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Dwell Timer Based Vertical Handoff Scheme for Heterogeneous

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					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 [3].
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 [4]. 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 [5]. 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
rkumar68@gmail.com).                                                       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 [6]. 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 [9] 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 [4]. 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
[1]   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
[2]   R. Chakravorty, P. Vidales, K. Subramanian, I. Pratt, and J.           Techniques and Wireless Communication.
       Crowcroft,“Performance Issues with Vertical Handovers - Experiences
       from GPRS Cellular and WLAN Hot-spots Integration”,IEEE               Dr. R. Kumar received the Bachelors Degree in Electronics and
       proceedings of Pervasive Computing and Communications                 Communication Engineering from Bharathidasan University in 1989. And
       (Percom’04), Orlando, USA, pp.34-47, Mar. 2004.                       Master of Science in 1993 from the BITS Pilani and Ph.D. degree from
                                                                             SRM University, Chennai in 2009. He is working as a Professor in the
[3]   N. Nasser, A. Hasswa, and H. Hassanein, “Handoffs in Fourth            Department of Electronics and Communication Engineering, SRM
       Generation Heterogeneous Networks”, IEEE Communications               University, Chennai, India. He has 13 publications in National and
       Magazine, vol. 44, no.10, pp. 96-103, Oct. 2006.                      International Journals. He is currently guiding four Ph.D students. His areas
                                                                             of interest include Spread spectrum Techniques and Wireless
[4]   M. Ylianttila, J. Makela P. Mahonen, “Supporting Resource Allocation   Communication.
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      Portugal, vol. 1, pp. 64–68, Sep. 2006

[5]   SuKyoung Lee, K.Sriram, Kyungsoo Kim, “Vertical Handoff Decision
      Algorithms for Providing Optimized Performance in Heterogeneous
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      pp.1-16, Nov. 2006.

[6]    A. Doufexi, S. Armour and A. Molina, “Hotspot Wireless LANs to
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[7]   Pahlavan, P. Krishnamurthy, A. Hatami, M. Ylianttila, J. Makela,
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[8]   A. Hatami, P. Krishnamurthy, K. Pahlavan, M. Ylianttila, J.Mäkelä,
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[9]   Ylianttila, Mika, “Vertical Handoff and Mobility System Architecture
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