High Speed Scalable Mobility ManagementArchitecture over Infrastructural WLAN

Syed Zubair Ahmad 1, Mohammad Saeed Akbar 2 Mohammad Abdul Qadir 31,2,3 Center for Distributed and Semantic Computing (CDSC), Mohammad Ali Jinnah University, Islamabad campus

1 szubair(jinnah.edu.pk, 2 saeed uaf@hotmail.com, 3 aqadir(jinnah.edu.pk

Abstract- Mobility management in a fast moving environment isconvoluted by issues like speed of movement, detection ofmovement, handoff processing and scarcity of media resources.Most of the proposed architectures and solutions for mobilitymanagement use complex processing to reduce handoff latencies.In this paper a light-weight fast mobility management scheme isproposed which is based on strategic deployment of WirelessLAN (WLAN) hotspots in a hierarchy of overlapped componentslike Handoff Anchor Points (HAP), Access Routers (ARs) andAccess Points (APs). The major handoff decision is taken in theHAP, a novel component in proposed architecture, which last fora significantly longer time due to reduced chances in probablemoving path and by sending Early Binding Updates throughactive ARs. Handoff latency has been reduced to almost linklayer handoff time with a high level of predictability andreliability. Simulation results highlight improvements achieved inthroughput, scalability, and handoff latencies.

Keywords- Fast Mobility Management, Movement Detection andPrediction, Link Layer Triggers, Wireless LAN (WLAN) Hotspots

I. INTRODUCTION

Fast mobility in a relatively small sized cell deployment cancreate serious confusion at handoff time because of the shortduration stay in a cell and multiple signaling from neighboringcells which may cause channel interference problems. Thissituation is much more predictable and reliable in cellularnetworks where the handoff decision is taken by the networkon the basis of signal quality and strength of neighboring cellsreported by the Mobile Node (MN). Wireless LAN has adifferent architecture as compared to the cellular networks;especially the frequency band used in this network is an IMSband which can easily be used by other devices in thesurrounding causing collisions and signal degradation [4].This problem is further complicated if we consider a multiplesignaling region due to overlapped WLAN cells as proposedby many researches [4] . Fast mobility in such scenario requireadditional processing overheads like cache management,profiles based handoff decisions or addition of another shimlayer for mobility management [1] [3] [5]. This additionaloverhead can become a bottleneck in situations where morethan one handoff is essential within a second. This highlightsthe need of a architectural framework that distributes somemobility and handoff decision making load on appropriatenetwork components such as Access Routers (AR) to save MNprocessing load and reduce handoff latency.

Presently wireless LAN (WLAN) hotspots are mostlydeployed in small community areas with a cell range ofmaximum 300 meter. Although this area may be good enoughfor a moderately fast moving MN for handoff completion butneeds to be predictable about its path and time it stays in thecell. An MN moving with a speed of 60km/hr may cover adistance of 17 meters in a second. Hence ifMN follows a paththat is the maximum in the cell i.e. cell diameter, as shown inpath A of Figure 1, then it can stay a maximum of 18 secondsin a cell moving in straight line. MN may have even longerstay in the cell if it follows a path like Path B of Figure 1.Considering a layer-3 handoff latency of about 1 second, itprovides a good ratio of total cell stay time to the handoff timeof 18:1. In case MN does not follow the maximum path in thecell by passing through two points close to each other in a cellperimeter, shall leave virtually no time for a handoff decisionmaking mechanism to work properly. Further it takes sometime before the Layer-2 of MN receives proper frames fromthe Access Point (AP) covering this cell as shown by thecentral dotted line of Figure 1. This delay may be due to eithernon availability of the media or weak signal quality.Eventually handoff process of layer-3 can be started at theearliest after crossing the inner middle circle which willreduce the cell stay time. If the signal quality threshold (SQT)is also a parameter for handoff decision then, handoff cannotstart before the inner most circle is crossed which furtherconstraints the stay time of MN in the cell. It is thereforeimportant to reduce the uncertainly of path and direction ofmovement ofMN so as to increase the certainty in the handoffdecision making. This highlights a need of a handoff loadbalancing which mean that some processing burden be takenaway from MN and may be placed on the networkcomponents such as ARs and APs. Future evolution ofWLANlike mobile Wi-Max based on 802.16 and Media IndependentHandoff (MIH) based on 802.21[4] shall provide much largercoverage in a heterogeneous environment shall need welldefined robust infrastructure deployment for proper handlingof fast mobility.

