An Improved Adaptive Decision Scheme for

Vertical Handoff in Heterogeneous Wireless

NetworksHuiling Jia, Peng Cheng, Zhaoyang Zhangt, and Shiju Li

Institute of Information and Communication EngineeringZhejiang University, Hangzhou, China 310027

Email: wannahealthy@hotmail.com, uptomorrow@ 163 .com, {ning-ming, leesj }lzju.edu.cn

Abstract-Vertical handoff is the switching process of a multi-interface mobile host between heterogeneous wireless networks.There have been many researches focusing on policy-enabledvertical handoff decision schemes, but no technical details onhandoff metric acquisition are provided in most of them. In thispaper, we introduce the MAC layer sensing technique, which isdesigned to estimate the handoff metrics in IEEE 802.11 WLAN(Wireless Local Area Network), into the policy-enabled decisionschemes. Furthermore, we improve the existing adaptive schemesfor stability period prediction, making use of the handoff metricsobtained from MAC layer sensing. Simulation results show thatthe proposed adaptive decision scheme outperforms the previousschemes in terms of sensitivity of stability period adjusting andability to avoid unnecessary handoffs.

I. INTRODUCTION

With the widespread success of wireless and mobile com-munications, a large variety of wireless systems have beencreated, including the second and the third generation cellularsystems, satellite systems, and wireless local area networks(WLANs), etc [1]. In addition, the emerging technologies offixed broadband wireless access (FBWA, IEEE 802.16 [2]) andmobile broadband wireless access (MBWA, IEEE 802.20 [3])are also expected to be promising in near future. Therefore,one of the most significant challenges for the next generationwireless networks is how to coordinate these heterogeneousnetworks, and how to manage user mobility among differentwireless technologies, which is referred to as inter-networkroaming or vertical handoff [4].

The major goal of mobility management in heterogeneousnetworks is to support seamless global roaming. Here, by"seamless", we mean that upper layer applications should beunaware of the roaming events when the mobile host (MH)changes its point of attachment. In order to achieve seamlessroaming, three problems must be properly resolved: systemdiscovery, handoff decision, and handoff execution [5], whichconstitute the entire process of vertical handoff. In this article,focus will be put on the second problem, i.e., to evaluate thereachable wireless networks and to select the best one as thehandoff target, according to a predefined decision scheme.One of the most well-known decision schemes is the policy-

enabled scheme proposed in [6], which consists of threeessential elements. First, a series of parameters called "handoffmetrics" are defined, which are measured to give an indication

tCorresponding author

of whether or not a handoff is needed. Second, a "costfunction" is proposed to quantify the cost of using a candidatenetwork at a certain time, which is a function of the handoffmetrics mentioned above. The network with the minimum costvalue is selected as the handoff target. Third, the MH shouldobserve if the target network is consistently better than thecurrent one for a "stability period" before performing handoff."Stability period" is defined as the interval between the time offinding the target network and the time of starting to performhandoff into it.

There have been several works focusing on the design ofcost function and the estimation of stability period. Wang et al.[6] presented the basic formulas of cost function and stabilityperiod. Then, in [7], several optimizations were put forward toimprove the calculation efficiency of the basic cost function. In[5], Chen et al. proposed two adaptive schemes to dynamicallyadjust the stability period according to the variances of thecost values. However, in these works, the method of handoffmetric detection is not fully addressed. Accordingly, a MAClayer sensing (MLS) technique [8] was developed to estimatethe available bandwidth and the mean access delay in WLAN.But, the handoff decision scheme adopted in [8] was notpolicy-enabled, and only a fixed observation window wasused to evaluate the necessity of vertical handoff. In thispaper, we introduce the MLS technique into the policy-enableddecision schemes performed in a WLAN/WWAN (WirelessWide Area Network) hybrid network, and improve the adaptiveschemes for stability period prediction proposed in [5], usingthe handoff metrics estimated from the MLS technique.

The rest of this paper is organized as follows. Section IIreviews some previous works related to decision schemes forvertical handoff. In Section III, we describe our improvedadaptive decision scheme in detail. Performance improvementsof the proposed scheme are presented in Section IV. Finally,Section V concludes this paper.

II. RELATED WORKS

In [6], the first policy-enabled decision scheme for verticalhandoff in heterogeneous wireless networks was proposed.This scheme introduced a cost function for candidate networkevaluation, which is composed of several handoff metrics, suchas available bandwidth, power consumption, and monetary

0-7803-9584-0/06/$20.00(2006 IEEE. 1816

cost, etc. Related notations are listed as follows:fn: The cost of using network n at a certain timeBn: The bandwidth that network n can offerPn: The power consumption of using the network accessdevice of network nCn: The monetary cost of network nwi: The weight or importance of the ith handoff metric(Zww = 1)Ni: The normalization function of the ith handoff metric.

