Efficient Vertical Handover Approaches

8/13/2019 Efficient Vertical Handover Approaches

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Efficient Vertical Handover Approaches for

Increased User Satisfaction in Next Generation

NetworksTabinda Salam 1, Samina Mushtaq 1, Taleaa Khalid 1, Muhammad Ali 2, Muhammad Amin 3

1Department of Computer Science

Frontier Women University

Peshawar, Pakistan.2,3

Institute of Management Sciences

Peshawar, [email protected]

[email protected][email protected]

Abstract The next generation of wireless networks willintegrate the current co-existing wireless networks to allow the

users to connect to the network that best suits their needs. The

users will be enabled to roam among different networks by a

process known as vertical handover. Vertical handover can be

imperative or alternative depending upon the reason of handover

initiation. In this paper, we have proposed algorithms to perform

imperative and alternative handovers. Our proposed algorithms

are efficient in terms of decrease in handover call drop rate and

avoidance of ping-pong effect. To elongate the battery life of

mobile station, we have proposed a power-based algorithm that

will be triggered when the power of mobile station drops below a

certain threshold. All of our proposed algorithms will increase

user satisfaction in 4G networks in terms of received signal

strength, QoS and conservation of battery life.

Keywords 4G networks, alternative handover, imperativehandover, QoS, user preference, utility based handover, vertical

handover.

I. INTRODUCTIONIn the current decade, wireless data access networks like

IEEE 802.11x WLAN and IEEE 802.16e WMAN have been

enhanced to provide very high data rates to end users to

enable them to enjoy social network websites, play online

games, download their favourite music files or videos, cancommunicate with others through video-conferencing and use

other web 2.0 and 3.0 technologies. Another remarkableadvancement of the current era is the evolution of mobile

cellular networks from 1G analog (like AMPS) to 3G packet

switched networks (like WCDMA or UMTS). These 3G

networks provide both voice and data services. However, thetheoretical data rate of UMTS is 2Mbps for mobile users

which is quite insufficient for todays network traffic demands.

As wireless data access networks provide high data rate while

cellular networks support high mobility of users, therefore, the

researchers proposed the integration of these co-existing

networks to provide high speed Internet access along with

voice communication services to the users from anywhere andat any time [1].

To enable seamless roaming of users among these

heterogeneous networks i.e. 4G networks, a process known as

vertical handover is used [1], as shown in fig. 1 [2]. Thisprocess allows the users to be always best connected

because at any given time, the users will be connected to a

network that best suit their requirements. In 4G networks, the

users will need to connect to a network whose signal strength

is the strongest among the neighbouring networks; they may

desire to handover to a network that is offering best servicesfor the applications running on their Mobile Stations (MSs);

or they may even require to handover to a large coverage area

network to elongate the battery life of their MSs. When thehandover is triggered on the basis of Received Signal Strength

(RSS), it is known as imperative handover and when it is

triggered to satisfy QoS requirements of the user, it is knownas alternative handover [1].

Fig. 1 Horizontal and Vertical Handovers [2]

In order to satisfy users aforementioned requirements, we

have proposed three different algorithms to carry out

imperative handover, alternative handover and power-based

handover. The first algorithm triggers handover when RSS

drops below a threshold value. Thus, it will allow the users to

WLAN WLAN

Vertical Handover Horizontal HandoverGPRSWLAN WLANWLAN

WLAN

GPRS Network
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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connect to a network having strongest RSS among the nearby

networks. The second handover algorithm i.e. alternativehandover algorithm evaluates the need of applications running

on MS and handovers to a network that best fulfils the

application requirements. It will facilitate the users to get best

services for their MSs applications. The third algorithm is

triggered when the power of the MS gets low. It prompts the

user for performing handover and executes it on the user

preference. This algorithm is beneficial for the users who wantto conserve the battery power of their MSs. The rest of the

paper is organized as follows: Section 2 explains vertical

handover along with the existing approaches of vertical

handover decision. Our proposed algorithms and theirflowcharts are presented in section 3. The last section contains

summary and future work.