Considering this scenario a strategic deployment of certainWLAN components based scalable architecture is proposed inthis paper in which handoff load of MN has been shared bycomponents like ARs and new proposed component namedHandoffAnchor Point (HAP) which are deployed in an orderlymanner to cover the areas where fast mobility is possible.Network topology is anchored around HAP and hence passing

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through HAP ensures a specific path to be followed in the nearfuture. Rest of this paper is organized as follows. Section IIprovides a brief overview of related work and background offast handoff mechanism. Section III provides the detailsrelated to architecture and design of proposed scheme. SectionIV provides analysis of proposed architecture with someempirical values of network latencies. Section V provides thesimulation results of proposed scheme. Section VI presentsconclusions and future work in this direction.

Pather 2 triggers

Path A

Figure 1: A wireless signaling model with path information

II. BACKGROUND

Although handoff management in mobile networks has beenkeenly studied by researchers for quite some time now, fastmobility has been focused only recently. Fast handoff forMIPv6 [1] has been proposed to minimize latencies related tocell change during fast mobility. It is based on movementdetection with the help of layer 2 triggers through anticipationof expected handoffs. Although a timely anticipation ofmovement and location of a mobile node can greatly reducehandoff time, the accuracy of anticipation generally does notcompensate for complexity of anticipation process. It meansthat despite heavy processing overhead of anticipation, thechances of a different than anticipated location of a MN arestill high. Explicit proactive handoff with motion prediction[3] is another scheme proposed for mobile IPv4 in which MNrecord its movement pattern and uses it for prediction ofpossible future location [9]. On receiving a link layer trigger,network layer sends a list of forward networks to the existingforeign agent (FA) which is in advance informed about thearrival of a new MN along with duplicate forwarding ofexisting sessions of MN packets to both present and expectedfuture FA. MN on receiving a duplicate packet discards onarrival in new FA. This scheme is proposed for MIPv4 but canbe adopted in MIPv6 environment as well. This scheme can bevery useful under good bandwidth availability or lesser mobiledevices but can proliferate additional path frames on the mediaresulting in high collision chances and reduced handoffthroughput at high node density. This not only constraints thelimited bandwidth but also duplicates information processingat different nodes. Early binding fast handoff (EBFH) is aproposed scheme under MIPv6 for ensuring rather thatpredicting a mobility event [6]. It used IEEE 802.16 triggerslike link-up link down, QoS threshold etc. to decide a handoffevent. It assumed an underlying infrastructure of

geographically linked access routers (AR) to send fast bindingupdate through existing AR. This can ensure reliable way ofknowing the next subnet rather then predicting. Overlappedcell architecture is used to provide a breathing time for thehandoff operation but the uncertainty still prevails on thepossibility of stay in the overlapped region. The path that anMN follows and the moving speed are two crucial factors thatmay restrict the utility of this scheme.

III. PROPOSED FAST HANDOFF SCHEME

In this paper, major emphasis has been given to a healthyratio of the time an MN stays in a cell to the time required forhandoff completion. This is termed as cell stay time to handofftime ratio which has been used to identify good handoff underfast mobility. It is obvious that this ratio should besignificantly high to make a seamless handoff and support realtime applications. Proposed scheme is based on the four basicparameters that can be significant in the future mobile wirelessnetworks. These include moving speed of MN, cell size of thewireless hotspot, density of mobile nodes in a specific cell andstrategic placement of hotspots to provided maximumcoverage along with good signal quality. First of all the speedof an MN is crucial with respect to the probable stay inside acell with good signal quality. Cell size can be different underdifferent wireless LAN/MAN standards. Considering IEEE802.16 with a broadband service under 10km coverage rangethe cell size is significantly large and an MN moving with aspeed of 120 km/hr may stay around 300 seconds in a cell.This leaves good space for the smooth handoff with a soundratio of cell stay time to handoff time. In case of 802.11 basedWLAN, this ratio shall be significantly low as discussed inintroduction section. Cell density is important because of thepossible limitation of a number ofMN in cells along with theincreasing load of mobility management in that cell that cangreatly increase handoff latencies. Increased MN density in acell can also cause buffering problems which is majormechanism for seamless handoff. Strategic deployment of cellis important from the fact that the signal strength reducesdirectly with the square of the distance from the access point.So despite a theoretical range of 10km in Wi-Max and 300meter in Wi-Fi, signal quality can be significantly low after aloss of 3dB. Hence we need a certain level of overlappingbetween the successive cells and be sure upto a reliablecertainty about the path which is more common for thepresence of MNs. Since fast mobility can only be possiblealong well defined road infrastructure, it is proposed to haveaccess points deployment along road side to provide a highsignal quality at high speeds. This setup can greatly enhancethe maximum exposure to overlapped region and provide ahigher degree of certainty of path followed by MN.

