Handoff Schemes in Mobile Environments: A

Comparative Study

Libin Thomas

Dept. of Computer Science

Christ University

Bengaluru, India

libin.thomas@christuniversity.in

J Sandeep

Dept. of Computer Science

Christ University

Bengaluru, India

sandeep.j@christuniversity.in

Bhargavi Goswami Joy Paulose

Dept. of Computer Science Dept. of Computer Science

Christ University Christ University

Bengaluru, India Bengaluru, India

bhargavi.goswami@christuniversity.in joy.paulose@christuniversity.in

Abstract— Vehicular Ad-hoc Networks is one of the most

popular applications of Ad-hoc Networks, where networks are

formed without any sort of physical connecting medium and can

be formed whenever required. It is an area in networks that has

enjoyed a considerable amount of attention for quite some time.

Due to the highly mobile environment where these networks find

their usability, it can be understood that there are a lot of

problems with respect to maintaining the communication links

between the moving vehicular nodes and the static

infrastructures which act as the Access Points (AP) for these

moving vehicular Mobile Nodes (MN). The coverage area of each

AP is limited and as such, the connections need to be re-

established time and again between the MNs and the closest

accessible AP. Handoff is the process involved here, which deals

with selecting the optimal APs as well as the best network

available for data transmission. In this paper, we compare

various handoff methods and categorize them based on the

different approaches they follow.

Keywords— Cost Based Methods, Wireless Communications,

Seamless, Ad-hoc networks, Handoff, VANETs, Switching,

Routing, Access Points, Cross Layer, Vertical Handoff, Horizontal

Handoff

I. INTRODUCTION

Ad-hoc networks are formed by the collaboration of one or more communicating nodes. These networks do not have any pre-determined architecture. It is applicable for a scenario that requires particular operations and after its scope, it simply cannot be adopted for any other purpose. In Wireless ad-hoc networks, the decisions are made dynamically. These decisions are based on the network it is connected to and the routing protocol it makes use of, to select the node that would store and further forward a packet. There are many applications that are based on ad-hoc networks. Nodes that are in a mobile environment make use of the ad-hoc networking

concept to be interconnected on the move at all times. One of the best examples of networks that provide communication support in a continually changing or moving environment are Vehicular Ad-hoc Networks or VANETs and these kinds of networks are used in inter-vehicular communications. VANETs can have communication nodes which can be Vehicle to Vehicle (V2V) and Vehicle to Infrastructure (V2I) [1]. Intelligent VANETs have already made an impact with their ability to make decisions in case of emergencies. VANETs are said to be made up of three attributes [2]. The first attribute are the vehicles themselves and they are considered to be the nodes in the network. Secondly, base stations along the roadside make up the infrastructure, thus giving a definite backbone to the network. The third set of components are radio waves which act as the communication channels for transferring data.

VANETs help mobile vehicles that move at great speeds to communicate effectively due to their ability to form intelligent networks that cater to the various needs of applications. A major aspect about these networks to be noted is the fact that these vehicles are highly mobile and are moving at great speeds. When there are mobile nodes that are travelling very high speeds, it may prove challenging to provide connectivity without distortions or network breakages. In scenarios such as these, the importance of handoff comes the fore. The delay caused by handoff dictates the QoS in VANETs [3]. Handoff is the process by which a link that was previously established between a node and an access point is switched to another point, in the direction in which the node is travelling in or is the closest to.

In this manner, a vehicle, which can be said to be a mobile node, can exchange information easily without getting disconnected from the access points. Re-routing is the most widely accepted and implemented method for handoff [4]. In

re-routing, a new path is identified and selected to be the route which the node must take in order to get connected to the next optimal access point, without much delay, therefore ensuring that there is no breakage in the communication link.

