An Efficient Decision Algorithm For Vertical Handoff Across 4G Heterogeneous Wireless Networks by ijcsis

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									                                                      (IJCSIS) International Journal of Computer Science and Information Security,
                                                      Vol. 8, No. 7, October 2010




         An Efficient Decision Algorithm for Vertical
             Handoff Across 4G Heterogeneous
                      Wireless Networks
                          S.Aghalya                                                         P. Seethalakshmi
                       Research Scholar,                                                      Anna University
                        Anna University                                                            India
                             India
                     stmaagh@yahoo.co.in                                                  auropansee@yahoo.co.in

Abstract - As mobile wireless networks increase in popularity,            (VHD) algorithm is essential for 4G network access. As the
we are facing the challenge of integration of diverse wireless            mobile users move in an environment with different
networks. It is becoming more important to arrive at a vertical           networks supporting different technologies, the VHD
handoff solution where users can move among various types of              depends on different criteria such as bandwidth, cost, power
networks efficiently and seamlessly. To address this issue, an
efficient vertical handoff decision(EVHD) algorithm has been
                                                                          consumption, user preferences and security [3].
proposed in this paper to decide the best network interface and                All the existing approaches mainly focused on the
best time moment to handoff. An overall gain function has been            vertical handoff decision, assuming that the handoff decision
utilized in this algorithm to make the right decision based on            processing task is performed on the mobile side. Such
various factors, the network characteristics such as usage cost,
bandwidth, power consumption and dynamic factors such as                  process requires a non negligible amount of resources to
Received Signal Strength (RSS), velocity and position of mobile           exchange information between MT and neighbor networks
terminal (MT). The effectiveness of the EVHD algorithm has                in order to accomplish the discovery of the best network to
been verified by carrying out simulations. The results show               handoff. The main issues of the handoff decision :
that EVHD achieves 78.5% reduction in number of                           combining decision criteria, comparing them and answering
unnecessary handoffs compared to static parameter based                   the user needs anytime and anywhere. Several proposals and
algorithm. The increase in throughput is about 60% compared               approaches considering VHD algorithms were proposed in
to static parameter based algorithm for all the types of traffic.         the literature.
The overall system performance has been improved by the
proposed efficient VHD algorithm and outperforms the three                     This paper proposes a vertical handoff decision
well known VHD algorithms including static parameter based,               algorithm in order to determine the best network based on
RSS based and RSS-timer based algorithms.                                 dynamic factors such as RSS, Velocity and Position of the
     Keywords - Heterogeneous network, Seamless handoff,                  mobile terminal and static factors of each network. Thus,
Vertical handoff, Handoff decision, Gain function.                        this algorithm meets the individual needs and also improve
                                                                          the whole system performance by reducing the unnecessary
                      I. INTRODUCTION                                     handoffs and increasing the throughput.
     Nowadays, there are various wireless communication                                       II. RELATED WORK
systems existing for different services, users and data rates
such as GSM, GPRS, IS-95, W-CDMA, Wireless LAN etc.                            An efficient vertical handoff (VHO) is very essential in
Fourth generation (4G) wireless systems integrate all                     ensuring the system performance because the delay
existing and newly developed wireless access systems. 4G                  experienced by each handoff has a greater impact on the
wireless systems will provide significantly higher data rates,            quality of multimedia services. The VHD algorithm should
offer a variety of services and applications and allow global             reduce the number of unnecessary handoffs to provide better
roaming among a diverse range of mobile access networks                   throughput to all flows. Research on design and
[1].                                                                      implementation of optimized VHD algorithms has been
                                                                          carried out by many scholars using various techniques.
     In a typical 4G networking scenario, mobile terminals                Based on the handoff decision criteria, VHD algorithms are
equipped with multiple interfaces have to determine the best              categorized as RSS based algorithms, Bandwidth based
network among the available networks. For a satisfactory                  algorithms, User Mobility based algorithms and Cost
user experience, mobile terminals must be able to                         function based algorithms
seamlessly transfer to the best network without any
interruption to an ongoing service. Such ability to handover                   In RSS based algorithms, RSS is used as the main
between heterogeneous networks is referred to as Seamless                 criterion for handoff decision. Various schemes have been
Vertical Handoff (VHO) [2]. As a result, an interesting                   developed to compare RSS of the current point of
problem surfaced on how to decide the best network to use                 attachment with that of the candidate point of attachments.
at the best time moment.                                                  They are: Relative RSS, RSS with hysteresis, RSS with
                                                                          hysteresis plus dwelling timer method [4,5]. Relative RSS is
     Vertical handoff provides a mobile user great flexibility            not applicable for VHD, since the RSS from different types
for network access. However, the decision on which                        of networks can not be compared directly due to the
network to use becomes much more complicated, because                     disparity of the technologies involved. In RSS with
both the number of networks and the decision criteria                     hysteresis method, handoff is performed whenever the RSS
increase. Thus an intelligent vertical handover decision



