1

Vertical Handoff in Heterogeneous Networks

Sandeep Sivvam

1, T.S. Jyothi Lakshmi

2,

C. Dharma Raj3

1Dept. of E.C.E, GITAM Institute of Technology, GITAM University,

Visakhapatnam, Andhra Pradesh, India 2Dept. of E.C.E, School of Engineering,

Gayatri Vidya Parishad College for Degree and P.G. Courses, Rushikonda,

Visakhapatnam, Andhra Pradesh, India. 3E.C.E & Vice Principal, GITAM Institute of Technology, GITAM

University, Visakhapatnam, Andhra Pradesh, India.

Abstract

In today’s world, wireless networking is pretty significant and popular way of giving worldwide information access to users wherever they move. One of the important tasks for continuous mobility is to have a simple, strong vertical handoff. The main objective of handover/handoff is to maintain the ongoing calls without any call drop even when the user is in motion. Generally, it is initiated whenever the mobile is crossing the boundary of a cell or by drop/fall in quality of the signal in the current channel. Supposing if the user is in motion and as the user leaves the cell and the user is in call during that time, if handoffs are not used or unavailable then its ongoing call is disconnected. One of the major concerns in heterogeneous wireless networks is to give the support of robust vertical handoff/handover. It occurs whenever a mobile shifts from one network to another (e.g., from WLAN to CDMA, 3G to GPRS etc.).

Key Words and Phrases: Handoff, Vertical Handoff, Handover, Horizontal handoff.

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1 Introduction

In cellular phone system, large number of base stations covering a

small geographic area known as cells. Mobility[1][19] is the

fundamental feature of a wireless cellular communication system. It is

provided by cell phone systems. So, it is the essential requirement of

the system that, as the mobile unit switches from one cell area to

another cell area, it must be able to transfer the call from the base

station of the first/primary cell, to that of the base station of the

next/new cell without any drop in call. It is similar in satellite

communications;[2] it is the method of relocating satellite control

responsibility from one earth station to another earth station with no

loss or interruption of service. In cellular communications, handover

means the process of reassigning an ongoing call/data session from

one channel connected to the core network to another channel on

every occasion that the mobile is in motion or the mobile is in hole

(weak spot or weak signal zone).

2 Handoff/Handover Process

Handoff or handover is a process of changing the channel, which is

linked to current connection whenever a call is in process, if the

mobile is in motion during the call. The handover process is initiated

[3][16] with a request for handover. When the power received by the

mobile unit from the base station of neighboring cell goes above the

power received from the base station of the current cell for a precise

value, it is known as the handoff threshold and it is a fixed value. For

successful handoff, it must be implemented quickly before the current

connection is lost by grabbing a channel. The Fig. 1 illustrates the

basic handover process.

Fig. 1 Occurrence of Handoff

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The handover process[4] requires number of factors to be considered

e.g. which handoff scheme are we using, how many channels in the

cell are free for call. In the handoff process the quality of service have

to be kept up to the standard. Handoff schemes which are poorly

designed incline to generate thick signalling traffic and, therefore, the

quality of service (QoS) dramatically decreased.

3 Types of Handoff

Handoffs/Handovers are broadly classified[5] into two forms- hard

and soft handoffs. These are also referred as “make before break” and

“break before make”. In the soft handoff, during the handoff process,

not only the existing resources even the new resources are used but in

hard handoff, current resources are freed before the new sources are in

use. Generally, Hard Handoff occurs in GSM systems and soft

handoff in CDMA systems.

A hard handover (or handoff) is a “break before make” handover [6].

In hard handover/handoff, the connection or the call gets dropped

before the user is linked to the new cell’s base station, this means that

the mobile station is linked to one base station at a given time.

Fig. 2 Different types of Handoff

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Hard handoff request must be served instantly in order to reduce the

interruption to the call. Handoff Initiation begin when the signal

strength at the mobile received from base station 2 is more than that of

base station 1. The signal strength measures are truly the signal levels

averaged over a chosen amount of time. The major problem [7] with

this approach to handoff decision is that the received signal strength of

both the base stations changes frequently. When the mobile is between

the base stations, the effect is to cause the mobile to wildly switch

links with either base station. The base stations bounce the link with

the mobile back and forth. Hence the phenomenon is called ping-

ponging.

Fig.3 Illustrates the conditions at which handover or handoff is

successful or not.

In Code Division Multiple Access (CDMA) soft handover [8] (or

handoff) is commonly used technique of Mobile Cellular Network. A

Soft handover is a handover were the switching and establishing of

connection with another base station will be completed before getting

disconnecting from the existing base station in the network, so it is

also referred as “Make-before- Break” Handoff. The main advantages

of soft handover technology are, during the mobile crosses between

the base stations the frequency/timing will not change due to this the

dead zones (weak spots or hole) will be minimized. Due to this effect

there is a small interruption in the connections face also the weak spots

will not exist. When compared with hard handover, Soft handover

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offers stable access continuity in network connection and very less

possibilities of call termination in the course of switching between

base stations. This is because of its intrinsic characteristic to handle

instantaneous frequency channels, which rarely suffer from

fading/interference at the same time and together. In soft handoff

technology, the connections are equally permanent and the

communication is very firm in comparison with other cellular

technologies because in CDMA technology, all the repeaters use

similar frequency channel for each mobile unit, regardless of the

location. When compared with hard handoff, the practical execution of

a Soft handoff is more costly and complex [11].

3.1 Horizontal Handoff

Horizontal handoff or handover is a handoff between two different

wireless networks operated by same network service providers. Here

mobility is performed on the same layers. In this handover technique

the on-going calls are to be continued, even though the IP address

changes due to the mobile node movement. This handover may be

categorized according to the direction of handover invocation. This is

the type of Handoff implemented till 3G technology.