Handoff can be broadly categorized into two types based on the network being made use of. The two types are horizontal handoff and vertical handoff. Horizontal handoff is the process of ‘handing over’ taking place within a particular network itself. In this process, the access points are switched within the same network. In vertical handoff, node switching takes place between access points of different enabling technologies like LTE, WiMAX etc. This process not only focuses on the network allocation, but also used as a decisive agent to select the optimal channel through which data packets can be exchanged. Handoff is also differentiated based on the number of access points a mobile node is connected to at a particular instance of time [3]. The first type, known as the hard handoff (break before make). In this approach, handover takes place after the mobile node has been disconnected from the current access point. The second type of handoff is called the soft handoff (make before break) where the handover process takes place much before the connection between the current access point and the mobile node is terminated. In such a handoff process, it is necessary to identify the target access point beforehand.

This paper has been categorized into four sections. Section 2 gives an overview of the various methods proposed for optimal handoff in mobile environments. Section 3 gives comprehensive summary about the various handoff methods discussed and the different aspects covered by each method. Section 4 provides conclusive findings and the future scope.

II. OVERVIEW VARIOUS METHODS OF HANDOFF

This section will go through various methods that

were proposed to make the handoff process efficient in mobile

environments. Handoff, as discussed in the previous section, is

a part of the communication process that aims for establishing

communication links between a mobile node and an access

point, when the node moves out of the range of a cell’s

coverage area. This process can be better understood by

observing Fig.1, which shows a simple handoff mechanism.

As the mobile node is approaching the coverage area of the

second access point along its route while reaching the

boundary of the current access point, the access point will

send out a request message to the next access point to see if it

can accommodate the mobile node. If the reply received is

affirmative, the communication link is handed over to the

target access point. If the reply is negative, the access point

begins to look for suitable target access points in the direction

of the mobile node’s movement.

FIG. 1 (Attached at the bottom of this paper)

Handoff is divided into different categories based on the

type of networks involved and based on the number of access

points involved. Handovers can be either horizontal or vertical

based on the selection of the communication network. Fig.2

will help us to visualize the various methods categorized into

vertical and horizontal handoff methods. If the handoff

process takes place within the network in which the node and

the access point is situated in, it will be horizontal handoff. If

the process of handoff takes place in such a way that multiple

networks are considered then that handoff is said be a vertical

one. In section, we will consider the various aspects,

procedures, categories etc., of handoff before the various

methods for handover is discussed.

A. Heterogeneous Networks

Heterogeneous Wireless Network (HWN) is a part of the

continuously evolving radio access-based networks [5].

Heterogeneous wireless networks can consist of a number of

wireless technologies such as IEEE 802.11 standards or Wi-Fi

for LAN, IEEE 802.16 standards or WiMAX for MAN, IEEE

802.15 standards or Bluetooth for PAN, 4G LTE for WAN

and a variety of other future Radio Access Networks. Each of

these access networks have their own set of characteristics that

may be similar to the others technologies or may be entirely

different from the others. These characteristics are data packet

rate, the area of coverage, amount of energy the technology

consumes, the various protocols supported, the storage

required etc., Subsequently, a mobile node that makes use of

multi-mode devices or cognitive radio network technologies

will be able to make use of the complementary features of

these heterogeneous radio access networks under various

circumstances [6].

Standards organizations such as 3GPP have brought forth

proposals for smaller cells such as picocells, microcells and femtocells and these smaller cells prove to be an efficient way to enhance the capacity of the system in question and bringing the access points closer to the mobile nodes, thus helping to decrease the amount of energy consumed by the networks. These cells also help in the migration of LTE towards heterogeneous wireless networks [7]. A framework proposed by IEEE 802.21 standard [8] helps in enabling vertical handoff without having to lose communication links, by replacing the type of connectivity the mobile node makes use of, to access the infrastructure [5].

