and a delay of 150 ms per way

4SMC Networks SMC2532W-B

(300 ms Round Trip Time (RTT)). The above PPP chan-nel models the 3G channel when the Protocol DataPacket (PDP) context is activated. The disconnectionand connection times were measured in different loca-tions of an office building with a commercial UMTSdata card. The round trip time is tuned to a typical valueof delay in this kind of channel under the same con-ditions. The connection time is measured as the timeelapsed between bringing up the card and the momentwhen an IP address is assigned to the Mobile Node (ac-tivation of a PDP context). Although the model takesinto account the connection time, we have assumed thatthe PDP context is always active, so the value of theconnection time does not have any impact on the simu-lations.

Our simulations are based on i) full 3G coverage andii) 3G link always on, which we argue that are realisticassumptions in typical scenarios.

VI. Evaluation of the results

The results obtained can be classified in three differ-ent categories. First, an analysis of theWireless LANutilization time versus the number of handoversis pre-sented, as a metric of the performance of the algorithm.Second, an analysis of theprobability of losing a Bind-ing Update is performed, to understand the effect of thealgorithm on the control plane.The packet loss due tosignal variationand its behavioras a function of the threshold are analyzed to detect theimpact of the different thresholds in a realistic environ-ment modelled by the wireless signal model used. Fi-nally, thepercentage of the different contributions to thepacket lossis studied to find the right tradeoff betweenseamlessness requirements and packet loss. The studyis complemented with ananalysis of the performanceof the algorithm in the configuration for zero packetloss. The simulations have been divided in two stages.First, we evaluated a wide interval for both thresholds(i.e. 3G → WLAN in the interval [-80, -65] dB andWLAN → 3G in the interval [-80, -70]) to understandthe trend of the algorithm’s behavior. In a second stagewe selected relevant points to find the threshold valuesthat achieve zero packet loss, this metric being the finalgoal of simulation study.

VI.A. Wireless LAN utilization time

Figure 5 shows the time the wireless LAN is used perhandover and the number of handovers performed forseveral combinations of both thresholds.It can be seen that as the threshold3G → WLAN (indBm) increases, the number of handovers decreases, but

Figure 5: Wireless LAN Time usage and number ofhandovers

the time a station stays in the WLAN increases. Thisshows that, by setting the3G → WLAN thresholds toa value large enough the algorithm can be configuredto avoid useless handover. In this way, only handoversthat allow the Mobile Node to be connected for a longertime to the WLAN APs are performed while excludingshort stays. This feature is desirable and ensures thatonly handovers increasing user WLAN experience areperformed.Although the number of handovers only depends on the3G → WLAN threshold, the wireless LAN utilizationdepends on theWLAN → 3G threshold too. As ex-pected, in the figure we can observe that the wirelessLAN utilization time increases as the3G → WLAN

threshold increases (in dBm). This growing trend ismaintained while increasing (in dBm) theWLAN →3G threshold.

VI.B. Binding Update loss probability

The main reason for BU losses is caused by the factthat the MN tries to perform a handover to wirelessLAN when the signal level is not good enough. In thissituation the BU or BACK can be lost. The data lossesassociated to this event occurred because the HomeAgent cannot send data through the wireless LAN link,when the MN is not present on the cell or even when itis present but the signal level is poor.

Figure 6 plots the probability of losing a BindingUpdate for varying thresholds. For all the configura-tions simulated there is one threshold that allows allhandovers to be performed without losing any Binding

Update. For completeness all the possible configura-tions of the thresholds have been simulated. In Figure6 (and all successive) it must be noted that the numberof packets lost is always minimized under the condition3G → WLAN(dBm) > WLAN → 3G(dBm).This is because when3G → WLAN(dBm) <=WLAN → 3G(dBm) there is a ping pong effect inthe 3G to WLAN handover, the mobile device executesa handover from 3G to WLAN because the WLANsignal level is greater than the3G → WLAN thresh-old, but immediately handovers back to 3G becausethe WLAN signal level is below theWLAN → 3Gthreshold.