The cost of using network n at a certain time is defined as

fn = Wb N(l/Bn) +wp N(Pn) +wc N(Cn) (1)As soon as the target network is decided, a stability period

(Ts) must be calculated to determine whether incurring the costofhandoff is worthwhile. According to the common practice ofpredicting the future from the recent past, if the target networkis consistently better than the current one for a duration of Tsbefore performing handoff, the target network is very likelyto keep in this state for another Ts after the handoff, thus thevertical handoff is worthwhile. The final form of Ts given in[6] is written in (2).

Ts = ihandoff + lhandoff fcurrent (2)rr-1 ftarget

where, ihandoff denotes the handoff latency, and r (cost ratio)is the ratio of the cost value of the current network to that ofthe target network. The better the target network, the largerthe value of r, and the shorter the duration of Ts.

However, the cost values of the current and the targetnetworks vary dynamically with the condition changes in thesenetworks or in the MH. As a result, Chen et al. [5] proposedtwo adaptive decision schemes in order to adjust the stabilityperiod according to the newly updated cost values.

In the first adaptive decision scheme, a vertical handoff willbe performed only when the target network can successivelyexcel the current one in cost evaluation for N times. Theevaluation interval between the mth evaluation operation andthe m+lth evaluation operation is defined as

T+Ihandof handoffTm N N .(rm -1) (3)

where, rm is the cost ratio obtained in the mth evaluationoperation. Accordingly, the total length of stability period isgiven by

TS = E (i o + N .(r1T)) (4)

During the N evaluation operations, whenever the targetnetwork becomes worse than the current one (i.e., rm < 1),the decision procedure will be terminated, and will not berestarted until the target network excels the current one again.

In the second adaptive decision scheme, the initial stabilityperiod (Tis), determined by (2), is divided into Total Indexadvertisement intervals (Ta). During each advertisement in-terval, a cost ratio evaluation is performed, and the valueof Total Index will be adjusted if the varying rate of cost

ratio exceeds a predefined precision (e), where the varyingrate in the mth advertisement interval is defined as (rm-rm 1) /rm1l. The follows summarize the main idea of thisscheme:

1) InitializationSet m = 1, Total Index F=1T;

2) While (m < Total Index)if >1 )Total Index m + LTotal-Index

else if ( rm 1Total Index m + F Total Inde/

V-ml11I2

V-ml.I

elseTotal Index = Total Index;

endm = m +1;

3) T, = Total Index Ta;Both of the two adaptive schemes can dynamically adjust

the stability period according to the latest updated cost values.Therefore, handoff decision can be made faster than the non-adaptive scheme [6] when the cost ratio is increasing, andmany unnecessary handoffs can be avoided when the costratio is decreasing. However, acquisition method of handoffmetrics, which is the foundation of vertical handoff decisionschemes, is not fully addressed in [5]. Furthermore, handofflatency is assumed to be fixed, which may vary dramaticallywith the changes in network conditions in the target network.In the next section, we propose an improved adaptive decisionscheme to solve these problems.

III. IMPROVED ADAPTIVE DECISION SCHEME

A. Challenges in Handoff Metric AcquisitionIn order to achieve seamless vertical handoff in hetero-

geneous wireless networks, the network conditions shouldalways be obtainable by appropriate handoff metrics. How-ever, this is quite challenging because there does not existcomparable signal strength to be utilized as physical layerhandoff metric due to the different physical techniques. Thisis further complicated by the fact that different wireless accesstechnologies offer different QoS parameters, such as availablebandwidth, access delay, and packet error rate, which aredifficult to obtain compared with the physical layer parameters.As a result, in most researches on vertical handoff decision[5][7][9], no technical details on handoff metric acquisitionwere provided, which incurs doubts about the practicabilityof the decision schemes, because performance of verticalhandoff may be discounted if some handoff metrics can notbe obtained, or can not be obtained so frequently in practice.

B. MAC Layer SensingIn [8], a MLS technique is proposed to detect network

conditions in V/LAN, such as available bandwidth and ac-cess delay, when a MH moves into the overlap area of aWLAN/WWAN hybrid network. The MLS technique providesa handoff metric acquisition method according to the end-to-end principle [8], therefore policy-enabled handoff decision

1817

TTn / TTn 1

schemes can be utilized in practical heterogeneous wirelessnetworks without introducing new network entities.