B. Cognitive Radio Based Network

It has been said that low spectrum utilization can be solved

with the effective usage of Cognitive Radio Networks [9]. The

available spectrum can be used effectively to its full extent

with the help of Cognitive radio technology, which allows

users who aren’t licensed (known as the secondary users), to

transfer data in bands that are licensed without causing any

sort of obstructions to the primary licensed users of the

bandwidth [10-12]. TCP/IP protocol stack is used with

additional functionalities in the physical and MAC layers for

data transfer between nodes that belong to a cognitive radio

network. Spectrum sensing and data transmission

functionalities are the newly added features in the physical

layer of cognitive networks [12] while spectrum decision

making, mobility of the spectrum, spectrum sensing and

spectrum sharing are the added functionalities of the MAC

layer for cognitive radio networks [13]. Cognitive radio’s

network layer features and functionalities are the same as the

network layer of a traditional network with the only difference

being that spectrum availability affects the routing function

[14] [15].

FIG. 2 (Attached at the bottom of this paper)

C. Vertical Handoff Methods

Vertical Handoff methods dealt with handoff processes

that occurred between different types of networks in a

spectrum. The bandwidth spectrum is usually depicted to be in

a vertical manner and as such, every network is allocated a

different band of frequencies from the spectrum.

a. Vertical Handoff in Heterogeneous Networks

In [16], the method focuses on Vertical Handoff where

handoff takes place between networks of various types. A

cross network vertical handoff is something that can improve

transmission of data as the most optimal network is selected

for the communications. A Vertical Handoff (VHO) decision

method can be seen here, in which the focus is on balancing

the overall load across all the attachment nodes and

maximizing the battery life of the mobile nodes involved. The

algorithm also includes a path-selection method for when

heterogeneous networks are involved, to select the most

optimal access point. It has been suggested that this VHD be

implement across vertical handoff decision controllers

(VHDCs). These controllers are located in the access networks

and they are responsible for providing the VHD function for a

particular region that contains one or more base stations. The

decision inputs are envisioned to be obtainable from the

media-independent handoff function (MIHF) that is defined by

IEEE 802.21 [17]. Vertical handoff decision controller is a

network-controlled mobility management system that is

controlled by IEEE 802.21 MIHF. MIHF can be used for

exchanging messages between the different access networks

that shares information about the load on the nodes, traffic,

link-layer conditions etc.

The algorithm starts the process of selection of optimal

network as soon as the link-layer trigger (LLT) is activated.

The LLT becomes active due to 1. RSS becoming lower than a

specific threshold for a node 2. When the RSS of another base

station has become significantly more than the currently

servicing access point. A modified version of Dynamic Source

Routing (DSR) is the protocol that is being used here for

optimal route discovery/ node selection.

Route request message is sent across the network, in order

to select the optimal access point. Route request travel is

limited by the Time-To-Live (TTL) field in the route request

message and this prevents the route request packets from

being sent to all of the access points in the network. After a

candidate node receives the request, it sends a relay message

to its access point. On receiving the relay message, the VHDC

selects the access point and the route optimal for that access

point. Once this is done, the node updates its routing table for

the node while sending relay acknowledgement to the proxy

node. The proxy node then sends a route reply message to the

node that initiated the route request message. The proxy node,

upon receiving a data frame, forwards the same to the next

node using IEEE 802.11 technology.

The DSR routing protocol being used here is modified in

such a way that if the downlink channel rate goes below a

certain threshold for the proxy node, it will enable

piggybacking for the degraded channel such that another

round of route selection is launched and a different optimal

route is selected [16].

b. Vertical Handoff based on MIH Standards

Media Independent Handover (MIH) is a standard

proposed by the IEEE 802.21 working group to provide

support for vertical handoff amongst heterogeneous wireless

networks and it considers signal strength as the only

characteristic in identifying the target network. The method in

[18] is a handover decision making module that includes more

number of parameters in addition to those that are considered

by MIH. These parameters include the node’s mobility and the

environment of the network during the handoff decision

making period so as to improve the quality and performance

of the network. MIH based handoff method given here is used

for evaluating vertical handoff performance involving

multimode terminals with Wi-Fi/WiMAX interfaces and MIH

entities. Link layer and application layer information is taken

into consideration for making handoff decisions. This way, the

mobile node is always connected and the switching process

becomes seamless in vertical handoff. The MIH protocol

defines the information shared amongst the MIH entities

during handoff and this produces messaging packets that are

common across various media.

