Future Generation Computer Systems 26 (2010) 1403–1408

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Future Generation Computer Systems

journal homepage: www.elsevier.com/locate/fgcs

Seamless handoff scheme in Wi-Fi and WiMAX heterogeneous networksFei Shi, Keqiu Li ∗, Yanming ShenSchool of Computer Science and Technology, Dalian University of Technology, No.2, Linggong Road, Dalian 116023, China

a r t i c l e i n f o

Article history:Received 28 December 2009Received in revised form12 April 2010Accepted 16 April 2010Available online 7 May 2010

Keywords:Vertical handoffHorizontal handoffHeterogeneous networksMobile IPv6

a b s t r a c t

With the development of wireless communication technology, various wireless networks have beendeployed. Heterogeneous networks will be dominant in the next generation wireless networks. In suchnetworks, providing a seamless handoff and quality of service (QoS) guarantees is one of the key issuesfor real-time services. In this paper, we propose a horizontal handoff scheme that reduces the horizontalhandoff latency based on the location and movement pattern of a mobile node (MN). We also present avertical handoff scheme for providing seamless services betweenWi-Fi andWiMAX networks. Finally, byperformance evaluation using computer simulations, we show that the proposed scheme can reduce thehandoff latency significantly.

Published by Elsevier B.V.

1. Introduction

With the development of wireless communication technology,wireless Internet access has become a trend. People want to havemultimedia services anytime, anywhere. However, these applica-tions have higher QoS requirements. Due to the limited coverageof a wireless network, a mobile node (MN) has to switch to otherAccess Point (AP) when it moves out of the current AP. There arehandoff delay and packet loss during this process. This is particu-larly important for VoIP and other real-time services. Seamlessmo-bile handoff requires uninterrupted communications while an MNis moving in the wireless networks. Providing a seamless handoffis one of the key issues in real-time services. There are two mainaspects to compare handoff schemes: switching delay and packetloss rate. A lot of work has focused on reducing delay and packetloss rate during handoff.However, there is not only one standard protocol in wireless

networks, as next generation mobile networks consist of multi-ple heterogeneousmobile networkswith different coverage range,quality and cost. There are often several different wireless net-works in the same region, such as IEEE 802.11 (Wi-Fi) and IEEE802.16 (WiMax). WiMAX uses spectrum which needs to be au-thorized to access to the Internet. Wi-Fi uses unlicensed spec-trum to provide access to a network. A Wi-Fi enabled devicesuch as a PC or mobile phone can connect to the Internet free ofcharge within the coverage. Therefore, Wi-Fi is more popular inend user devices. A typical Wi-Fi router might have a range of32 m indoors and 95 m outdoors. A commonly held conception isthat WiMAX will deliver 70 Mbit/s over 50 km. WiMAX and Wi-Fi

∗ Corresponding author.E-mail address: keqiu@dlut.edu.cn (K. Li).

0167-739X/$ – see front matter. Published by Elsevier B.V.doi:10.1016/j.future.2010.04.011

have quite different QoS mechanisms. WiMAX uses a mechanismbased on connections between the base station and the user de-vice. Each connection is based on specific scheduling algorithms.Wi-Fi cannot guarantee the QoS requirements. In the real scenario,a mobile user is often at the overlap signal range of WiMAX andWi-Fi at the same time as shown in Fig. 1.There are two kinds of handoffs, horizontal handoff and ver-