In IEEE 802.11 V/LAN [10], network allocation vector(NAV) is the main scheme to avoid collision by setting abusy duration on hearing frame transmissions from other MHs,therefore it can well reflect the channel busy status. In [8],it is proved that there exists a relationship between NAVand the available bandwidth in WLAN, as well as the meanaccess delay of VWLAN. Consequently, by listening to andcollecting NAV in MAC layer, available bandwidth and meanaccess delay during the observation window can be calculated.According to Appendix A of [8], the available bandwidth(BW) is calculated as

L NAVBW B=BoT + Tn,c,(M- 1) /2

where, NAV is the average of NAV in the observationwindow; Bo is the total system bandwidth of WLAN; L isthe mean frame size; M is the average number of trials of atransmission; Tn is the NAV duration for a successful frametransmission; Tn,c is the duration for a collision.

In Appendix B of [8], the mean access delay (Delay) indistributed coordination function (DCF) mode is given by

Delay = b/2 (PcTc,l + (1 -Pc)Tsuc + V(CWmin))S-1

+ S Pi (PoTsucP'1 + Tco0 + v(2jCWmin)) (6)j=1

where, b is the channel busy time per second; Tsuc is thetime length of a successful transmission; Tcoi denotes thelength of a collision; Pc is the collision probability; S denotesthe maximum number of trials before a frame is discarded;CWmin is the initial contention window range defined inDCF mode; P1 denotes the probability that a transmissionfrom other users is inserted in each time slot; accordingly, Podenotes the probability that no transmission is inserted; v(D)is defined as the mean of u(D) (see equation (A-12) in [8]),where D is uniformly distributed in [0, D].

C. Proposed Adaptive Decision SchemeIn this paper, we propose an improved adaptive decision

scheme of Chen's schemes [5], and our research is focusedon the WWAN to WLAN roaming direction, leaving the casein opposite direction to be studied in future work. In Chen'sschemes, two handoff metrics, i.e., available bandwidth andMH's moving speed (Sn), are considered in the cost function(see equation (7)), and it is assumed that there exists a locationservice server (LSS), which can provide all the necessaryhandoff metrics of the candidate networks. However, in theproposed scheme, the available bandwidth in WLAN canbe obtained by the MLS technique, and because the MHis attached to WWAN before it moves into the coveragearea of WLAN, the available bandwidth in WWAN can beestimated from the obtained data rate in WWAN. In addition,measurement of MH's moving speed can be realized byDoppler frequency shift measuring or the Global Positioning

System (GPS) equipment in the MH. Therefore, the same costfunction (7) can be used in the proposed scheme, without anyassumptions on technical details.

fn = Wb N(I/B,) + w5 N(Sn) (7)

In addition, when we introduce MLS into these policy-enabled decision schemes, several modifications are made toboth of them:a) In the MLS technique, only an observation window isdefined, and if the available bandwidth and mean access delaycalculated in this window satisfy the predefined thresholds,roaming into V/LAN is granted at once. Therefore, we intro-duce the concept of stability period into MLS.b) In Chen's second scheme, the stability period is divided intoseveral advertisement intervals, and cost values are updated ineach of them. Accordingly, there will be several observationwindows in the stability period of our scheme, and theavailable bandwidth and mean access delay will be calculatedin each window.c) In Wang's scheme, handoff latency is used in order todenote the period in which the MH can neither transmit norreceive data packets. In fact, for a MH roaming into a WLANunder DCF mode, the main source of handoff latency is themedia access delay, because the MAC protocol in V/LAN isbased on carrier sense multiple access with collision avoidance(CSMA/CA). As a result, we substitute the handoff latency in(2) with the mean access delay, which is easier to obtain bythe MLS technique.d) In Chen's schemes, the notion of "adaptive" is realized byperiodically updating the cost ratio and adjusting Total Indexaccording to the varying rate of cost ratio. Whereas in theproposed adaptive scheme, in addition to the cost ratio, themean access delay is also updated per observation window,then the stability period is adjusted according to the variationtrend of both cost ratio and mean access delay.

The summary of the proposed adaptive decision scheme canbe described as follows:

1) InitializationSet m = 0, Total Index FDe,laYm + Delay,1,;m = m+ 1;

2) While (m < Total Indecx)if >1 )Total Index LD,layT + DelayT;

else if (rm r' <1e)Total Index FD,lay- + DelayT 1;

elseTotal Index Total Index;

endm = m +1;

3) T, = Total Index T,;where, T, denotes the duration of an observation window;Delaym is the mean access delay calculated in the mthobservation window; c is the precision as defined in Chen'ssecond scheme.