The algorithm is made up of handoff initiation and

handoff decision modules. The first module increases

throughput and reduces packet loss by avoiding unnecessary

handovers in a network. The handoff initiation phase or

module consists of methods that consider and make decisions

about the necessity of handoff in a network and in that case,

selecting the optimal target access points across different

networks. The link up, link down or link going down events

trigger the implementation of The Vertical Handover

Algorithm (VHA).

The second part of the algorithm involves the handoff

decision making module. A potential target is selected from a

collection of eligible networks, during the decision-making

phase in handoff. Generally, there are two cases. The first is

that, if there is only one network available which incidentally

is the current network, then the handoff process takes place

within the network (horizontal handoff). If there are more

number of candidate networks, then Vertical Handoff will take

place once the node makes the decision to do so. The second

case is that, if there is more than one network available, then

the algorithm must identify the most prevalent or optimal

network. Once the selection is done, the traffic is routed from

the service network to the target network. Selecting the

optimal network is on the basis of numerous attributes

including PLR, delay, latency, throughput etc.

The method has numerous advantages over MIH such

as performance improvement in terms of throughput,

improving the handoff latency and the PLR. Mobile nodes are

known to have issues such as the ping pong effect, which

occurs when the node does not get attached to just one access

points and keeps switching between multiple access points.

This can lead to unnecessary multiple handovers, leading to

transmission delay and loss of packets. The method reduces

ping pong effect on the mobile nodes.

c. Spectrum Handoff for Network Selection in NEMO

based Networks

In the method from [19], the concept of Network

Mobility (NEMO) is being used to manage and perform the

handover process in cognitive radio (CR) vehicular networks.

CR based vehicular networks are intelligent networks that can

distinguish between channels which are unused and those that

are allocated, and use them to establish links between the

nodes and access points. The CR mobile node or the vehicular

node which is a constantly moving node, has the capability to

manage two or mode types of non-safety services (voice calls,

videos, best effort etc.) at the same time. This makes it

difficult for the mobile routers to select the best network that

needs to be used for handoff. For this, Multiple Attributes

Decision Making (MADM) method can be utilized for aiding

in handoff. In this paper, MADM methods such as grey

relational analysis and cost-based methods are used. The

reason why grey relational analysis (GRA) methods are used

is because the data in consideration is original information and

the calculations are simple. In cost-based method, it takes

multiple attributes directly for any sort of calculation that

needs to be performed. It is easier to make decisions in a

scenario that involves handoff in heterogeneous networks and

that is the reason why this method is included. The algorithm

includes the following five steps for selecting the optimal

network for handoff operation in a heterogeneous network

environment:

1. A matrix that is made up of the different attributes is

formed. This matrix contains data that is derived from NEMO

based cognitive networks in the form of

Dij = (Data Rate, PLR, Jitter, Price, Traffic Density, Direction,

Power Consumption),

Where i represents the access network while j represents the

multiple attributes.

2. The attributes are then converted into benefit

attributes and cost attributes. Benefit attributes are those which

need to be maximum. An example would be the data rate,

where maximum data rate means optimal network. Cost

attributes focus on the attributes that need to be minimum. An

example for this would be delay or PLR where the minimum

these values, the better. The value derived are based on cost-

based method, GRA method and entropy method.

3. Vector normalization and max-min normalization

needs to be used for obtaining normalized attributes. Cost

based methods make use of vector normalization values while

GRA method uses max-min normalization values.

4. Entropy method is used to compute the subjective

weights with or without taking into consideration the cognitive

node’s choices.

5. Optimal network is selected by making use of the

GRA and cost-based methods. GRA method can be used to

select the optimal network from the descending order of

weighted average values and the GRC values. Optimal

network can be selected using the cost-based method by

making use of the highest cost value derived [19].