tical handoff. Horizontal handoff is the handoff between differ-ent wireless access points that use the same technology. To over-come handoff latency, two horizontal handoff schemes, hierarchi-calMIPv6 (HMIPv6) [1] and FastHandoff forMIPv6 (FMIPv6) [2] areproposed by IETF. Combination of HMIPv6 and FMIPv6 motivatesthe design of Fast Handoff for HMIPv6 (F-HMIPv6) [3], which leadsto more efficient network bandwidth usage. However, simulationexperiments [4] show that the main latency occurs in MovementDetection (MD). Those above proposedmethods cannot reduce thehandoff latency significantly. Ref. [5] proposed a method based ondynamic region to reduce the movement detection delay, whichdynamically adjusts the interval according to the bandwidth uti-lization. However, the AR might send advertisements frequentlyeven though the MN does not need handoff, a large number ofadvertisements are useless and wasted. NDPR [6] (No uniformDetection and CoA Pre-register) introduces a nonuniform detec-tion pattern. It can be inferred that the MN has moved into a newAR once MN finds that the marked MAC is changed. The interval isset according to the distance between the MN and the current AR.But an MN’s movement pattern is also very important to forecasthandoff and adjust the interval appropriately.Vertical handoff refers to a mobile node changing the type of

connectivity it uses to access a supporting infrastructure, usually tosupport node mobility. It is very important and more complicatedin the heterogeneous wireless communication. Paper [7] proposed

1404 F. Shi et al. / Future Generation Computer Systems 26 (2010) 1403–1408

Fig. 1. Heterogeneous networks.

a method that used two independent handoff triggers. The Signalto Interference-plus-Noise Ratio (SINR) is used to trigger handoffif communication is going to lose. And the throughput triggershandoff when the quality of current network degraded. However,a minimum time between two handoffs needs to be manually setand these two parameters are decisive metrics in vertical handoff.Another handoff procedures was proposed in paper [8]. But it didnot consider the handoff trigger, which is an important componentto reduce handoff latency. Paper [9] used the available bandwidthand received signal strength index to be the handoff metric. Butit didn’t provide simulation result to prove its efficiency. [10]presented a handoff method that used the IEEE 802.21 informationelements and the GPS location information. However, only fewmobile devices are equippedwithGPS and the IEEE 802.21 protocolis not fully defined and really commercialized yet. [11] introduceda generic vertical handoff decision function for heterogeneouswireless networks. But it did not describe how to determine theweight of each parameter.In this paper, we presents a horizontal handoff method and

a vertical handoff method by taking the MN’s location and itsmovement pattern into account. The MN can detect its positioninformation by using devices such as GPS and save it in its buffer.According to the location information, the movement pattern, thespeed and the direction ofmovement can be figured out.When theMAP receives the handoff preparation request (HPReq) sent fromthe MN, it selects an appropriate NAR to handoff [12], and notifiesthe MN by handoff preparation reply (HPRep). The MAP gets theARs’ information within its domain by exchanging messages withthem. The detection interval is adjusted by the speed.The rest of this paper is organized as follows. In Section 2, we

propose our low latency horizontal handoff method (LLHH). InSection 3, we fist introduce some previous vertical handoff pro-tocols, and present our vertical handoff scheme in heterogeneousnetworks. We evaluate the performance of our proposed schemevia simulations in Section 4. Section 5 concludes this paper.

2. Horizontal handoff

In this section, we first introduce four main ICMPv6-type [13]messages for handoff management:(1) Handoff Preparation Request (HPReq) message: This messageis sent from anMN to theMAP for handoff request preparation.It contains information about user preferences/profile, appli-cations required QoS capabilities, L2 information of ARs, IP ad-dress of the MN, signal strength of the MN and the AR’s ID forMN location tracking.

(2) Handoff Preparation Reply (HPRep) message: This message issent by theMAP to theMN and contains prefixes of subnet, thelist of candidate ARs and their capability.

(3) Handoff Preparation Notification (HPN) message: This mes-sage is sent by the MN to the MAP to notify the possibility ofimpending handoff. It contains the information about the se-lected new AR that the MN will switch to. HPN includes therequest for verification of pre-configured new on-link care-ofaddress (NLCoA) and for establishment of bi-directional tun-nel between the MAP and the NAR in order to prevent routingfailure during handoff.

(4) New Link Attachment (NLA) message: This message is sent bythe MN to the NAR to announce its presence on the new linkand to confirm the use of NLCoA.