1818

IV. PERFORMANCE EVALUATION

A. System Model

The network environment utilized in our simulation isa WLAN/GPRS-EDGE (General Packet Radio Service-Enhanced Data rates for Global Evolution) hybrid network.The IEEE 802.11 WLAN system, which is under DCF modeand with request-to-send/clear-to-send (RTS/CTS) enabled, isconsidered in the simulation model. The total system band-width of WLAN is 3.6 Mbps. The average number of MHsattached to V/LAN simultaneously is set to 10, therefore,variation in NAV occupation is mainly caused by the changesof traffic load in these MHs. As for the GPRS-EDGE system,the supported operation modes are listed in Table I. Therandom constant bit rate (CBR) traffic model in [8] is adoptedin our simulation, and the average length of the MIAC framesis set to 1K bytes.

Table I. Operation modes in GPRS-EDGE

4

3.5

Q-c 3--0Q

- 2.5

.DC:- 2V

-0a)E 1.5v.7

< 1

0.5

0&0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

NAV occupation

Fig. 1. Available bandwidth versus NAV occupation.Transmit Rate

(kbps)1 timeslot2 timeslots3 timeslots

GPRSCS-i9.0518.127.2

GPRSCS-213.426.840.2

EDGDECS-133.066.099.0

EDGDECS-240.182.0123.0

B. Numerical ResultsNumerical simulations are performed to four decision

schemes in all, i.e., the proposed adaptive decision scheme,Chen's two adaptive schemes, and Wang's nonadaptivescheme. In order to achieve a fair comparison, in all of theseschemes:a) the same cost function as presented in (7) is used;b) related handoff metrics are obtained via the same method,where the available bandwidth and the mean access delay aredetermined by the MLS technique.c) handoff latency is substituted by mean access delay, andthe only difference is that the mean access delay used in ourscheme is updated per observation window, while in the otherschemes it is fixed at the initial value calculated in the firstobservation window.

In addition, the evaluation count N in Chen's first schemeis set to 5, and the duration of observation window and theprecision (e) used in Chen's second scheme and the proposedscheme is set to 0.1 second and 500 respectively, in accordancewith the values adopted in Chen's simulations.

Simulation results on the estimated available bandwidth andmean access delay versus observed NAV are illustrated inFig. 1 and Fig. 2, respectively. It can be observed in Fig. 1that the system available bandwidth decreases almost linearlywith the NAV occupation, while in Fig. 2, we can see thatthe mean access delay increases more and more dramaticallywith the increase of NAV occupation, which accords with thesimulation results given in [8].

In Fig. 3, the relationship between stability period and thevarying rate of cost ratio is presented. In order to saving space,in Fig. 3, Wang's scheme is denoted as "Wang", and Chen's

0.7

0.6

-a

o 0.5-0a)

X 0.4V

a) 0.30Cum 0.2-

0.1 0.2 0.3 0.4 0.5NAV occupation

0.6 0.7 0.8

Fig. 2. Mean access delay versus NAV occupation.

two adaptive schemes are denoted as "Chenl" and "Chen2"respectively, while our scheme is denoted as "proposed". Itcan be seen that all of the three adaptive schemes can shortenthe stability period when the cost ratio is increasing, whileextend the stability period when the cost ratio is decreasing.However, the proposed scheme achieves the highest sensitivityin stability period adjusting. That is to say, in all of theseschemes, our scheme can make the vertical handoff performingtime advanced or delayed most dramatically. This performanceimprovement is achieved because the mean access delay in ourscheme is adjusted in each observation window. According tothe results in Fig. 2, mean access delay does not increaseslinearly with the NAV occupation, but in Chen's secondscheme, the access delay is fixed, while the remaining part ofstability period is adjusted linearly by rm i/rm, which can

1819

0.1_-10 0 10 20 30

Average varying rate per observation window (%)

Fig. 3. Stability period versus the average varying rate of cost ratio. Theinitial cost ratio is set to 3.