D. Horizontal Handoff Methods

a. Early Binding Handover on IP

The method from [20] implements early binding for

faster handover in MIPv6 nodes in connectionless packet radio

links. It is not always possible to make sure that the mobile

nodes have enough time to send and receive messages before

performing handoff, due to the network’s highly mobile

nature. MIPv6 was introduced so that they could provide a

mobile communication system in vehicular IPv6-based

networks.

The actual handoff process involved in MIPv6 was

composed of Duplicated Address Detection (DAD), care-of

address (CoA), movement detection, and binding update [21].

To reduce the overall latency during handoff, the latency in

DAD and movement detection must either be reduced or

excluded completely. A slight yet remarkable modification

made to the original handoff in MIPv6 is to expect the third

layer handover (the network layer) just before the handover in

the second layer (link layer). This way, the latency in

handover can be theoretically be reduced with the inclusion of

an anticipating scheme. Even then, this anticipatory method of

handoff could not promise faster connectivity and lower

latency period.

Using simulations, the average staying time of a

mobile node was found to be travelling at around 80km/h

within a cell of radius 3km was around 170 seconds. This time

included the time required for handoff as well. Early binding

update meant that mobile nodes soon acquired the required

data about the upcoming network upon entering the cell area

of another access point and had sent fast binding update

messages before a trigger was delivered to its network layer.

Thus, ample amount of staying time within the radius or

coverage area of a cell is provided. The nodes that are slow

moving do not require a faster process for handover as they

have ample time for establishing links and for exchanging

information. To differentiate between slow moving nodes and

fast-moving nodes, flags have been implemented. The access

points can detect whether the nodes are moving or not from

the amount of time it stayed in the previous network or from

the time it takes to cross the boundary [20].

b. Handoff Mechanism using CoA in VANET

In [22], a handoff method in VANETs that makes use

of Care of Address (CoA) for pre-allocating an address to the

mobile node before the actual handoff takes place such that it

reduces the delay that occurs during handover is discussed.

The Internet Protocol (IP) made sure that when a mobile node

was about to change its access point, the network gateway was

made aware of the switching that was going to take place and

information about the node in question, the current access

point and the next access point were all stored in the gateway.

Network mobility is a requirement so that it supported the

movement between networks and carry forward the sessions

created for the mobile nodes, when the points of attachments

changed to fixed infrastructures [22].

Every node in NEMO has two addresses assigned to

it. Home address is the first type, which is assigned by the

home network agent and the second type of address is the Care

of Address (CoA) which gets assigned to the nodes by the

access points to which they get connected to. Thus, every time

a mobile node gets attached to an access point, CoA is

requested by the mobile node, which is then added into the

home network agent’s registry.

The Incessant Handoff Mechanism tries to reduce the

delay in handoff and to improve PLR. Mobile Anchor Point

(MAP) has been made use of, to take care of managing the

different routers and to assign the CoA to the mobile nodes.

Every mobile router has two addresses assigned to it which

are, the Regional Care of Address (RCoA) and the onLink

Care of Address (LCoA). RCoA stands for an address of the

MAP that takes care of the domain while the LCoA is the

address of the router [22]. If there is movement from the

mobile router, MAP assumes that a handoff process is about to

take place although, it will not know if the handoff will be

inter-domain or intra-domain. MAP pre-allocates an LCoA to

every access routers within a domain for every moving mobile

router. Inter-domain-pre-allocated address table stores this

information, which in turn, is then forwarded to all of the

routers that lie within the range of that particular domain using

the method of limited broadcast routing.