2.1. Dynamic interval adjustment

When the MN knows that it moves into a NAR’s coverage, itsends a message to the PAR and NAR, to let them know that theMN is in the overlap coverage. This also means handoff may hap-pen later. As the MNmoves at the threshold switch decided by thedistance, it sends the handoff request to the NAR and PAR. Thenthe handoff process initiates. Therefore, if the MN wants to knowthe exact time when the handoff happens, it must detect its po-sition more frequently. But too many detections will cost a lot ofoverhead and aggravate the MN’s burden. Furthermore, since eachnetwork has different situations (like bandwidth, the AR’s cover-age and other configurations of the network), we cannot set thedetection interval to be static.For the proposed scheme, we define x to be the distance be-

tween theMN and the PAR, and y to be theMN’s speed. Let T be themaximum interval, γ be the compression radio. γ is a non-linearfunction, which depends on x. β is a variable determined by y. µand ϕ are compression parameters used to find the best γ and βin different networks respectively. Let h(x) be the switch intensity,which can be obtained from Eq. (6). Then we have

t = βγ T . (1)

γ is defined by

γ = g(x) =ln[1+ µ(1− h(x))]

ln(1+ µ), (µ > 0, d > x > 0). (2)

Then we can calculate β by

β =

ln(1+ ϕy)ln(1+ y)

, 0 < y < 3,

ln(1+ y)ln(1+ ϕy)

, y ≥ 3.(3)

Let d be the maximum coverage of the AR. We have

l =x/dd− x

, (d > x > 0). (4)

Supposes L follows the negative exponential distribution withparameter l. We can get L’s distribution function as follows:

FL(l) = 1− e−λl, (l > 0). (5)

Let the distribution function be the switch intensity, so we have

FL(l) = h(x). (6)

Then we can get t from (1), (2), (8), (4) and (5), that

t = βln[1+ µe−λ

xd(d−x) ]

ln(1+ µ)T . (7)

As shown in Eq. (7), the detection interval is adjusted dynam-ically. When the MN is far from the PAR, the interval will be setsmall. On the contrary, the MN will have a long interval if the MN

F. Shi et al. / Future Generation Computer Systems 26 (2010) 1403–1408 1405

is close to the PAR. At the same distance, the intervals are also dif-ferent with different speeds. By taking the speed into account, wecan avoid the situation that theMNmoves a long distance betweentwo detections when the speed is high, since the intervals will besmall at high speed.

2.2. Handoff process

Unlike FHMIPv6 [3] and NDPR [6], the MN can initiate thehandoff when it moves at the threshold for our scheme. The wholehandoff process is shown in Fig. 3.(1) When an MN detects that it is moving at the edge of thecurrent AR range, it sends HPReq to MAP for handoff requestpreparation. The MN attaches its location information toHPReq.

(2) The MAP obtains the MN’s trace and the latest location. Thenit selects the most appropriate NAR for the MN switch, accord-ing to the distance between the MN and the NAR and the MN’smoving direction. TheMAP gets the AR’s routing table informa-tion and maintains them by router information eXchange (RIXrequest/Reply) message [5].

(3) The MAP sends a HPRep message to the MN, containing an ad-vertisement of the NAR. This message also contains informa-tion about the Next Link Care-of-Address (NLCoA) for the MNto use in the NAR region (i.e., NAR’s network prefix for statelessauto-configuration or NLCoA for state configuration). It notifiesthe best candidate AR’s prefix of subnet to the MN.

(4) After the MN obtains the candidate AR, it sends HPN to theMAPwith the request for verification of pre-configured NLCoA.When the MAP receives HPN, it forwards this message to theNAR. The NAR sends an NLAck message to the MAP, includingwhether the NLCoA can be used or not. If the request NLCoA iscollided, NAR will attach a valid NLCoA. This message will beforwarded to the MN by the MAP.

(5) Once L2 handoff occurs, the packets are forwarded to the NARand the PAR. And these packets are saved in the NAR’s buffer.When the MN receives the message from the NAR, it sends aNLA message to notify the NAR to send those saved packets.

(6) The MN makes a Local Binding Update (LBU) and Local Bind-ing ACK (LBACK) exchange with the MAP to inform its arrivalat NAR.