Figure 6: Probability of losing a Binding Update forseveral thresholds

The Binding Update losses depend heavily on the3G → WLAN threshold. It can be seen in Figure6 that there is a level of this threshold (-74 dBm) inwhich Binding Update losses are reduced to zero. Inthe figure we can see also a dependency of Binding Up-date losses with theWLAN → 3G threshold, this isonly because the ping pong effects explained before if3G → WLAN(dBm) <= WLAN → 3G(dBm).

VI.C. Losses due to signal variation

The losses due to signal variation appear as an effect ofthe oscillation in the signal level. Even when the wire-less signal level is not sufficiently weak to trigger a han-dover to the 3G leg, fading or a high negative variationof the signal produces a loss of several packets. Theselosses depend on the distance to the Access Point. Fad-ing can happen by a third moving object (e.g. a personwalking near the AP) or by the movement of the MN(i.e., when going through a metal door or wall).

Figure 7 shows several histograms for the packet lossdue to variation of the signal. The X axis represents the

Figure 7: Effect of the thresholds in the packet loss dueto variation of the signal level

number of packets lost (each time a loss due to signalvariation occurs). The Y axis represents the probabilityof this packet loss. As the threshold3G → WLAN (indBm) is increased for a fixedWLAN → 3G thresholdthe losses due to signal variation vary from the positivevalues to zero. This behavior is the expected one sincethe signal variation depends on the distance between theMobile Node and the AP. If the3G → WLAN thresh-old increases, these losses decrease. In Figure 7, theright hand histogram shows how the probability of zeropacket loss increases when the3G → WLAN thresh-old is configured to trigger handovers only when theterminal is close to the AP.

Figure 8: Study of the different contributions to thepacket loss

VI.D. Study of the different contributionsto packet loss

Figure 8 shows a study of the different contributionsto the global packet loss for three different thresholds.The major reason for packet loss in all the configura-tions is the handover to the 3G leg. Losses due to signalvariation, that start just before a handover to 3G andfinish after the Home Agent has received the BindingUpdate (and the packets are sent through the 3G chan-nel), are accounted. The contribution to the packet lossbecause a Binding Update procedure fails, is less rel-evant. It tends to disappear after the threshold of -74dBm is crossed. The loss due to signal variation ap-pears for all thresholds but its effect decreases whenthe3G → WLAN threshold increases (in dBm). Thelosses in the case of handover to the 3G channel aremostly affected by theWLAN → 3G threshold, ascan be seen in the bottom graph of Figure 8.

In all the studies performed the minimum packet lossis of 14 packets/handover, a quantity that can be sup-ported by an appropriate buffer in the application layerin most scenarios, although it can be a problem for ap-plications with delay requirements. In next sub-sectionwe explore the needed thresholds to achieve zero packetloss.

Note that the time required by the mobility signalingdoes not have an impact on the packet loss, since theMN keeps using the old interface until the handover iscompleted. Hence, there is no interruption on the packetflow while the handover is ongoing.

VI.E. Zero Packet Loss

Figure 9 shows the wireless LAN utilization time,the number of handovers performed and the packetloss trend when a the threshold configuration forzero packet loss is considered. Zero packet losscan be achieved when a high3G → WLAN (indBm) (namely3G → WLAN values is -55 dBm)threshold is used, to eliminate the losses due toBinding Update losses and signal variation. Also ahigh WLAN → 3G (in dBm) threshold is needed toreduce packet losses due to signal variation, althoughthis threshold must be lower than the3G → WLAN

threshold to avoid ping-pong effects. In the resultsobtained, with the values3G → WLAN = −55dBm

andWLAN → 3G = −66dBm, we achieve seamlesshandovers.

The penalty imposed for the use of such highthresholds is clear since the number of handoversis drastically reduced. Note that the plots showncorrespond to samples where handovers had taken

Figure 9: Study of the performance obtained for 0packet loss

place. A 30% of the samples obtained, did not containany handover, in contrast with the other configurationsshowed previously where all samples presented han-dovers. The plots therefore represents relative values tosuch conditions.