                                                                    124                           http://sites.google.com/site/ijcsis/
                                                                                                  ISSN 1947-5500
                                                     (IJCSIS) International Journal of Computer Science and Information Security,
                                                     Vol. 8, No. 7, October 2010



of new Base station (BS) is higher than the RSS of old BS                                      Gn = f (Bn, Pn, Cn)
by a predefined value. In RSS with hysteresis plus dwelling
timer method, whenever the RSS of new BS is higher than                 Gn is the Gain function for network n. The Gain function is
the RSS of old BS by a predefined hysteresis, a timer is set.           calculated by using Simple Additive Weight (SAW)
When it reaches a certain specified value, handoff is                   algorithm.
processed. This minimizes Ping pong handoffs. But other                           Gain function Gi = wb fb,i + wp fp,i + wc fc,i
criteria have not been considered in this method. EVHD
algorithm makes use of this method for RSS comparison.                  Where wb is weight factor for offered bandwidth, wp is
                                                                        weight factor for power consumption by network interface
     In bandwidth based algorithms, available Bandwidth for             and wc is weight factor for the usage cost of network.
a mobile terminal is the main criterian. In [6], a bandwidth
based VHD method is presented between WLANs and a                       fb,i , fp,i ,and fc,i represent the normalized values of network i
WCDMA network using Signal to Interference and Noise                    for bandwidth, power consumption and usage cost
ratio (SINR). It provides users higher throughput than RSS              respectively. Based on the service requirement, the weights
based handoffs since the available bandwidth is directly                are assigned to the parameters.
dependent on the SINR. But it may introduce excessive                       Calculation of Overall Gain function provides the best
handoffs with the variation of the SINR. This excessive                 network to handoff. A candidate network is the network
handoffs is reduced by a VHD heuristic based on the wrong               whose received signal strength is higher than its threshold
decision probability (WDP) prediction [7]. The WDP is                   and its position is less than the threshold. The RSS of MT is
calculated by combining the probability of unnecessary and              measured. using the path loss and shadowing formula that is
missing handoffs. This algorithm is able to reduce the WDP              widely adopted for ns-2. The RSS of MT can be expressed
and balance the traffic load. But in the above papers, RSS              as
has not been considered. A handoff to a target network with
high bandwidth but weak received signal is not desirable as                          RSS = PL(d0) – 10nlog (d/d0 ) + Xσ
it may result in connection breakdown.                                  Where PL(d0) is the received power at a reference distance
     In user mobility based algorithms, velocity information            (d0). The simple free space model is used to compute PL(d0).
is a critical one for handoff decision. In the overlay systems,         d is the distance between servicing BS and MT. n is the path
to increase the system capacity, micro/pico cells are                   loss exponent. Xσ is a Gaussian random variable with zero
assigned for slow moving users and macro cells are assigned             mean and standard deviation of σ.
for fast moving users by using velocity information [8]. It                  Fluctuations in RSS are caused by shadowing effect.
decreases the number of dropped calls. An improved                      They lead the MT into unnecessary ping-pong handoffs. To
handoff algorithm [9] has been presented to reduce the                  avoid these ping-pong handoffs, a dwell timer is added. The
number of unnecessary handoffs by using location and                    timer is started when the RSS is less than RSS threshold.
velocity information estimated from GSM measurement data                The MT performs a handoff if the condition is satisfied for
of different signal strengths at MT received from base                  the entire timer interval.
stations. From these papers, it is seen that velocity and
location information are also having great effect on handoff                The position of the MT is measured. It is based on the
management. They should also be taken into account in                   concept that a handoff should be performed before the MT
order to provide seamless handoff between heterogeneous                 reaches a certain distance from the BS. This is known as
wireless networks.                                                      position threshold [8].
    Cost function based algorithms combine network                                        Position threshold r = a-ντ
metrics such as monetary cost, security, power consumption              Where a is radius of the service area of the BS, ν is velocity
and bandwidth. The handoff decision is made by comparing                of the MT and τ is estimated handoff signaling delay.
the result of this function for the candidate networks
[10,11,12]. Different weights are assigned to different input               The priority for each network is based on the difference
metrics depending on the network conditions and user                    which is measured for each network.
preferences. These algorithms have not considered other
                                                                        RSS difference = RSS-RSS threshold
dynamic factors, such as velocity, position of the MT.
                                                                        Position diff = position threshold-position of the MT
 III. PROPOSED VERTICAL HANDOFF DECISION ALGORITHM
                                                                             Higher the difference means higher the priority. It is so
     EVHD algorithm is a combined algorithm that
                                                                        because higher difference indicates that the MT is more
combines the static parameters of the network such as usage
                                                                        nearer to the BS of that network. Hence the MT can stay for
cost, bandwidth and power consumption and dynamic
                                                                        more time in the cell of the respective network before asking
parameters such as RSS, velocity and position of the MT.
                                                                        for another handoff. Thus it is possible to reduce the
The main objective of EVHD is to maximize the throughput
                                                                        unnecessary handoffs and improve the performance of the
by reducing the number of handoffs. The EVHD algorithm
                                                                        system.
involves two phases: the calculation of Gain function and
the calculation of Overall Gain function.                                   The priority levels pi are assigned to the networks
                                                                        according to the difference. Overall Gain (OG) is calculated
     Calculation of Gain function provides cost
                                                                        by multiplying Gain function by this priority level.
differentiation. The Gain function calculates the cost of the
possible target network. It is a function of the offered                                           OG = G*pi
bandwidth B, Power consumption P and usage charge of the
network C.                                                              A candidate network which has the highest overall Gain is
                                                                        selected as the best network to handoff.