As the MAP does not know about the kind of handoff

that is about to take place, it sends out messages that contain

the regional address (RCoA) and the link address (LCoA) of

the moving mobile router to the other routers. Identifying the

individual nodes in scenarios where there are more than one

node being handed off at the same time is the reason behind

including both the addresses. Inter-domain pre-allocated

address tables are created by the neighboring MAPs and these

tables store the reserved LCoA for all of the mobile routers

within the various access points/routers. The MAP, in whose

range the moving MR is present, will create an Intra-domain-

pre-allocated address table. In the first column of this table,

the current link address (LCoA) of the mobile router is stored,

the second column of the table contains the access point’s ID

denoting its IP address and the third column stores the link

address (LCoA) which happens to be the reserved address of

the mobile router under every access router. Limited broadcast

routing is used to then distribute this table to all of the routers

within the domain. On receiving the packets, the access router

tries to match its IP address with values from the table. The

row with the matching value of the IP address stored in the

second column of the table is selected and stored separately in

a fast access memory [22]. The aim is to reduce the load on

the access router and only a small amount of storage is

required in the case of multiple handoff processes taking place

simultaneously.

c. Handover for IPv6-based VANETs

In [23], a handover method with a new format for

IPv6 can be seen. Angle of Arrival (AoA) technique is made

use of by a current access point for predicting the approaching

target access point (TAP). The orientation of a vehicular node

or any static access point can be determined by AoA, with

respect to any infrastructure. Mobility handover of a vehicular

node within the same subnet is taken care of. During this first

step, the vehicle maintains the data link layer connection while

the network layer is getting handed off, which means that the

node will still be able to communicate with the source access

point (previous access point) while trying to gain access to the

access point that is determined to be the target. At this time,

the router which controls both the access points, that is the

TAP and source access point (SAP), will have its information

modified by the current access point about the switching

between the networks that will take place. Ideally, the handoff

process is started by the source access point.

The second step focuses on the mobility handoff of

the moving vehicular node, between the two subnets that are

being addressed to [3]. Support from the vehicle that is

connected to TAP is required. As the vehicle approaches a

different subnet that is being controlled by another router, the

node pings the neighboring vehicle that is attached to TAP

which in turn leads to the neighboring vehicle alerting its

router about the mobile node that is approaching its vicinity.

The method of having the neighboring vehicular node aiding

in handoff is aimed at making the handover process less

tedious while also cutting costs.

d. Handover Management in IP based Mobile Networks

In [24], a handoff method for mobile vehicles using

MIPv6 where every mobile router (vehicle), access points and

the routers are assigned with a particular global IPv6 address

by the operator of a particular network. The node retains this

address when it moves, switching from one road side unit to

another road side unit, within the range of that particular

operator. The vehicular node sends a VRS (Vehicular Route

Solicitation) as a proxy [3], which works as a pinging packet

or a message that tells the service access point that the node is

about to leave the range of that access point, when the node is

about to cross the boundary of its subnet. Handover Assist

Information (HAI) is shared within this proxy message and it

contains the access point IDs, node IDs and the router IDs.

The serving/service access point then makes the choice with

respect to selecting a target access point or router. One thing

to note here is that a router can be an access point but an

access point cannot be a router. The selection of a target

access point is based on the information shared by the HAI,

thus selecting optimal router which leads to an efficient

handover. Once the selection is made by the serving access

point, the mobile node that is about to switch receives a proxy

router advertisement message [3] containing the target access

point’s IPv6 address. At the same time, the target access point

receives a Handover Initiation message containing the IPv6

address of the vehicular node. After this step, the service

access point then sends a request message for binding, which

contains the target access point’s IPv6 address, to the node’s

home agent and the corresponding nodes. An

acknowledgement is then forwarded from the target access

router to the currently servicing access router as a reply for the

handover initiation message sent.

A bidirectional route is thus set up between the

currently servicing access router and the target access router.

The target router then begins to buffer the packets that is being

forwarded by the service access point, which were meant for

the vehicular node. During the same time, the reply for the

binding request message is given out by the home agent of the

vehicular node and its correspondent nodes. The node then

detaches from the service access point’s link layer and gets

attached to the second layer or the data link layer of the target

router. This way, the communication link is not broken and

there is a seamless handover. The buffered packets which are

at the target access point are then forwarded to the global IPv6

address of the mobile node. Now, the correspondent nodes can

also be a part of the node to node process of packet

transmission [3].

e. Proactive Approach based Handoff

The method in [25] follows a proactive approach for

handoff by making use of access points in vehicular ad-hoc

networks. The newly implemented scheme is a faster handoff

method that makes use of access point graphs in multiple

nodes, improving the re-association latency and separating the

re-association latency from context transfer process.