The Mean Wireless Utilization Time, shownin Figure 9, depicts a decreasing slope while theWLAN → 3G increases (in dBm). This behaviorwas noticed previously. The Wireless UtilizationTime for a given3G → WLAN (in dBm) thresholddecreases while increasing theWLAN → 3G (indBm) threshold.

The number of handovers presents a stable shape, asthe difference between the thresholds is greater than thesignal variation, no ping pong effect occurrs. The ab-solute number of handovers is small (approximately 2handovers) and depends on the3G → WLAN thresh-old. If this threshold is high (in dBm), the Mobile Nodemust be very near the AP to perform a handover.

The packet loss per handover decreases dramaticallyuntil theWLAN → 3G threshold reaches -70 dBm. Itthen tends to zero forWLAN → 3G = −66dBm.

VII. Outlook and conclusions

There is a growing trend in mobile communicationstowards overlay networks and mobile devices with thecapability of using different access technologies. In thisscenario the mobile device must choose the right accesstechnology taking into account user preferences and

guaranteeing service continuity. In the short term, thecombination of WLAN and 3G (UMTS) technologiesis of the utmost importance.

The IEEE is working on the specification of the802.21 standard, that defines an architecture forterminals to support handovers between heteroge-neous networks in a technology independent way.The architecture is based on an intermediate layerbetween layer 2 and upper layers; this is the layerwhere handover decisions are taken. This intermediatelayer interacts with upper layers and with technologydependent lower layers. On the other hand, the IEEE802.21 draft standard does not define how the handoverdecisions should be made. In this paper we analyzeand implement architectural issues and integrate, ina simulation environment, layer two and layer threefunctionalities. Within this framework an algorithm isevaluated taking into account several metrics such asWLAN utilization time while minimizing the numberof handovers and packet loss.

The work presented in this paper studies, by meansof simulation (using OMNET++), a typical scenariowith UMTS universal coverage and islands of WLANcoverage. If WLAN coverage is available, it is pre-ferred because of cost and bandwidth reasons. Wehave defined an architecture for the mobile terminalbased on the IEEE 802.21 work and the use of MobileIPv6 to manage IP mobility. Handover decisions aretaken by means of two WLAN signal level thresholds,one to decide a handover from 3G to WLAN, and adifferent one to decide when to handover from WLANto 3G. The algorithm for handover decision has twoobjectives: maximize WLAN utilization and minimizeservice discontinuity. In the paper we have exploredthe influence of the thresholds for handover decisionon these parameters, namely, WLAN utilization andpacket loss, taking into account the interaction withMobile IPv6 behavior.

The results obtained allow to observe the trade-offbetween service continuity and WLAN utilization.The two defined thresholds proved flexible enough tomanage this trade-off, allowing configurations withpacket loss and high WLAN utilization, and alsoconfigurations with zero packet loss although achievinga much lower WLAN utilization. In this way, weshowed that, by configuring the two thresholds, wecan adapt the mobile terminal behavior, with respect tohandover decisions and to user preferences. Anotherinteresting result of the work was to see the flexibilitythat the IEEE 802.21 architecture showed for imple-

menting policies for handover decisions managing theinteraction with upper and lower layers.

The mobile terminal speed used in the simulationstudy (10 m/s) can be considered a worst case for thestudied scenario (pedestrian speed or slow moving ve-hicles are below that speed). We are currently workingon studying the behavior with lower speeds and tryingto extract a general relationship between speed, signallevel thresholds, and achieved WLAN utilization andpacket loss. This would allow us to define algorithmsfor handover decision that are based on signal levelthresholds and terminal speed, and how they must beconfigured to achieve some level of WLAN utilizationand service continuity.

VIII. Acknowledgements

The work described in this paper is based on results ofIST FP6 Integrated Projects DAIDALOS II. DAIDA-LOS II receive research funding from the EuropeanCommunity’s Sixth Framework Programme. Apartfrom this, the European Commission has no responsi-bility for the content of this paper. The authors wouldlike to thank Dr. Jose Felix Kukielka for his helpfulcomments.

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