A. ArchitectureProposed scheme is centered around a decision making routerwhich acts as the anchor point of the handoff operation. It isnamed as HandoffAnchor Point (HAP) due to its macro levelmobility decision making role. Although its role is anchoringof handoff operation, its functionality is much different from

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other such components proposed in micro-mobilitymanagement schemes like distributed Hierarchical MIP [7]. Inthis architecture all ARs between two HAP lying on a pathsegment form a neighborhood subnet. The HAP may be linkedwith multiple subnets and handovers an MN to a specificsubnet on the basis of its direction of movement. This decisionabout the probable direction of movement is made throughsignal strength at the specific interface of HAP, withoutsupport of any positioning or tracking system. As an MN takesmovement on a specific route and HAP finalizes its direction,forward ARs are informed about its future arrival till the nextHAP in the path. A mobile node sensing its signal strengthwith a specific AP decreasing; shall send a Binding Updatefor the next AR through present AR without knowing next ARaddress. Alternatively HAP shall also send the future ARaddresses to the MN on its entry into a specific path. Thisgreatly reduces the overhead of cache and databasemanagement along with any uncertainty of future point-of-attachment. Figure 2 presents an architectural view ofproposed scheme. HAP is placed at the intersection of allpaths resulting in a multi-coverage area from all surroundingsubnets. The placement of components like AR, HAP and APis chosen with the prime motive of simultaneous signalingavailable from two different subnets and no region withoutgood quality signaling is left. The functional issues related tomajor components in the scheme are discussed in nextsubsections.

B. HAP FunctionalityHAP is equipped with multiple interfaces linked withsurrounding subnets with good link characteristics. Itscoverage area is kept low with significant size of overlappedregion with neighboring subnet ARs. HAP will not be actingas a handoff node rather performs a handoff coordinator role.Since MN has still good link characteristics when it entersHAP coverage area, its second interface detects HAP beaconsand responds with a Beacon Reply message identifying itselfand the AR with which it is currently attached. After that MNstarts sending its signal strength information to the HAP afterevery 2OmSec. This information helps HAP in deciding itsdirection of movement. At a specific point where signal levelof the current AP decreases and new AP increases to athreshold value, HAP takes a handoff decision and sends MNa list of ARs that it will travel till the next HAP in the path.HAP also sends packets to both previous AR (pAR) and newAR (nAR) about the handoff. This message contains MNMAC identity and identity of nAR. This message will servedual meanings to the two ARs. The pAR will complete itshandoff operation and forward any packets for the MN to thenAR. Additionally HAP is responsible for Detection of MNwithin a coverage area of 300m along with probability ofmovement direction, finalizing decision of forward path,maintenance of signal strength threshold values, maintenanceof subnets in each direction and forwarding MN informationto probable future subnets. The signaling sequence betweenMN, ARs and HAP is presented in Figure 3. It is important tonote here that IMS band is used in WLAN 802.1 1x standards

and channel allocation is contention based which may causevariable delay in directional assessment. Multiple interfacecards have a problem of interference but provide a means forsoft binding.

Figure 2: Architectural view of HAP along with surrounding subnets

C. Access Router FunctionalityAR performs the activates related to Router Advertisement,relaying Early Binding Update and Registration Request tothe Home Agent (HA) and Correspondent Node (CN) whichare defined in the standard MIPv6[2]. One important additionis the HAP's Node Approach message that is used to informthe next AR about the arrival of an MN. In response to thismessage AR sends a Router Advertisement to the mobile node.When MN leaves the coverage area of this AR it receives anEarly Binding Update message from the MN to forward it tothe next router in the forward path of the MN. This achievesmake-before-break binding with the next AR in the path.Previous AR also provides bi-casting of packets to both MNand next AR router after receiving Early Binding Update fromthe MN. ARs are also overlapped to provide handoffcompletion time to minimize the bi-casting which is expensivein term of scarce media resource. Figure 3 and 4 presentmessages received and sent by the AR to complete handoffoperation.