                                                                  125                             http://sites.google.com/site/ijcsis/
                                                                                                  ISSN 1947-5500
                                                       (IJCSIS) International Journal of Computer Science and Information Security,
                                                       Vol. 8, No. 7, October 2010



                      IV. SIMULATION                                    of handoffs is reduced by 78.5% in EVHD algorithm
                                                                        compared to static parameter based algorithm and 25%
                 A simulation model where two cellular                  compared to RSS-timer based algorithm. The huge reduction
systems GSM and CDMA and a WLAN forming an overlay                      in number of handoffs, is one of the major achievements of
structure is considered for simulation given below.                     EVHD algorithm.
                                                                             The number of packets serviced by static parameter
                                                                        based, RSS based, RSS-timer based and EVHD algorithms
                                                                        have been observed and is shown in fig.2. EVHD algorithm
                                                                        is able to service more number of packets in a given period
                                                                        of time compared to other algorithms because of its
                                                                        reduction in number of handoffs.
                                                                             The results show that the EVHD algorithm performs
                                                                        better in terms of number of handoffs and throughput
                                                                        compared to static parameter based, RSS based, RSS-timer
                                                                        based algorithms.

The MT can be in any one of the regions A, B, C, D. For                                                                                                 Figure 1

this simulation, the following values are assigned for the                                                16

parameters: PL(d0) = -30dB,     n = 4, d0 = 100m σ =                                                      14

8dB τ = 1sec                                                                                              12




                                                                          n m e o h d ffs
                                                                           u br f no
                                                                                                          10                                                                            static
                     WLAN       GSM             CDMA                                                                                                                                    RSS
                                                                                                          8
                                                                                                                                                                                        RSS-timer
  Offered
                     2Mbps      100kbps         150kbps                                                   6                                                                             proposed
  bandwidth                                                                                               4

  Power
                     3hrs       2.5hrs          2hrs                                                      2
  consumption                                                                                             0
                                                                                                                   1       2       3       4     5      6    7     8   9   10    11
  Usage cost         10Rs/min 5Rs/min          2.5Rs/min                                                                                       simulation time

  RSS threshold      -60dB    -80dB            -70Db
  Velocity
                     11m/s      13m/s           12m/s
  threshold
                                                                                                                                                         Figre 2

      The simulation has been performed for static parameter                                              700

based, RSS based, RSS-timer based and EVHD algorithms.                                                    600
In Static factors based algorithm, static parameters alone
have been considered and hence causes lot of false handoffs.                                              500
                                                                                      number of packets