3. Vertical handoff

In vertical handoff, the situation ismore complicated. The hand-off metrics in this situation should include RSSI, user preference,network conditions, application types, cost etc. Based on the hand-off metrics mentioned above, the decision about how and whento switch the interface to which network will be made. In thispaper, we mainly study the vertical handoff between Wi-Fi andWiMAX. Vertical handoff includes two kinds of handoffs: fromWi-Fi to WiMAX and from WiMAX to Wi-Fi. For users, cost is themost attractive reason to use Wi-Fi. Furthermore, there are smallusers in each access point (AP), so the actual translation rate willnot be smaller thanWiMAX. Therefore, user should access toWi-Fias much as possible. We have proposed a new approach to achievethe low latency vertical handoff. According to the state of the net-work environment, as well as the different MN, respectively con-figure the handoff in order to achieve the fastest handoff. In thewireless network, MN’s most important characteristic is mobility,therefore we have introduced the velocity of the MN as an impor-tant parameter to trigger the vertical handoff.(1) Handoff from Wi-Fi to WiMAX. When the connection is goingdown, if there is other Wi-Fi AP meet access request, thepreferred choice is to handoff to the NAP. If there is no NAParound can be accessed, MN must handoff to WiMAX oncereaching the trigger.

(2) Handoff fromWiMAX toWi-Fi. Usually, aWiMAXnetworkmaycover one or more Wi-Fi networks. Because of small range ofWi-Fi, MN will quickly move out of a base station’s coveragewhen at high speed, so handoff should not be triggered. Thesecommunications will not last long, but lead to unnecessaryhandoffs. While MN doesn’t move fast, handoff will happenwhen reaching the trigger.

3.1. Dynamic trigger setting

One method to improve seamless handoff is the ability tocorrectly decide when to carry out vertical handoff. The key ofvertical handoff performance is tomakehandoff decision appropri-ately. Signal strength, which is the main metric in traditional hori-zontal handoff, cannot be used to vertical handoff decisions due tothe overlap of heterogeneous networks and the different physicaltechniques used by each network. The velocity has a larger effect invertical handoff. Because the user would pass the original networkin a short time when he travels at high speed. Therefore, we musttake the velocity into account. We set the handoff trigger dynami-cally with the speed. based on the moving direction, we can makecorrect decision of handoff or not.Let RSSIstand be the stand RSSI of Wi-Fi. If the MN receives signal

strength bigger than RSSIstand, MN can receive packets from thisnetwork with high quality service. Let RSSItrigger be the handofftrigger in Wi-Fi, and let RSSIaverage be the average RSSI that the MNreceived fromWi-Fi (RSSIreceived). If RSSIaverage > RSSItrigger , thenMNcould handoff to this network. Let β be the parameter determinedby velocity, α be a weigh parameter. We have

β =

ln(1+ αy)ln(1+ y)

, when leave Wi-Fi,

ln(1+ y)ln(1+ αy)

, when enter Wi-Fi.(8)

Then we can get

RSSItrigger = RSSIstand ∗ β (9)

RSSIaverage = γ RSSIreceived ∗ (1− γ )RSSIaverage−1. (10)

1. Handoff from WiMAX into Wi-Fi. The handoff will carry outonce RSSIaverage > RSSItrigger .2. Handoff from Wi-Fi into WiMAX. RSSIstand < RSSIaverage <

RSSItrigger means that the link of Wi-Fi is going down, and MNshould prepare the handoff in advance. This predicable handoff canfinish before leaving original network, which reduces latency ofpassive handoff. And RSSIaverage < RSSIstand means the link of Wi-Fiis down, handoff must be carried out immediately.