D. Mobile Node FunctionailityMN has its core responsibility to detect the link layer triggerslike Link-up, Link-down, Link-Quality-Low, Signal-Threshold-Gain, Signal-Threshold-Lost etc. to decide about the handoffinitiation. On the Detection of a Signal-Threshold-Gain trigger,MN estimates a possible stay time in this cell on the basis ofits previous cell experience. Since the cell sizes are same to alarge extent, MN becomes vigilant about the next handoffinitiation and possible stay time in this cell. MN sends anEarly Binding Update message to the present AR as it detectsSignal-Threshold-Lost trigger from the link layer. Whilepassing through coverage area of HAP, MN receives aHandoff Initiation message from HAP to be ready for a RouterAdvertisement from the approaching AR. Figure 3 and 4

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present messages received and sent by the AR to completehandoff operation.

Threshold Lost

Li,-Down

Node Detectd

Router Solicitation

Next Subnet Info

Router Advertisement

Move Detection

Node Arrival

Node Detetl l

Registration Request

Figure 3: A timing diagram for sequence of exchange messages by HAP

THandoff= TDiscovery + 2 * T Processing.MW +2 * T WirelessDelay + 2 *TProcess AR + 2 *TInternet Delay + TProcess HA

(1)

Where TDiscovery, TProcessing_N'N TWireless Delay, TProcess AR , T InternetDelay, and Tprocess-HA are Agent discovery, MN Processing,Wireless transmission, AR Processing, Internet passage andHA processing delays respectively. This is the case of normalhandoff procedure under IPv6. Empirical values of thesedelays yield a delay of 1.25 Sec as given below

T Handoff = I sec+ 2*1mS +2*lOms +2 * ims +2*100ms+lms) = < 1.25 Sec (2)

In proposed scheme the average handoff time has beenreduced near to the time it takes to complete layer-2 handoffalong with Early Binding Update (EBU) Messaging which arecompleted in the overlapped region by using existing AR link.This reduces delays of Discovery process which has thehighest contribution in overall handoff delay. This increasesthe cell stay time to handoff ratio significantly eventuallyallowing smooth handoff.

Data Transfer

| Threshold Lost

EBU Message

Bi

Routef

.Regist

V Link-Down

EBU Relay

Casting --

Aertsement

ation

i t~~~~~~~~~~~0

NodeArrival

Figure 4: Timing diagram representing message exchange between ARsand MN

IV. ANALYSIS

The main idea behind this research is to increase the cell staytime to handoff time ratio in a WLAN mobility environment.It has already been discussed in the introduction section thatcell stay time shall be in the range of 10 to 20 secondsdepending on the path followed by the MN in case of Wi-FiLAN [10]. Considering a file transfer application going on andfile transfer takes 10 minutes then there can be around 30 to60 handoffs during this file transfer activity. Normal layer-3handoff time is in access of 1 second which means that thereshall be 50 to 90 second consumed in handoff in a total of 600second session. This is around 10 to 18 % loss of time anddata. A breakdown of handoff time under normalcircumstances is given in (1).

Data losses are also handled in proposed handoff scheme.Data losses are due to a temporary discontinuity of the sessiondue to change in POA. There are two common strategies tohandle this situation one is enabling buffering and the other isbroadcast in a specific domain. Broadcast is expensive withrespect to radio resource where as buffering can be expensivein terms of host resources and may lack scalability. Bufferingcan have serious problems if the handoff time is high and thereare multiple MN processing handoff operation. Since thisscheme takes under 1200 mSec to complete handoff, a veryhigh data rate of 512 Kbps will require around 96 buffers of1KB each (about 20 % buffering) for each mobile nodehandled in the cell. Although this is a high amount as far asbuffering is concerned but is manageable under high memorysizes of modem devices. Further such high data rates areseldom used during mobility. This also rules out and multicastpossibility because of its high bandwidth consumption. Thisalso raises questions about how much node density can beaccommodated in a cell for providing seamless handoff. Earlybinding updates can reduce buffering requirementconsiderably as mostly buffering will be used under 5 % oftraffic of handoff time. Following simple relationship canmodel a buffer management strategy.

Buffer Size = Bandwidth * HandoffDelay (3)

Here, Frame length of the MAC along-with its relatedoverhead has been ignored for simplicity. This equationsuggests that reduced handoff latency can greatly decrease thebuffering requirement. Since handoff latency is in the range ofa few hundreds of milli-sec, buffering support for proposedscheme remains scalable even under relatively high nodedensity of about 10 nodes in a cell.

