MASAUM Journal of Computing, Volume 1 Issue 2, September 2009 136 Dwell Timer Based Vertical Handoff Scheme for Heterogeneous Wireless Networks K.Ayyappan, R.Dhanraj, P.Dananjayan and R.Kumar are adopted for different application environments. It is well Abstract— Next generation mobile communication systems recognized that next generation wireless networks will aims at meeting the increasing demand for services with higher integrate heterogeneous technologies to achieve enhanced data rates and enhanced service quality. Instead of developing performance. a new uniform standard for wireless communications systems, the next generation communication networks strive to A complementary network to a 3G cellular system is seamlessly integrate various existing wireless communication WLAN. It offers more bandwidth for smaller coverage area. networks with IP as backbone network and thereby provide Moreover, larger bandwidth available for WLAN makes it anywhere, anytime connectivity with high data rate and possible to achieve higher data rates. For example, IEEE enhanced service quality. A typical scenario of this network 802.11b, WLAN can have a bandwidth more than 20 MHz. integration is an interworking between wireless local area IEEE 802.11b operates at license exempt industrial, scientific network (WLAN) and third generation (3G) cellular networks and medical (ISM) frequency band from 2.4 GHz to 2.483 (CN). The 3G cellular networks provide ubiquitous GHz. It extends the physical layer based on direct sequence connectivity but low data rate, whereas WLAN can offer much spread spectrum (DSSS) specified in the original 802.11 higher data rates but only cover small area. With combined standard and supports a higher data rate up to 11 Mbps . strengths, the integrated networks will provide both wide area The subsequent revision such as 802.11a and 802.11g adopt coverage and high-rate data services in hotspots. Also the orthogonal frequency division multiplexing (OFDM) and varying characteristics of these integrated networks degrade offers a maximum data rate of 54 Mbps at unlicensed 5 GHz the service quality during frequent handoffs. To minimize the and 2.4 GHz bands, respectively. However, it has been service quality degradations like handoff delay, packet losses, designed as a wireless extension to the wired Ethernet. decreased throughput and network disconnection, a dwell timer based vertical handoff scheme for CN and WLAN For instance, an 802.11b access point (AP) can integrated networks is proposed in this paper. This handoff communicate with a mobile station (MS) up to 60 m at 11 algorithm will be very much useful to minimize the handoff Mbps and up to 100 m at 2 Mbps with omni-directional delay and maximize the throughput. antennas. Consequently, with lower cost and much higher data rates, WLANs can effectively supplement the 3G networks in Index Terms— Vertical handoff, Dwell timer, Heterogeneous hotspot areas, where bandwidth-demanding applications are wireless networks, Received signal strength, Quality of service. concentrated. As a result, by effectively combining 3G cellular networks and WLANs into an integrated wireless data access I. INTRODUCTION environment, mobile users can be provided with both ubiquitous connectivity and high-rate data services in hotspots. Next generation wireless networks are characterized by The interworking principle enables user to access the anywhere, anytime connectivity, enhanced data services and particular network depending on the application needs and higher data rates to end user [1, 2]. New technologies such as types of radio access networks (RANs) available ( e.g., UMTS IEEE 802.11 Wireless local area network (WLAN), Bluetooth, and WLAN). High performance radio local area network (HIPERLAN), General packet radio service (GPRS)/ Enhanced data rates for In hotspot areas, user may relocate among these global evolution (EDGE), Code division multiple access heterogeneous networks. When user wants to relocate, (CDMA2000) and Wideband Code division multiple access networks, lot of issues should be considered . One of the (WCDMA) aim to achieve this. These different technologies main issues is handoff management, which deals with making handoff from WLAN to 3G or vice versa. The type of handoff K. Ayyappan. is currently Professor in ECE department of Rajiv Gandhi which takes place in heterogeneous network is called vertical College of Engineering and Technology, Pondicherry, India. (Phone: handoff. When the user relocates between the cells so often, 9345466411; e-mail: aaa_rgcet@ yahoo.co.in). existing received power based vertical handoff algorithm will R.Dhanraj is currently Lecturer in Electronics and Communication Engineering Department of Mailam Engineering College, Pondicherry, India results in frequent handoffs. These frequent handoffs will Dr.P.Dananjayan is working as a Professor in the Department of result in degradation in throughput . So by introducing the Electronics and Communication Engineering, Pondicherry Engineering dwell time in this algorithm, frequent handoffs can be avoided. College, Pondicherry, India This