It depends on the Inter-Access Point Protocol (IAPP)

for context and state transfer processes between APs. The

method has five sections with each section having an

algorithm that performs a specific task. The first one focuses

on AP Graph. Defining a directed graph G = (V, E) where V is

the collection of all access points and E is the collection of

edges between the access points. The relation between all

access points is said to be an equivalent relation as it is

reflexive, symmetric and transitive. APs are directional and as

direction is an issue in moving vehicular handoff, AP Graphs

are used. Minimum Spanning Tree is applied to the network to

identify the shortest path for each of the components. The first

section ends with identifying the topological ordering graph

and moves on to the next set of algorithms. The second section

of the algorithm focuses on computing the DFS for network

such that all AP components that are connected strongly is

selected to be the output. The third section focuses on the

Propagation algorithm. The fourth section deals with a

Handoff Algorithm for Elimination. The final algorithm works

for insertion of the Access Points into the graph [25].

f. QoS-Enabled Handoff Using CoMP

In the method from [26], a handover scheme that is

based on Co-ordinated Multiple Point transmission for

femtocell networks. Femtocells are the latest technology being

used to overcome the issues faced by conventional

communication networks due to improvements over existing

networks such as extended coverage, improved Signal-to-

Interference-plus-Noise Ratio (SINR), lower power

consumption. 3GPP Release 12 for LTE Advanced [27]

defines CoMP as of its features that was formed to satisfy the

need of IMT Advanced Framework and has since been a focus

of research with respect to high speed moving femtocell

techcnology [26]. It is an eNodeB-to-eNodeB handoff method

using CoMP for highly mobile VANETs that deploys

femtocell networks. eNodeB or e-UTRAN Node B which is

also called as evolved node B happens to be the air interface (a

communication link between two stations in a wireless

communications scenario) for LTE and it evolved from Node

B which was a part of UTRAN in UMTS technology. When a

transceiver that is outside a network needs to perform handoff

operation, the transfer of information amongst the mobile

users that was handled by that transceiver and the currently

serving node will be controlled by other transceivers outside

the network which is under the management of the central

control femtocell (CCF) access point [26]. Once the

connection between the target eNodeB and the outside

transceiver has been established, the mobile users begin to use

that transceiver again. The information about that particular

target eNodeB is made use of by other transceivers to aid in

their handover processes. The use of CoMP in handoff spans

over four different steps. They are:

1. The moving train nears the edge of the area being

served by eNodeB, to which it is connected, and the strength

of the signal weakens. The transceiver that is on the first

compartment requires handoff and begins scanning the

network to identify the best eNodeB neighbor. To ensure

transmission of data for the users within the first compartment,

the control femtocell access point (CCF) connects the

femtocell access point of that compartment with one of the

other remaining outside transceivers that is still connected to

the serving eNodeB.

2. Once the transceiver completes scanning its

surroundings, it gets connected to the best neighboring

eNodeB. CCF receives data about that neighbor eNodeB. The

connected target eNodeB and the transceiver start to

communicate after the connection is established. At this point,

the users within the first compartment who were connected to

other transceivers get connected back to the first transceiver.

3. As the second compartment crosses the area covered

by the serving eNodeB, it needs to perform handover. By

making use of the information stored in the CCF, it gets

connected to the target eNodeB without having to scan for the

best neighbor eNodeBs, thus making the handoff process

becomes faster for the second compartment.

4. Step 3 is continued until all the compartments

perform handover and the process starts over again once the

area covered by that eNodeB is crossed.