II. VERTICAL HANDOVER DESCRIPTIONA. Vertical Handover Phases

Traditionally, vertical handover takes place in three phases,

namely: system discovery, handover decision and handover

execution [1]. However, [3] and [4] have discussed several

limitations when the handover occurs according to above

mentioned phases. Therefore, they suggested reordering of

vertical handover as: handover decision, system discovery and

handover execution. Furthermore, for conserving power of

MS, they proposed that the MS should activate interfaces for

those networks under whose coverage area it is present. The

information about nearby networks can be obtained by

installing location aware functionality in both the MS and

Base Stations (BS) or Access Points (AP); usually Global

Positioning System (GPS) is used for the said purpose [5].

B. Vertical Handover Decision Parameters

In 4G networks, multiple networks with varying

characteristics are available to the users. Therefore, vertical

handover can be triggered on the basis of several decision

parameters like QoS, cost, user preference, network condition(like RSS, bit error rate, signal to interference and noise ratio

(SINR) etc) and MSs velocity, its moving pattern and power

[6].

C. Prevention of ping-pong effect

In 4G networks, the MS may perform unnecessaryhandovers whenever it detects a network better than the

current one. This leads to a phenomenon known as ping pong

effect [7]. To prevent the occurrence of ping pong effect, the

concept of dwell timer is proposed in [8]. The dwell timer isused to check the stability of a network. If the network is

stable i.e. it can provide its services even after the expiry ofdwell timer, the vertical handover to this network is executed.

It is proposed in [9] that the duration of dwell timer should not

be static; its value should be adjusted in accordance with the

movement of MS. In [10], the authors have proposed a

mobile-velocity adaptive vertical handover that avoids pingpong effect by considering the velocity of MS. However, the

authors have defined dwell time as the time a MS may spend

within the coverage area of target network. A threshold value

for this dwell time is also defined and a MS performs

handover to the target network, if the dwell time of thatnetwork is larger than the threshold value.

D. Types of vertical handover

The authors of [1, 11, 12] have proposed several types of

vertical handovers. Among these types, the two prominent

types of vertical handover are imperative and alternative

handover. Imperative handover is an obligatory handover [12]as it is needed to maintain the connection of a MS with the

network. It is triggered mainly on the basis of RSS, SINR or

bit error rate [11]. Alternative handover, on the other hand, is

important from users perspective. It is triggered to satisfyuser preferences on the basis of QoS or cost [11].

Contrary to the traditional concept of imperative and

alternative handovers mentioned above, [13] has defined

imperative and alternative handover in a different way.

According to [13], imperative handover is triggered on the

basis of RSS, coverage area and QoS. The authors proposedthat alternative handover is triggered on the basis of non-

technical issues like user preference in terms of price and

incentives, services and context.

E. Existing approaches for vertical handover decision

Several schemes have been proposed that are used to takehandoff decision on the basis of user preference, network

condition (RSS or SINR, etc) and battery power. Like in [12],

a vertical handoff decision function (VHDF) is proposed that

takes into account network parameters including cost, security,

power consumption of a network, network condition and

network performance. The users assign weights to these

factors and all the reachable networks are evaluated on the

basis of VHDF. The network with highest VHDF value is

selected as the target network for performing handover. In [14]

a surplus based network selection algorithm is defined which

enables the users to select a network which is offering lowest

price among all the candidate networks for the users required

services. The authors claim that their proposed scheme avoids

network congestion and at the same time provides users

required QoS and optimally utilizes resources of all the

networks.

In [15], a time adaptive vertical handover decision schemeis proposed that selects a target network on the basis of user

preference and service requirements. For discovering

reachable networks, interface activation is performed on the

basis of two algorithms that dynamically adjust the time for

activating interfaces on the basis of beacon signals. However,

if a MS is not under the coverage area of a particular network,activating its interfaces for the corresponding network will be

wastage of power. Moreover, the authors proposed three

interfaces, one for network discovery, second for network

selection and third for communication with a particular

network provider for negotiation about the services he/shemay provide. The authors claim that this process will avoid

network congestion. However, a major limitation of this

negotiation is that the user has to explicitly communicate with

each network operator to determine the network that suits


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his/her needs. This negotiation will consume quite lot of time

and may cause delay in the handover process. Secondly, if allthe candidate networks reject the handover request, the user

again has to search the new network, which may lead to

significant handover delay. We propose an alternative

handover algorithm that selects a network according to users

perceived requirements without bothering the user to negotiate

about his/her required services with each network operator.