means that the mobile node will wait for this amount of Dr.R.Kumar is working as a Professor in the Department of Electronics and Communication Engineering, SRM University, Chennai, India., (e-mail: dwell time before it makes handoff. It makes handoff from one firstname.lastname@example.org). network to another network only when this dwell time expires. MASAUM Journal of Computing, Volume 1 Issue 2, September 2009 137 Thus various factors degrading the throughput could be avoided and able to get maximum throughput with minimum Through border gateways in the IP backbone network, handoff delay. So the performance between these two different WLAN terminals are provided IP connectivity to external IP networks can be optimized. networks such as the public Internet or a corporation intranet. To propose an optimal handoff scheme for maximizing the Instead of providing continuous coverage over wide areas, mean throughput and minimizing handoff delay during handoff WLANs are usually deployed in public or private hotspots between WLAN (IEEE 802.11b) and 3G cellular networks. An such as cafes, airports, and offices. Users in these areas efficient handoff algorithm will try to minimize delay and normally have a very low mobility level, as most of these areas maximize throughput. The most critical area is extreme edge of are located in indoor environments. Also, cellular coverage is the cell, where received signal strength (RSS) varies around the available in these areas. As a result, a non-uniform overlay sensitivity threshold of receiver . RSS can go temporarily topology structure has to be considered for 3G/WLAN under the receiver sensitivity threshold, and then come back. integration. This area is referred to as a transition region where ping-pong The network architecture to integrate a WLAN and a effect will takes place. cellular network is shown in Fig 1. The 3G cellular network In this dwell timer based algorithm, initially the mobile node covers a wider area and the WLAN is used for a hot spot area. in WLAN cell will take samples of received signal strength from the access point (AP) and compares with the predefined threshold. If the consecutive samples during predefined dwell time are below the threshold then mobile node initiates the handoff to 3G. Otherwise it will persist with WLAN. The main aim of this dwell timer based algorithm is to make the mobile node persists with a higher data rate system even after the received signal strength falls below the predefined threshold. The objective of this paper is to analyse the two critical parameters, mean throughput and handoff delay for different degrading factors in the transition region. This paper is organized as follows; Section 2 discusses basic interworking architecture and the challenges to be considered while interworking Section 3 outlines the introduction to the dwell time algorithm and its implementation during transition region. Section 4 brings out Fig 1 Interworking Architecture the simulation results obtained for mean throughput during transition region. Section 5 concludes the paper. The handoff process can be intra or intersystem. The need for inter-system handoff (vertical handoff) between II. UMTS/WLAN INTERWORKING heterogeneous networks may arise in the following scenarios: The heterogeneous technologies employed in cellular i. When a user is moving out of the serving network and networks and WLANs bring many challenges to the enters into the overlaying network shortly. interworking. Based on different radio access techniques, the cellular networks and WLANs present distinct characteristics ii. When a user is connected to a particular network, but chooses to be handed off to the underlying or overlaid in terms of mobility management, security support, and quality network for its future service needs. of service (QoS) provisioning. In order to achieve seamless integration, these issues should be carefully addressed while iii. When distributing the overall network load among developing the interworking schemes [7, 8]. different systems is needed (this may optimize the performance of each individual network). After third generation, relatively mature and complete technologies have been established in cellular networks to III. PERFORMANCE ANALYSIS address issues such as mobility, security, QoS, etc. With The performance of these algorithms is analyzed in widely deployed infrastructure from radio access networks to transition regions for both moving-in and moving-out core networks, ubiquitous connectivity is provided to mobile scenarios  as shown in Fig 2. The performance, measured as users over wide areas. In contrast, the WLAN specifications the mean throughput (bits/s), is a function of the terminal only focus on the physical layer and medium access control velocity (v), the handoff delay (∆ ) and the ratio of the (MAC) layer. As for the upper layers, it assumes to adopt the effective data rates (Ω). same protocols as those in wired networks, e.g., the internet protocol (IP) suite, with some adaptation for wireless links to If the two overlapping systems have significantly different avoid performance