3.2. Vertical handoff process

When the MN enters a Wi-Fi network, it detects that the newnetwork signal. The MN sends handoff request message to AP, andthe information of MN is sent to the AP. Then the AP configuresthe required information according toMN’s information to registerthe MN in this network. The AP responds the configuration to theMN. Once the new network reaches handoff trigger, handoff can becarried out using the configuration information.When the MN leaves a Wi-Fi network and enters a WiMAX

network, it finds the signal of Wi-Fi is going down, and it detectsa new WiMAX network using 802.16 interface. Before the linkis down, MN communicates with the BS to configure registerinformation. After that, vertical handoff will be carried out. Fig. 4shows the vertical handoff process.We save the commonly used AP’s subnet prefix in the cache of a

MN.When theMNdetects a newWi-Fi network, it should comparethe subnet prefix with those saved in the cache. Because of cached

1406 F. Shi et al. / Future Generation Computer Systems 26 (2010) 1403–1408

Table 1System parameters configuration in horizontal handoff.

Parameters Values

Wired link bandwidth 10 Mbps802.11 Wireless link bandwidth 1 Mbps802.16 Wireless link bandwidth 15 MbpsData package size 512 bytesAdvertisement package size 48 bytes802.11 AR cover range 40 m802.11 AR overlap range 10 m802.16 BS cover range 1000 mMaximum interval 5 msData rate 4 Mbps

Fig. 2. Horizontal handoff scenario.

information are small and the local computing speed is fast, thetime of comparison can be neglected. The cache information willbe classified as the blacklist andwhitelist. For example,we save theAPs which we often pass by but not stay in it in the blacklist, andsave the APs which set in the office or home in the whitelist. Whenthe MN detects a newWi-Fi network, it first searches the blacklist,andmaintain the current connection if amatch is found.Otherwise,it searches the whitelist. If the match is successful, handoff shouldbe carried out immediately. If the network detected is not in thewhitelist or the blacklist, then the handoff process is carried out asdescribed above.

4. Performance evaluation

The proposed low-latency handoff scheme has been imple-mented using the ns2. The standard ns distribution version ns-allinone2.29 is patched with the freely ns wireless extensionmodule, and the Mobile IPv6 is available.In horizontal handoff evaluation, parameters and default values

used in performance evaluation are listed in Table 1, and thenetwork topology considered for our analysis is illustrated in Fig. 2.In the network, there are two MAPs and three ARs. AR1 and AR2are in the MAP1 and AR3 is in the MAP2. At the beginning of thesimulation, the MN is close to the AR1. The MN moves towardsthe AR2. The handoff happens as the MN moves at the thresholdswitch. The handoff delay is defined as the interval between thelast packet from PAR and the first packet from NAR that the MNreceives. Fig. 5 shows the handoff latency at different speeds.In the low speed pattern, all the switches happen in the overlap

of the two AR’s coverage. For LLHH, according to forecast handoffwith the distance and the speed, the handoff happens much closerto themiddle of the PAR and the NAR. And theMN rarely leaves thePAR’s coverage, but just moves in the overlap, such as people walkaround.When theMNmoves slowly, there is enough time to judgewhether the MN will leave and complete handoff before the MNleaves the PAR’s domain. So LLHH can reduce many unnecessary

Fig. 3. Horizontal handoff process.

Normal operate

Detect new Wi-Finetwork

Detect a newWi-Fi

Search in theblacklist

Search in thewhite list

Reach thetrigger

Reach thetrigger

Handoff

Detect WiMAX

Original Wi-Finetwork going down

Yes

Yes

Yes

YesYes

No

No

No

No

No

Fig. 4. Vertical handoff process.

1.2

1

0.8

0.6

0.4

0.2

1.4

0

Han

doff

late

ncy

(s)

1 3 5 10 15 20 25 30 40MN move speed (m/s)

Fig. 5. Comparison of horizontal handoff latency.

switches. It is more efficient in reality. While the speed is high, thelatency increases apparently for FHMIPv6 and NDPR. Although thelatency is high for LLHH, it is also nearly half of the other two. And

F. Shi et al. / Future Generation Computer Systems 26 (2010) 1403–1408 1407

Pac

ket l

oss

rate

1 3 5 10 15 20 25 30MN move speed (m/s)

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.2

0.15

0.25

Fig. 6. Comparison of horizontal handoff packet loss.