In RSS based algorithm, RSS of the MT has been compared                                                   400
                                                                                                                                                                                       static

with the signal strength threshold of the respective network.                                                                                                                          RSS
                                                                                                                                                                                       RSS-timer
If it is lesser than the threshold, handoff is performed. But                                             300
                                                                                                                                                                                       proposed
because of some shadowing effects, the signal strength is                                                 200
used to fluctuate and cause a lot of false handoff trigger. In
RSS- timer based algorithm, RSS has been recorded over a                                                  100

period of time. This timer is applied to reduce the                                                            0
fluctuations of RSS caused by shadowing effect and hence,                                                              1       2       3   4      5      6   7     8   9   10   11
to reduce ping-pong handoff. In the Proposed EVHD                                                                                              simulation time
algorithm, static parameters, RSS, velocity and position of
the MT have been considered for handoff decision. A                                                                                            VI. CONCLUSION
handoff is carried out whenever the position of the MT
reaches to a certain boundary, regardless of RSS. This                      Efficient vertical handoff decision algorithm is a
reduces the handoff failure. The boundary is a safety                   combined algorithm that combines the static parameters of
distance of MT from the BS to assure a successful handoff               the network such as usage cost, bandwidth and power
and this boundary is not fixed and is varying according to              consumption and dynamic parameters such as RSS, velocity
the position and velocity of MT.                                        and position of the MT. The algorithm has been
                                                                        implemented successfully using ns-2 simulator. The results
                V. RESULTS AND DISCUSSION                               show that EVHD achieves about 78.5% reduction in number
     In this study, the performance evaluation of the efficient         of handoffs compared to static parameter based algorithm
vertical handoff decision algorithm (EVHD) has been                     and 25% reduction compared to RSS-timer based algorithm
carried out and the metrics number of unnecessary handoffs              and it is clear that EVHD provides better throughput with
and throughput have been compared with static parameter                 minimum number of handoffs compared to other algorithms.
based algorithm, RSS-static parameter based algorithm,                  Thus EVHD has outperformed the other algorithms by
RSS-timer-static parameter based algorithm.                             providing less number of handoffs and high throughput and
                                                                        hence it is efficient in enhancing QoS for multimedia
     The number of handoffs experienced by the algorithms               applications.
is shown in fig.1. The obtained results show that the number



                                                                  126                                                                                 http://sites.google.com/site/ijcsis/
                                                                                                                                                      ISSN 1947-5500
                                                               (IJCSIS) International Journal of Computer Science and Information Security,
                                                               Vol. 8, No. 7, October 2010



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[5]   A.H. Zahran, B. Liang, A. Saleh, Signal threshold adaptation for                   Georgia, USA, March 2004, pp. 867–872.
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      Networks and Applications 11 (4) (2006) 625–640.                                                       AUTHORS PROFILE
[6]   K. Yang, I. Gondal, B. Qiu, L.S. Dooley, Combined SINR based                  Mrs. S.Aghalya has received her B.E. degree in Electronics
      vertical handoff algorithm for next generation heterogeneous wireless         and Communication Engineering from Madras university ,
      networks, in: Proceedings of the 2007 IEEE Global
      Telecommunications Conference (GLOBECOM’07), Washington,                      India in 1991 and M.E. degree in Optical Communication
      DC, USA, November 2007, pp. 4483–4487.                                        from Anna University ,India in 2001. She is an Assistant
[7]   C. Chi, X. Cai, R. Hao, F. Liu. Modeling and analysis of handover
                                                                                    Professor at St.Joseph’s College Of Engg, Chennai ,India.
      algorithms, in: Proceedings of the 2007 IEEE Global                           She has 16 years of Teaching experience. She is currently
      Telecommunications Conference (GLOBECOM’07), Washington,                      pursuing her research at Anna University Trichy, India . Her
      DC,USA, November 2007, pp. 4473–4477.                                         research interest is in wireless networks.
[8]   Xiao.C, K.D.Mann and J.C.Olivier,2001, Mobile speed estimation for
      TDMA-based       hierarchical  cellular  systems,      Proc.Trans.
                                                                                    P. Seethalakshmi has received her B.E. degree in
      Veh.Technol,50, 981-991.                                                      Electronics and Communication Engineering in 1991 and
                                                                                    M.E. degree in Applied Electronics in 1995 from Bharathiar
[9]   Juang.R.T, H.P.Lin and D.B.Lin,2005, An improved location-based
      handover algorithm for GSM systems, Proc of wireless                          University, India. She obtained her doctoral degree from
      communications and networking conference, Mar.13-17, pp-1371-                 Anna University Chennai, India in the year 2004. She has
      1376.                                                                         15 years of Teaching experience and her areas of research
                                                                                    includes Multimedia Streaming, Wireless Networks,
                                                                                    Network Processors and Web Services.




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