III.PROPOSED VERTICAL HANDOVER ALGORITHMSThis section describes our proposed vertical handover

algorithms. Our proposed algorithms require that the GPS

module should be installed on the MS to enable it to get the

information about its location, moving direction and velocity[16]. The location information will be sent by the MS to its

current Point of Attachment (PoA) (BS/AP) which in turn

informs the MS about neighbouring PoAs, using its own

location aware functionality [4]. The MS will then activate the

interfaces for those networks only which are in its vicinity.

This will prevent the activation of unnecessary interfaces and

also conserve the power of MS. The information about the

velocity and moving direction of MS will be used by our

proposed algorithms to calculate the estimated residence time

of the MS in a network. A network, for which the estimated

residence time of the MS is less than the favourable residence

time (), will not be considered as a candidate for handover.

We suggest that the BSs/APs must broadcast the

information about their coverage areas, services available and

network load in their beacon signals. A high load signal from

these PoAs will alert the MS about network load or congestion

and thus the MS will not consider this network as a candidate

for handover. This will surely decrease the handover call drop

rate which mostly occurs when a congested network rejects

the handover call and as a result, the call is dropped and the

MS gets disconnected from the network.

To ensure a seamless transfer of MS information from one

BS/AP to another, the CTP [17] will be used. The informationabout header compression, QoS and Authorization,

Authentication and Accounting (AAA) is sent to the new

BS/AP by the old one through CTP before handover execution.

Thus, it decreases the amount of time consumed which the

MS needed to perform protocol exchange with the new BS/AP

for allocating resources, after the handover is executed [18].We have proposed three different algorithms namely

imperative, alternative and power-based algorithms. Section

III- A describes imperative handover; III-B explains

alternative handover while III-C throws light at power-basedhandover. All these sub-sections contain the relevant

flowcharts as well. Table 1 contains the legend for all theflowcharts and algorithms.

Table 1: Legend for flowcharts and algorithms

RSScur Received signal strength of

current network

RSSTh Threshold Received signal

strength

RSSCN Received signal strength of

candidate network

Ucur Value of utility function for

current network

Ureq Required utility function value

Unew Utility function value of

reachable network

ResTime Residence time in a network

Favourable residence time

Critical value of battery

DTRSS Dwell timer for checkingnetwork stability in terms of

RSS

DTres Dwell timer for checking

network stability in terms of

ResTime

A.Proposed Imperative HandoverImperative handover is triggered when the network

condition changes and the MS does not get sufficient signal

strength from the current PoA (BS/AP) [11]. To continue an

on going call, it is necessary to handover to a nearest PoA. If

the handover is delayed, the call may be dropped and the MS

may get disconnected from the network. It is also possible that

only for a very short amount of time, the MS gets degradationin signal strength; may be due to an obstacle or a temporary

network load. If the MS start triggering handover to another

PoA as soon as signal strength degrades, it may lead to

unnecessary handover calls for the network and a ping pong

effect for the MS. Therefore, the MS should wait for a certain

dwell time and check the network condition. If even after the

expiry of dwell timer, the MS does not get sufficient signal

strength, it should initiate the handover process. As mentioned

earlier, the dwell timer should not be static rather it should be

adaptive [9]. We have proposed an equation to adjust the

amount of dwell time adaptively, according the signal strength

it receives from the current network. The proposed equation is

as follow:

DTRSS= Tmin+ (1- ((RSSThRSScur)/RSSTh)) Eq. 1

where:

DTRSS = Dwell timer according to RSS

RSSTh= Threshold RSS

RSScur= RSS of the current PoA

Tmin= Default minimum amount of time that MS should

wait for triggering handover

Imperative handover takes place between PoAs of similar

networks. It is possible that there is no homogeneous PoA in

the vicinity of the PoA to which MS is currently connected.

Therefore, to avoid disconnection for the MS, a verticalhandover needs to be performed. For example, a MS may be

connected to a WLAN when it experiences degradation in

signal strength and another WLAN is not available in its

vicinity. Therefore, the MS has to perform a vertical handover

to a 3G or any other network.

Our proposed imperative handover algorithm caters forboth of these possibilities and performs a horizontal or vertical

handover according to the situation. Algorithm 1 explains the

proposed imperative handover.