2 3 4 5 6 7 8 91 10

Velocity

0.006

0.005

0.004

0.003

0.002

0.001

0.007

0

Han

doff

Late

ncy

The Latency of Handoff from Wi-Fi to WiMAX

Fig. 7. Handoff latency fromWi-Fi to WiMAX.

when the speed is under 10m/s, the latency does not increasewiththe speed for LLHH.Fig. 6 shows the packet loss during handoff process.We observe

that package loss ismuch less for LLHH than for FHMIPv6 andNDPRwhile speed is under 10 m/s, especially at the average walkingspeed (about 1.5 m/s). In high speed pattern, more messages willbe sent by the MN. So when the network bandwidth is low, theremay be congestions. For LLHH, packet loss increases, but still lessthan FHMIPv6 and NDPR.In vertical handoff evaluation, we change the 802.11 Wireless

link bandwidth to 11Mbps, 802.11 AP cover range to 20mand datapacket size to 4960 bytes. The network configuration is shown inFig. 1.Fig. 7 shows the handoff latency from Wi-Fi to WiMAX. While

the MN moves from WiMAX into Wi-Fi, our scheme can reducethe handoff latency, especially at high speed. When MN movesfast, the handoff latency of our scheme is nearly half of the MIH.And Fig. 8 shows the handoff latency from WiMAX to Wi-Fi. Asthe MN leaves Wi-Fi and enters into WiMAX, it will take moretime to finish handoff. Because the BS is usually far from the MN.The situation may be worse if the MN leaves Wi-Fi before thehandoff is complete. Therefore, as we make sure that the handoffcan be complete as theMNmoves out ofWi-Fi, the vertical handofflatency can be reduced.As a whole handoff process, the MN passes a Wi-Fi network

from WiMAX. Fig. 9 shows the packets loss rate in the wholehandoff process. In our scheme, theMN should stay inWi-Fi as long

2 3 4 5 6 7 8 91 10

Velocity

The Latency of Handoff from WiMAX to Wi-Fi

0.11

0.108

0.106

0.104

0.102

0.1

Han

doff

Late

ncy

0.112

0.098

Fig. 8. Handoff latency fromWiMAX to Wi-Fi.

0.026

0.024

0.022

0.02

0.018

0.016

0.014

0.012

0.01

0.008

2 3 4 5 6 7 8 91 10

Velocity

The Packets Loss Rate0.028

0.006

The

Pac

kets

Los

s R

ate

Fig. 9. Packets loss rate in whole handoff process.

as possible. As the bandwidth of Wi-Fi is lower than WiMAX andlack of QoS,most packets losses are caused inWi-Fi. So packets lossrate of our scheme is a little bit higher than the MIH at low speed.However, the handoff happens earlier in our scheme at high speed.This will reduce packet loss rate as the MN moves fast.

5. Conclusions

In the next generation wireless networks, providing a seamlesshandoff and quality of service (QoS) guarantees is one of the keyissues in real-time services. Several handoff schemes have beenproposed in the literature. However, these schemes cannot handleall scenarios. This paper provides a low latency handoff scheme,including horizontal handoff and vertical handoff. In homogeneousnetworks, an MN can detect its movement and forecast thehandoff, which alleviates the communication cost between theMNand its Access Point (AP). Furthermore, by setting the dynamicinterval in an appropriate range, the MN’s burden can also bealleviated. In heterogeneous networks, the vertical handoff schemeuses the velocity to be an important metric to trigger handoff.Our scheme classifies the 802.11 APs into blacklist and whitelist.The scheme can avoid unnecessary vertical handoff. Finally, byperformance evaluation using computer simulations,we show thatthe proposed scheme can reduce the handoff latency significantly.

Acknowledgements

This work is supported in part by Program for New CenturyExcellent Talents in University (NCET) of Ministry of Education of

1408 F. Shi et al. / Future Generation Computer Systems 26 (2010) 1403–1408

China, and also in part by NSFC under Grant Nos. 60903154 and60973117.