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V. SIMULATION RESULTS

The proposed scheme has been simulated in ns-2 simulationenvironment. Figure 5 presents a layout of simulation setup.As shown, the placement of ARs and HAPs has been keptfixed along the movement paths with suitable overlapping.Since these ARs work as hotspots, the numbers of nodes thatcan be attached to a cell are generally scalable up-to the limitof WLAN segment, though larger node density increasescontention and relativity complex back-off contention windowmanagement may arise. For an object moving with a speed of60 Km/hr (17m/s) and having a handoff time of 500 mSecsuitable hotspot overlap distance comes out to be around 8.5meters. In case of a longer handoff time like 1 second or morehotspot coverage distance approaches 20 meter or so which iseasily managed under the deployment shown in Figure 5. It isassumed that the WLAN interface activation takes place aftera sizable distance of about 10 meters covered in the hotspotcoverage area. Hence a total distance needed to be covered inthe hotspot to ensure a seamless handoff comes to be around20-30 meters. TCP and UDP traffic was generated for asimulation time of 100 second with application of CBR andFTP type. The path of movement was restricted in specifiedrouts which are labeled in the Figure 5 as movement paths.The speed of movement was varied from 10 to 25 m/s in a stepof 5 m/s. One important focus point of this simulation is thestudy of node density behavior on handoff and throughput. Forthis purpose number of nodes varied from 1 to 16 while theirmovement was relatively stationary to each other as all thenodes moved with same speed at the same time to test theimpact of node density on handoff latencies.

Figure 5: A General simulation scenario of proposed scheme

Graph shown in Figure 6 presents a basic TCP throughput atdifferent node density. It can be seen that the average TCPthroughput declines at a sharp rate as the number of nodeswith different TCP session increase. This is attributed to thelarger handoff latencies along with more congested paths forhigher node density. The higher handoff latencies at highernode density increase Round-trip Timeout (RTO) whichcauses congestion window decrease sharply. The packet dropdue to either buffer overflow at ARs or non-availability of

path may also result in decrease in congestion windowresulting in reduced throughput. Since TCP Tahoe model wasused for this experiment, possible oscillation of congestionwindow size may also be a cause of low throughput at highnode density. Since RTOs are most probably due to handoffdelays so congestion window may readjust as per its algorithmdemand. It's important to note that even at maximum nodedensity and high speed throughput does not fall to zero andTCP still manages some bandwidth at a very high competitionand fast speed. Figure 7 presents a comparison between TCPand UDP traffic under fast mobility. A single node movingwith a speed of 20 m/s was evaluated and the throughputachieved is shown. The basic object of this experiment was tofind the contribution of TCP overhead in traffic throughputunder fast mobility environment. The graph has a gap ofaround 10 to 15 kbps during the normal and handoff situationwhich shows sizeable load ofTCP overhead along-with its selfsustained congestion management scheme. UDP traffic fall isdue to the bi-casting of the packets which may arrive late dueto their path from new AR and may be out of-order whichshow a decrease in the first case but result in rise in the nextcell binding. Figure 8 presents a signaling load against thenumber of handoff events per minute. The signaling loadincreases with the increased rate of handoff events since the amultiple node at fast speed generate higher signaling. Thegraph shows linear increase of signaling overhead with someupward variations at the HAP. Since HAP has larger signalingdue to the multiple interfaces then nodes from different pathconverge at this point, so when mobile node passes throughthis region signaling load is high. Once the MN has movedthrough the HAP region the signaling load is minimum duethe optimized organization of ARs in an overlapped hierarchy.Figure 9 presents a relationship of handoff latency againstvarying node density. The graph shows a linear increase ofhandoff latency up-to node density of 8 mobile nodes and thenrises exponentially for higher node density. This behavior isdue to the increased processing overhead as multiple soft-statehave to be managed resulting in a thrashing like situation atthe AR. The multiple soft-state management is generallyconsidered as a means for seamless mobility but required extraprocessing power and optimized algorithms for properoperation. In addition to this, chances of media accesscollisions are also high at high node density which may causedelayed binding updates resulting in higher handoff latency.