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1. If(RSScur < RSSTh) then2. //Check the instability of current

network

3. wait for DTRSSunits4. If (unstable) then5. Discover the nearest PoAs6. if(similar PoAs exist) then7. Determine the network condition8. Include non-congested PoAs into N9. else If(nearest similar PoAs does

not exist)then

10. Discover reachable networks11. Determine the network condition12. Include non-congested networks

into N

13. For each Ni N14. Calculate ResTime15. If (ResTime > ) then16. add Niinto RT17. End Loop18.

Sort RT into descending order19. For each RTi RT

20. If(RSSRTi>RSSTh) then21. wait for Tminunits22. If(stable && not congested) then23. Target network= RTi24. Perform Context transfer25. Execute HO26. End loop27. Else // Network is stable28. Stay with the current network29.Else // RSScur> RSSTh30. Stay with the current network

B.Proposed Alternative Handover4G network aims at providing the services that are required

by a user. Therefore, it facilitates the user to perform an

alternative handover to get better services for the applications

running on his/her MS. Our proposed alternative handover

algorithm evaluates the services required by an application. If

the application is not getting its required services, a handover

is triggered. All the non-congested, reachable networks are

discovered by activating only the relevant interfaces. The

estimated residence time of the MS in each network is

calculated. The networks for which the estimated residence

time is greater than threshold value, , are then evaluated by

our proposed utility function, as follows:

U= X * ((B*WB)+(1/C* WC)+(1/P* WP)) Eq. 2

where:

X= SA * HLsX= exclusion factor

SA = service available

HLs= High load signal

B = bandwidth available at the network

C = cost of the service

P = power consumed at the network

WB, WC, WPare weights assigned by the user to

Fig 2. Flowchart for imperative handover.

bandwidth, cost and power consumption respectively.

The utility function takes into account the bandwidth,

power consumption and cost of service of a network. User

preferences are also involved in the utility function by

allowing them to assign weights to these parameters. The sum

of these weights will always be equal to one. Through aninterface on the MS, the user can assign weights and he/shecan also change these weights according to his/her

requirements. For example, for non real time data, a user may

prefer cost over bandwidth, but for smooth running of real

time data, he may prefer bandwidth over cost.

Our proposed Eq. 2 also includes exclusion factor (X) i.e. if

the MS receives high load signal from a network, the value ofHLswill be set to zero. As a result, the utility of that network

will become zero. This indicates that the network is congested

and can not provide the resources required by the application.

Thus, the exclusion factor is used to exclude a network from

the set of candidate networks. All the networks are evaluated

and a set of candidate networks is made. Among the candidatenetworks, the network with highest value of utility function is

selected and its stability in terms of RSS and available

services is determined after the expiry of dwell-timer.

Algorithm 1.Imperative Handover ( )

RSScur

Target Network

Discover non-congested,

reachable networks

No

Yes

Stable

Unstable

Yes

NoIf Exist?

Evaluate CNs in

descending order

RSSCN > RSSTh& CN is Stable

Yes

Yes

No

Set of candidate

networks (CN)

Perform Context Transfer &

Execute Handover


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Algorithm 2.Alternative Handover ( )

1. If (Ucur < Ureq) then2. Discover reachable networks3. Determine network condition4. If (not congested) then5. Include network into N6. For each Ni N7. Calculate ResTime8. if (ResTime > ) then9. Calculate the utility of Ni10. If(UNi > Ureq)then11. Include Niinto M12. End loop13. Sort M into descending order14. For each Mi M15. IF (RSSMi > RSSTh) then16. Wait for DTresunits17. If (Stable) then18. Target network= Mi19. Perform Context transfer20. Execute handover and exit21. End Loop22. Else // No need of handover23. Stay connected with current network

This dwell timer is adaptive in terms of residence time and

value of utility function, and is calculated as follows:

DTres= Tmin+ (((ResTime- )/ ) * (1/UNi)) Eq. 3

Where

DTres= Dwell Timer according to residence time and

network utility

ResTime = residence time in the candidate network.

= favourable residence time

UNi = networks utility value

Even after the expiry of this dwell timer, the network can

still provide the required resources, the network is considered

as stable. Necessary context transfer is then performed using

CTP [16] and the handover is executed. All candidate

networks are not evaluated for checking stability and only the

network with highest utility is evaluated, therefore, lessamount of time and power is consumed. The proposed

alternative handover is explained in algorithm 2.