0.41

0.35

0.3Q

0.25 -Qm-00-0 0.2

~0.15C,,n_r_

Go 0.1

D

0.05

1

WangChen 1Chen2proposed

2 3 4 5Initial cost ratio

Fig. 4. Unnecessary handoff probability versus initial c

varying rate of cost ratio is -10%, i.e., the average d(ratio is 10%.

not reflect the variation trend in network con

Fig. 4 shows the comparison results in unr

probability, which is defined as the probabilMH waits for a period of Ts and performsinto WLAN, the network condition in WLATthan that in GPRS before the MH can profit fAs the initial cost ratio increases, the unn

probabilities of both Wang's scheme andincrease accordingly. This is because, wheiratio is high, the initial stability period isif the decision scheme is not sensitive eno

handoffs will happen. However, for the prop

unnecessary handoff probability remains in the lowest levelamong all the schemes even when the initial cost ratio is high,showing the best ability in unnecessary handoff avoidance.

V. CONCLUSIONIn this paper, we introduce the MLS technique [8], which is

designed to estimate available bandwidth and access delay inIEEE 802.11 WLAN, into the policy-enabled handoff decisionscheme adopted in a WLAN/WWAN hybrid network. Makinguse of the metrics obtained by the MLS technique, the problemof handoff metric acquisition is easily resolved, and further-more, we improve the adaptive schemes for stability periodprediction proposed in [5]. This improvement is achieved byadjusting the mean access delay periodically, and using thenewly updated access delay each time the stability period is re-calculated. Simulation results show that the proposed adaptivedecision scheme outperforms Chen's adaptive schemes andWang's nonadaptive scheme in terms of sensitivity of stabilityperiod adjusting and the ability to avoid unnecessary handoffs.

ACKNOWLEDGMENTThis work was supported by the National Natural Science

Foundation of China (No.60572115), by the High TechnologyResearch and Development Program (No. 2005C21005) ofZhejiang Province, and by the Natural Science Foundation ofZhejiang Province (No. Z 104252).

REFERENCES

[1] F. Zhu and J. Mcnair, "Vertical handoffs in fourth-generation multinet-work environments," IEEE Journal on Wireless Communication, vol. 11,no. 3, pp. 8-15, June 2004.

[2] IEEE 802.16-2001, "IEEE Standard for Local and Metropolitan AreaNetworks Part 16: Air Interface for Fixed Broadband Wireless AccessSystems," Apr. 8, 2002.

[3] IEEE 802.16e/D5-2004, Part 16: Air Interface for Fixed and MobileBroadband Wireless Access Systems Amendment for Physical and

E__E__E__E_ Medium Access Control Layers for Combined Fixed and Mobile Op-eration in Licensed Bands, Nov., 2004.

[4] M. Stemm and R. H. Katz, "Vertical handoffs in wireless overlay6 7 8 networks," ACM Mobile Network (MONET) Special Issue on Mobile

Networking in the Internet, pp. 335-350, Summer 1998.[5] W. T. Chen, J. C. Liu, and H. K. Huang, "An adaptive scheme for vertical

handoff in wireless overlay networks," in Proc. of ICPADS 2004, July,ost ratio. The average 2004, pp. 541-548.

ecreasing rate of cost [6] H. J. Wang, R. H. Katz, and J. Giese, "Policy-enabled handoffs across

heterogeneous wireless networks," in Proc. ofIEEE Workshop on MobileComputing Systems and Applications, 1999, pp.51-60.

[7] F. Zhu and J. Mcnair, "Optimizations for vertical handoff decisionditions algorithms," in Proc. of the Wireless Communications and Networkingdltonsprecisely. Conference (WCNC 2004), vol. 2, March 2004, 867-872.

[8] C. X. Guo, Z. H. Guo, Q. Zhang, and W. W. Zhu, "A seamless and

tecessary handoff proactive end-to-end mobility solution for roaming across heterogeneouswireless networks," IEEE Journal on Selected Areas in Communications,

lity that after the vol. 22, no. 5, pp. 834-848, June 2004.

vertical handoff [9] M. Ylianttila, M. Pande, J. Makela, and P. Mahonen, "OptimizationN becomes worse scheme for mobile users performing vertical handoffs between IEEE

802.11 and GPRS/EDGE networks," in Proc. ofIEEE GLOBECOM2001,'rom this handoff. vol. 6, Nov. 2001, pp. 3439-3443.

iecessary handoff [10] IEEE 802.11 WG Part 11: Wireless LAN medium access control (MAC)Clhon'qc cqhhomac and physical layer (PHY) specification, IEEE standard, 1999.%-11CIA1 3 31W1W

n the initial costrather short, andugh, unnecessary

tosed scheme, the

1820

-oc

0

0

C,,

I._Q

cn

Cl)I0.3

0.2