References

[1] H. Soliman, C. Castelluccia, K. El-Malki, L. Bellier, Hierarchical mobile IPv6mobility management (HMIPv6), IETF RFC, 4140, 2005.

[2] G. Koodli, Fast handovers for mobile IPv6, IETF RFC, 4068, 2005.[3] H.Y. Jung, E.A. Kim, J.W. Yi, H.H. Lee, A scheme for supporting fast handover inhierarchical mobile IPv6 Networks, ETRI Journal 27 (6) (2005) 798–801.

[4] Lijun Luo, Xinhua Jiang, Fumin Zuo, Experimental study ofmobile IPv6 handoffperformance, Computer Engineering 33 (16) (2007).

[5] Qinglin Zhao, Zhaongcheng Li, Yujun Zhang, Jun Li, Kai Wang, A method forreducing movement detection delay based on dynamic region, Journal ofSoftware 16 (6) (2005) 1168–1174.

[6] Bo Shen, Yun Liu, Hongke Zhang, A novel scheme of low-latency mobile Ipv6handoff for wireless LANs, ACTA Electronica Sinica 33 (4) (2005).

[7] Z. Dai, R. Fracchia, J. Gosteau, P. Pellati, G. vivier, Vertical handover criteria andalgorithm in IEEE 802.11 and 802.16 hybrid networks, ICC08, 2008.

[8] Yongqiang Zhang, Weihua Zhuang, Vertical handoff between 802.11 and802.16 wireless access networks, GLOBECOM08, 2008.

[9] J. Nie, J.C. Wen, Q. Dong, Z. Zhou, A seamless handoff in IEEE 802.16a andIEEE 802.11n hybrid networks, in: Communications, Circuits and SystemsProceedings International Conference, vol. 1, 27–30 May 2005, pp. 383–387.

[10] A. Garg, K.C. Yow, Determining the best network to handover among variousIEEE 802.11 and IEEE 802.16 networks by amobile device, in: 2nd Int. Conf. onMobile Technology, Nov. 2005.

[11] A. Hassawa, N. Nasser, H. Hassanein, Generic vertical handoff decision functionfor heterogeneous wireless networks, WCNC05, Mar. 2005.

[12] Christian Makaya, Samuel Pierre, Efficient handoff scheme for heterogeneousIpv6-based wireless networks, in: IEEE Conf. Wireless Communications &Networking Conference, WCNC, 2007.

[13] A. Conta, S. Deering, Internet control message protocol (ICMPv6) for theinternet protocol version 6 (IPv6) specification, IETF RFC, 2463, 1998.

Fei Shi is a master student at the Department of ComputerScience and Engineering, Dalian University of Technol-ogy, China. He received his bachelor degree from the De-partment of Computer Science and Engineering, ShandongUniversity of Technology in 2006. His research interests in-cludes QoS control, and handoffs in wireless networks.

Keqiu Li is a Professor at the Department of ComputerScience and Engineering, Dalian University of Technology,China. He received his bachelor and master degrees bothat the Department of Applied Mathematics, Dalian Uni-veristy of Technology in 1994 and 1997, respectively. Heobtained his Ph.D. degree at the graduate school of Infor-mation Science, Japan Advanced Insititue of Science andTechnology in 2005. He also has two year’s post-doctoralexperience in Univeristy of Tokyo, Japan from 2005 to2007. His general research interests include computernetworks and security, web technology, and distributed

computing.

Yanming Shen is an Associate Professor in the ComputerScience and Engineering Department at Dalian Universityof Technology, China. He received the Ph.D. degree fromDepartment of Electrical and Computer Engineering atthe Polytechnic University (now Polytechnic Institute ofNYU) and his B.S degree in Automation from TsinghuaUniversity, Beijing, PR China in 2000. He was a summerintern with Avaya Labs in 2006, conducting researchon IP telephony. His general research interests includepacket switch design, peer-to-peer video streaming, andalgorithm design, analysis and optimization.