C.Proposed Power-Based HandoverPower is an important resource for all kind of portable

devices like laptops, MSs and PDAs etc. Therefore, certainmeasures should be taken to conserve the power and to

elongate its life-time. Usually wireless networks (WLAN or

WiMax) consume more power as compared to cellular

networks. Therefore, if a MS is connected to the wirelessnetwork, it may wish to handover to a cellular network to

elongate its battery life. We have developed an algorithm that

is triggered when the battery life of MS drops below a certain

threshold (). The user may assign a value to this threshold or

the algorithm can use the default value for it. When the

power-based handover is triggered, first of all the current

Fig. 3 Flowchart for alternative handover.

network of the MS is determined. If it is a cellular network,

the user is prompted to recharge the battery. In case of

wireless network, the user is prompted to enter his/her choice

of discard, allowed or deferred. If he/she wants to defer the

handover, e.g. to complete the downloading of his/herrequired files; he/she then specifies the amount of time for

which the handover is postponed. After the expiry of this time,the algorithm executes the handover. In case, user allowed to

handover, the algorithm executes the handover immediately. It

is also possible that the user does not wish to handover to

another network. So, he/she may discard the handover. As aresult, the MS will remain connected to the wireless network.

Algorithm 3 represents our proposed algorithm. A flowchart

of this algorithm is also presented in fig. 4.

IV.SUMMARY AND FUTURE WORKCurrent cellular networks (including 2.5G and 3G networks)

do not involve user preference in handover process. For thisreason, user preference is an important decision criterion for

next generation networks. Users mostly require good QoS fortheir MSs applications. They also require that their MSs

should automatically connect to a network offering good QoS,

whenever such a network is available. However, the handover

process should be seamless enough without affecting the app-lications executing on MSs. Our proposed alternative

handover algorithm caters for user preferences in terms of cost,

bandwidth and power consumption of a network. Users are

allowed to set preferences for these parameters and the

reachable networks are evaluated according to users priorities.

Ucur< UreqStay with current

network

ResTime > &&

Unew> Ureq

No

Yes

Yes

RSSCN>RSSTh

& CN is Stable

Yes

No


homogeneous nearest PoAs

Set of candidate

networks (CN)

Evaluate CNs in

descending order

Target Network


Execute Handover


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Algorithm 3.Power-Based Handover ( )

1. If (power < ) then2. Determine current network3. If (cellular network)4. Prompt to charge battery5. Stay connected with current network6. Else // in case of wireless network7. Take user choice (Allow, Defer,

discard)8. If (discard) then9. Stay connected with current network10. If (allow) then11. Go to step no. 1412. If (defer) then13. Wait for deferred unit times14. Discover the reachable BSs15. Determine the network condition16. If (not congested) then17. Include PoA into N18. For each Ni N19. Calculate ResTime20. If (ResTime > ) then21. add N

iinto RT

22. End Loop23. Sort RT into descending order24. For each RTi RT25. If (RSSRTi> RSSTh) then26. wait for Tminunits27. If (stable && not congested) then28. Target network= RTi29. Perform Context transfer30. Execute handover and exit31. End Loop32. Else // power > 33. Stay connected with current network

The network with best QoS among all the networks is selected

for handover. We have also proposed an algorithm forperforming imperative handover which will allow the users to

always receive sufficient signal strength for their MSs.

Whenever signal strength decreases, imperative handover to

another PoA is performed. To facilitate the users to elongate

their MSs power, a power-based algorithm is proposed. Thisalgorithm will determine the power of MS and will prompt the

user to allow handover to a less power consuming network.

Currently, we have not simulated any of our proposed

algorithms. In future, we will analyse our proposed algorithm

in terms of processing time, efficiency, handover call droprate and handover delay. Our current utility function for

incorporating user preferences does not include user

preference for a particular network operator. Therefore, wewill improve our utility function to involve this parameter as

well. Moreover, power-based algorithm prompts user for

amount of delay for handover execution. The user usuallydelays handover for certain amount of time in which his/her

important files can be downloaded. It is possible to improve

the power-based algorithm to estimate the handover execution

delay by determining the files currently downloading and

estimating the time in which they can be fully downloaded

and thus defers handover execution until the the downloadingcompletes.

Fig. 4 Flowchart for power based handover

ACKNOWLEDGMENTS

The authors would like to thank Ms Mahrukh Fida and Mr.

Waheed-ur-Rehman for their moral support and valuable

suggestions during the course of this research.

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Efficient Vertical Handover Approaches

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