3.2 Vertical Handoff

Vertical handoff or handover is a handoff between two different

wireless networks operated by different network service providers. In

vertical handover the mobility is performed between the different

layers and the users can move between different network technologies.

In vertical handover the mobile travels across many heterogeneous

networks and not only changes the IP address but also the Quality of

Service(QoS) characteristics and even changes the network interface.

In 4G both horizontal and vertical handoff is used.

Fig. 4 Comparison of Horizontal Handoff vs Vertical Handoff

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4 Vertical handover process

The process of vertical handoff can be divided into three main steps

namely handoff initiation, handoff decision, and handoff execution[1].

i) Handoff Initiation Phase: In this phase, in order to start the handoff

event, information to be collected about the network from different

layers likes Link Layer, Application Layer and Transport Layer. These

layers provide the information such as RSS, power, link speed, cost,

bandwidth, jitter, user preferences and network subscription,

throughput etc. Based on this information handoff will be initiated in

an appropriate time.

ii) Handoff Decision Phase: In this step, mobile device decides

whether the connection to be continued with current network or to be

switched over to another one and the decision may depend on various

parameters, which have been collected during handoff initiation phase.

iii) Handoff Execution Phase:

In this phase, existing connections need to be re-routed to the new

network in a seamless manner. In this phase, Authentication,

authorization and the transfer of user’s context information are also

included.

5 Handoff Management Issues

Handoff management has proposed several challenges[2][3] in the

implementation of wireless technologies. The open issues are listed

below:

5.1 Quality of Service (QoS)

The main issue to be considered is ensuring of same Quality of

Service(QoS) as of that it is in the primary base station before handoff

takes place. The key factors that impact the QoS disturbance during

handover are - handover blocking due to inadequate resources, out-of-

order cell delivery, cell losses, delay and delay differences. The

minimization of Quality of Service (QoS) disturbance can cost

buffering. Provisioning of the QoS also needed to address the timing

and synchronization issues.

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5.2 Rerouting Connections

The issues remain in improvement of processes for discovery of new

route options, for reconfiguring the connection path and creation of

signaling protocols for determination of the possibility of proposed

solutions.

5.3 Point to Multipoint

It deals with the upgradation of protocols that address rerouting the

point-to-multipoint links.

5.4 Mobile-to-Mobile Handoff

For a mobile to mobile connection, there is a need to address

upgradation of current protocols in order to support connection routing

and Quality of Service.

5.5 Optimization

Optimizing[15] the Vertical handoff decision parameters (RSS,

Bandwidth and Power etc.,) for HetNets is as important as providing

the user with best QoS continuously. Optimizing plays an important

role in making use of the available resources in the best possible

manner.

6 Conclusion

In wireless networks, handoff between cells is inevitable because it is

very necessary to maintain the ongoing calls and thereby reducing the

dropped call rate. There are incidents where a handoff is ineffective

and lots of research is conducting in this area to reduce the dropped

call rate. The handover initiation methods are composed on the basis

of hysteresis, signal strength, and threshold. The basic concept of

handoff in mobile cellular systems has been presented. Four

conventional handoff schemes i.e., soft handover, hard handover,

vertical handover and horizontal handover are briefed in this paper.

Some of the important handoff management issues are also discussed

in this paper.

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Author Biographies

Sandeep Sivvam is currently a research scholar in

Dept. of E.C.E, GITAM Institute of Technology

(GIT), GITAM University, Visakhaptnam, Andhra

Pradesh, India. He received his B.Tech. in

Electronics & Communication Engineering and

M.Tech. in VLSI Systems Design from JNTUK in

2010 and 2012, respectively. Currently, he is

working as Assistant Professor in Dept. of E.C.E,

School of Engineering, Gayatri Vidya Parishad College for Degree & P.G

Courses, Rushikonda, Visakhapatnam, Andhra Pradesh, India. His area of

research includes Wireless Communication and Embedded Systems.

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Jyothi Lakshmi T.S is currently working as

Associate Professor in Dept. of E.C.E, School of

Engineering, Gayatri Vidya Parishad College for

Degree & P.G Courses, Rushikonda,

Visakhapatnam, Andhra Pradesh, India. She received

her B.Tech. in Electronics Engineering from REC,

Calicut and M.E. in Electronics Engineering from

Mumbai University in 1991 and 2004, respectively.

She is also life Member of Institute of Electronics & Telecommunication

Engineers. She worked in reputed engineering colleges for the past 18

years and served in many positions. Her area of research includes Smart

Antennas, Wireless Communication.

Prof. C. Dharma Raj is currently working as

Professor in Dept. of E.C.E & Vice Principal,

GITAM Institute of Technology (GIT), GITAM

University, Visakhapatnam, Andhra Pradesh, India.

He received his B.E. in Electronics &

Communication Engineering from Andhra University

and M.E. in Microwave & Radar Engineering from

Osmania University in 1985 and 1987, respectively.

He received his Ph.D in 2012 from Gitam University. He worked as

Scientist “B” in LRDE, Bangalore for about one year and later he worked

in Marine and communication India Ltd., as a Technical officer. Eversince,

after his work in industry he is working in GITAM University and served

many positions for the past 28 years. He had published 26 papers in

Reputed Journals like Springer, IEEE etc., and he also wrote 7 text books

in the field of E.C.E and guided 7 Ph.D studens and 28 M.Tech projects.

His area of research includes Electromagnetic field theory, Antennas,

Microwave & Radar Engineering and Wireless Communications.

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