degradation. A WLAN system connects data rates, it becomes important to utilize the system with the multiple APs, while access routers in turn connect the layer 2 maximum data rate. In the transition region, the RSS, distribution system to an IP backbone network. measured in the transceiver with certain sensitivity threshold MASAUM Journal of Computing, Volume 1 Issue 2, September 2009 138 goes up and down around this threshold. Transition region Fi is normalized time spent in the network. (TT) refers thus to time and corresponding distance where the It is seen that Fi is one factor of ηi. The total throughput during received signal strength dances around the threshold. the transition region TT is, I Stot = ∑ Si (4) i =1 where, Stot is total throughput Si is mean throughput Equivalently, the effective throughput Si in the system i is, Si = ηi R i (5) where, ηi is throughput reduction coefficient Fig 2 Different types of handoff R1 is the data rate available over the air in WLAN R2 is the data rate available over the air in 3G In the simulations the network load of both UMTS and The vertical handoff profitability between any two networks WLAN cells and protocol overheads related to mobility can be evaluated with parameterΩ, which is the effective management and inter-working functions are approximated. throughput ratio This is done by using abstraction ηi which stands for the throughput reduction coefficient. Index number i is the number Si = (6) for the wireless system. ‘ηi is a product of several factors Fi(k). i Si +1 There are a total of K factors affecting the throughput in network i in the transition region . These factors include where, packet losses (which cause retransmissions), packet Ωi is effective throughput ratio encapsulation and sending delays, packet delivery probability, Si is mean throughput protocol- payload ratio and the number of active users in the In the simulation environment, Ω values range from about 5 cell (network load) ηi can thus be approximated as shown in for UMTS, assuming a 1Mbps theoretical data rate at the edge ‘(1)’. of WLAN cell, cellular data rates 160 kbps for UMTS. The K throughput reduction coefficient is 40% for WLAN and 50% ηi = C Fi (k) (1) for cellular networks. k =1 A handoff from network ‘i ‘to ‘i + 1 ‘is profitable only if, Normalized throughput over the transition region TT when TD(i) + ∆ i using a dwell-timer TD(i) can be formulated as in’(2)’, Ti > (7) 1− i Ni R i ∑ (Ti − TD(i) − ∆i ) A handoff from network i + 1 to i is profitable only if, n =1 Si = (2) TT TD(i+1) + ∆ (i +1) Ti+1 > −1 (8) where, 1− i i is network ( i=1 for WLAN, 2 for 3G). Ni is number of timeslots where, TD is dwell time Ti is transition time TT is where RSS falls below the threshold first time i is Handoff delay IV. SIMULATION RESULTS The effective normalized time spent in the network i is given The objective of this simulation is to analyse the two critical as in ‘(3)’, parameters, namely throughput and the handoff delay when the user moves between the two different networks. The Ni simulation model is based on two different networks namely ∑ (T − T n =1 i D(i) − ∆i ) WLAN and 3G cellular networks (GPRS, EDGE, and UMTS). Fi = (3) TT The simulation parameters which are used are shown in the where, Table.1. MASAUM Journal of Computing, Volume 1 Issue 2, September 2009 139 The Fig 3 shows the variation of mean throughput for four TABLE I different WLAN modes in terms of data rates against the SIMULATION PARAMETERS throughput reduction coefficient ‘η’. Here modes are classified as high data rate (11Mbps), medium data rate (5.5 Parameter Details Mbps), standard data rate (2Mbps) and low data rate (1 Mbps). Number of networks 2 When the throughput reduction coefficient η1 drops down to 10%, the mean throughput in the transition drops to 80%. WLAN data rate (Mbps) 11, 5.5, 2, 1 The Fig 4 shows the effect of mean throughput for different GPRS modes as function of throughput reduction GPRS data rate (kbps) 40 coefficient during transition region. It is seen that the degradation of η has less effect with the lower rate system for EDGE data rate (kbps) 80 mean throughput in the transition region. For example, in case UMTS data rate (Mbps) 0.144, 0.384, 2 of GPRS and CS-4 with 3 timeslots, when η2 drops down to 10%, the mean throughput drops only about 5%. In the Figure Dwell time (ms) 500 4.2 CS-1, CS-2, CS-3, CS-4 indicates the different channel coding schemes available in the GPRS, where each coding Handoff Delay (ms) 650 scheme will be having different data rates depending on the usage of number of timeslots. Velocity (m/s) 1, 5, 15 Fig.5 Mean throughput for EDGE as a function of throughput reduction Fig.3 Mean throughput for WLAN as a function of throughput reduction Coefficient Coefficient Fig.6 Mean throughput for UMTS as a function of throughput reduction Fig.4 Mean throughput for GPRS as a function of throughput reduction Coefficient Coefficient The Fig.5 explains the effect of mean throughput for different EDGE modes as function of throughput reduction MASAUM Journal of Computing, Volume 1 Issue 2, September 2009 140 coefficient during