Throughput vs Node Density at varying speed

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< 25- 20Qs 15

2 10

50

* Speed ( 10 m/s)Speed ( 15 m/s)Speed (20 m/s)Speed (25 m/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

No. of Nodes

Figure 6: Comparison of TCP throughput under proposed scheme atvarying speeds and node density

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TCP/UDP Throughput of proposed scheme

30

-;, 25acom 20

a 15

0 100

1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97

Time (Seconds)

Figure 7: Comparison of TCP and UDP throughput of a sin'moving at a speed of 20 m/s

Signalling Load vs Handoff Events

35

,,, 300m 25

_ ; 20R _- 1550)en 10

0)0 5

0

---Sign(

1 2 3 4 5 6 7 8 9 10 11 12 13

Handoff/Minute

Figure 8: Handoff delay against varying load of mobile nodes

Handoff Latency vs Node Density

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movement direction is estimated using signaling strengthreported by MN of the surrounding APs and once the node hasattained sufficiently high signal strength in a new cell, it ishanded over to that AR. The geographic neighboring of theplacement of ARs, HAP and APs develop a natural IP

--*--TOP neighboring giving a robust routing infrastructure that persistso UDP for longer duration. The next policy decision about handoff

will be encountered till reaching next HAP and in betweentwo HAPs, MN will need only limited signaling to completehandoff. Recent research in mobile Wi-Max shows that soonera long range omni-directional WLAN will be available whichmay need similar infrastructure to provide a long distance highdata rate mobile network services. Experimental results have

gle node shown promising results for its deployment at large campusarea or moderate size community areas with known mobilitypaths. In future it is planed to study the impact of soft statemanagement and its processing overhead. It is also recognizedthat some interference issues complicate the channel allocationin overlapped regions for which a non-overlapped strategywith significant buffering is also in plan to be tested. Further,assistance from other network to facilitate handoff is also

alling Load planned to be tested for a study of vertical handoff underheterogeneous wireless network environment.

REFER1ENCES[1]. E.R. Koodli. Fast handovers for mobile IPv6. Technical Report

RFC 4068, IETF 2005.[2]. J.A.D. Johnson, C. Perkins, Mobility support in IPv6.

Technical Report RFC 3775, IETF 2004.[3]. Hocheal Kim, Young Kim. An Early Binding Fast Handover

for High Speed Mobile Nodes on MIPv6 over ConnectionlessPacket Radio Link. In proceedings of SNPD-06, 2006. pp. 237-242

[4]. D. J. Vivek Gupta. A generalized model for link layer triggers.Technical Report IEEE 802.21, IETF 2004.

[5]. J. Lee, S. Kimura, and Y. Ebihara. An Approach to FastHandoff Scheme for Mobile IPV6 in Cellular IP Networks. InProceedings of Communication Systems and Applications 2006

[6]. Norbert Jordan, Rainer Huber, Joachim Fabini, A simulativeStudy on the Performance of Fast signaling in a Mobile IPv6-Wireless LAN based Network Environment, Proceedings ofICMB 2005.

[7]. J. Xie and I.F. Akyildiz. A distributed dynamic regionallocation management scheme for mobile IP, in: IEEEINFOCOM 2002, Vol. 2 (New York, USA, June 2002) pp.1069-1078.

[8]. Akyildiz IF, Jiang Xie & Mohanty S. A Survey of mobilitymanagement in next-generation all-IP-based wireless systems.IEEE Wireless Communications 4: 16-28. (2004)

[9]. Sharma S, Zhu N & Chiueh TC (2004) Low-latency mobile IPhandoff for infrastructure mode wireless LANs. IEEE JSelected Areas in Communications 4: 643-652.

[10]. S. Shin, A. Forte, A. Rawat, and H. Schulzrinne, ReducingMAC layer handoff latency in IEEE 802.11 Wireless LANs.ACM MobiWAC,2004, Philadelphia, PA USA, pp. 19 - 26,Septermber 26 - October 1 2004.

Handoff Latency

01 2 3 4 5 6 7 8 9 10 11 12

No. of Nodes

Figure 9: Signaling load of handoff events

VI. CONCLUSIONS

In this paper an integration ofWLAN with a decision makingnode called HAP has been proposed to achieve a seamlesshandoff operation. Fast mobility requires robust infrastructurelike cellular networks to perform well. WLAN are providingmore bandwidth then cellular network but have a coveragelimitation of a few hundred meters. In this paper a scheme hasbeen proposed which is simple, lightweight and robust whichprovides low latency handoff under fast mobility. The corecomponent introduced in this paper is a Handoff Anchor Point(HAP) which acts as a facilitator for handoff operation andhandovers a mobile node from one subnet to a second subnetdepending on the direction of movement of node. The

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