transition region. In EDGE, the channel CS-1, CS-2, CS-3, CS-4 indicates the different channel coding coding rates are enhanced with higher capacity modulation and schemes available in the GPRS, where each coding scheme coding schemes. Here the channel coding schemes are will be having different data rates depending on the usage of represented as ECS-1, ECS-2, ECS-3, and ECS-4. Also each number of timeslots coding scheme will be having different data rates depending on the usage of number of timeslots. Fig 6 depicts the effect of mean throughput for different UMTS modes as function of throughput reduction coefficient during transition region. Here modes are classified as low speed (indoor), medium speed (pedestrian) and high speed (vehicular). The data rates corresponding to each of these modes are gives as 2Mbps, 384kbps and 144kbps respectively Fig 9 Mean throughput for EDGE as a function of handoff delay The Fig.9 shows the effect of mean throughput for different EDGE modes as function of handoff delay during transition region. When handoff delay exceeds 650 ms, the continue handoffs (ping-pong effect) suffocate the throughput. Here, the channel coding schemes are enhanced and Fig 7 Mean throughput for WLAN as a function of handoff delay represented as ECS-1, ECS-2, ECS-3, and ECS-4. Also each Fig 7 shows the effect of mean throughput for different coding scheme will be having different data rates depending WLAN modes as a function of handoff delay. As the handoff on the usage of number of timeslots. delay increases, the mean throughput decreases and vice versa. It can be roughly estimated that when handoff delay exceeds 650 ms, the continue handoffs (ping-pong effect) suffocate the throughput. This is more drastic for higher data rate modes. Fig 10 Mean throughput for UMTS as a function of handoff delay Fig 8 Mean throughput for GPRS as a function of handoff delay The Fig 8 describes how handoff delays effects to the Fig 10 depicts the effect of mean throughput for different performance of vertical handoff for different GPRS modes. It UMTS modes as function of handoff delay during transition is seen that when handoff delay exceeds 650 ms, the continue region. As the handoff delay increases, the mean throughput handoffs (ping-pong effect) suffocate the throughput. Here, MASAUM Journal of Computing, Volume 1 Issue 2, September 2009 141 decreases and vice versa. Here, modes are classified as low K. Ayyappan received the Bachelors Degree speed (indoor), medium speed (pedestrian), high speed in Electronics and Communication Engineering from Bharathidasan University in (vehicular) depending on the data rates 1989. He completed his Masters degree in Power Systems from Annamalai University in 1991. He is currently Professor in ECE department of Rajiv Gandhi College of V. CONCLUSION Engineering and Technology, Pondicherry, In this paper, main emphasis is to analyse two critical India. He is pursuing research in the area of parameters namely mean throughput and handoff delay internetworking in wireless communication. He has published three papers in international obtained in vertical handoff between IEEE 802.11b (WLAN) journals in the same area. His areas of interest and cellular network (UMTS). From simulation, it is inferred include signal processing and mobile that in many cases it is preferable to persist in high data rate communication. network as long as possible and dwell time can be used as a function of performance. It also shows the relation between R. Dhanraj received his B.Tech Degree from Sri Manakulakula Vinayagar handoff delay and throughput reduction coefficient to the Engineering College, Pondicherry University in 2006. He completed his throughput perceived by mobile user during transition region. M.Tech from Pondicherry Engineering College, Pondicherry in 2008. His The handoff delay caused by the frequent handoffs has much areas of interests include wireless communication and Computer communication. degrading effect for the throughput in the transition region. But by using the optimal value of a dwell time, effect of Dr. P. Dananjayan received Bachelor of Science from University of handoff delay can be compensated in order to maximize the Madras in 1979, Bachelor of Technology in 1982 and Master of throughput. Engineering in 1984 from the Madras Institute of Technology, Chennai and Ph.D. degree from Anna University, Chennai in 1998. He is working as a Professor and Head of the Department of Electronics and Communication REFERENCES Engineering, Pondicherry Engineering College, Pondicherry, India. He has  M. Lott, M. Siebert, S. Bonjour, D. von Hugo, “Interworking of more than 60 publications in National and International Journals. He has WLAN and 3G systems”, IEEE Communications Magazine, vol. 151, presented more than 130 papers in National and International conferences. pp.34-41, no.5, May 2006. He has produced 6 Ph.D candidates and is currently guiding eight Ph.D students. His areas of interest include power electronics, Spread spectrum  R. Chakravorty, P. Vidales, K. Subramanian, I. Pratt, and J. 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