Probability of outage for the link established between

transceivers placed on the outside of the compartments of a

train and the target eNodeBs is shown as

𝑃out,proposed = 𝑃[max (SINReNodeB (𝑘) , SINReNodeB

(𝑚)) < 𝛾out] ,

Where 𝛾out is a predefined threshold for indicating the

acceptable reception; 𝑘 and 𝑚 are identities of neighbor

eNodeBs i.e, the serving node and the target node respectively

[26].

g. Proxy MIPv6 Handoff Scheme in VANETs

The method from [28] tries to improve the efficiency

of Proxy MIPv6 addressing technique. Due to the highly

mobile environment in VANETs, there occurs frequent

handovers, which can result in a larger Packet Loss Ratio

(PLR) which leads to data packets being dropped and an

increase in handoff latency. An early binding update in MIPv6

can be used to counter these issues. An IP address pool is

stored and maintained by a Mobile Access Gateway (MAG)

which contains the addresses given by a network

administrator. This gateway contains a table that stores data

about other gateways or routers. This is an advantage for the

current or previous router to identify a new gateway or router

which can be an eligible candidate for handoff or to which

gateway the mobile node can get attached to next. When the

current gateway notices that the mobile node is moving out of

range of the current gateway’s reach, it will alert the next

mobile access gateway about the vehicle that is moving

towards it’s vicinity. Thus, the next mobile access gateway

gets ready for the newly inbound vehicle by assigning an IP

address for that node. Binding cache entry at the local mobility

anchor gets updated with the information about the new

mobile node [3]. The next mobile access gateway sends an

IRA (Information Request Acknowledgement) to the

approaching vehicle without any delay. This way, the vehicle

that is about to enter the next access gateway’s range can

configure its IP address with that gateway, while still being

connected to the previous mobile access gateway.

GPS is used by the vehicular nodes to share its

location coordinates with the Points of Attachments (PoA) so

that it makes it easier to identify the direction in which the

node seems to be travelling in. Uninterrupted internet

connection is the feasible feature brought in by the scheme

[28].

h. Handoff Mechanism using RFID Tags

In [29], a given method tries to perform handoff

using RFID tags. The most note-worthy characteristic of a

mobile node is its ability to move between networks or within

a network, with very less to zero breakage in the

communication links established between the mobile node and

the access point. The vehicular node can move around a

network or between networks but the infrastructure that

provides communication access to these mobile nodes are

static. Handoff is performed by making use of RFID tags that

are embedded onto a vehicular node. MAC address, which is a

unique address that gets assigned to every vehicle, is stored in

the RFID tag is embedded into the chassis of the vehicle.

RFID scanner strips are placed along the road side which

seeks for the RFID tags on the vehicles and these scanner

strips forward the MAC address found, to the access gateways

that are responsible for performing handoff. A subnet is

composed of a group of access points and it is maintained by a

single access router. The advantage here is that the access

router will be able to identify the vehicle which is approaching

its subnet, thereby reducing the delay in handoff. The subnet

makes use of a server known as the Local Vehicle Server

(LVS), for efficient maintenance of the mobile vehicular

nodes [29]. The LVS stores and updates a table containing

information about the vehicle relation. The updated table

contains MAC addresses of the vehicles, IP addresses of the

vehicles and the IP addresses of the access points. A different

server known as the Global Vehicle Server (GVS) is used to

store and maintain the vehicle locality table. This table has

two fields- IPv6 address of the vehicle and the IPv6 address of

the access router. GVS also maintains a separate table called

the vehicle address table to store the mobile nodes’ home

addresses. The home address of every mobile node is assigned

by the home agent during startup time. MAC address and

home address of the mobile node is stored by the vehicle

address table. The vehicle relation table manages the mobile

nodes falling under that particular subnet of an LVS while the

vehicle locality table maintains a record of the mobile node’s

position within the subnet.

i. Handoff in Wireless Mesh Network (FHT for WMN)

The method from [30] is a handoff technique in

wireless mesh networks. Handoff in a mesh network takes

place whenever the node leaves the boundary of its Home

Mesh Router (HMR). It tries to select or gain access to a new

router based on how good its Signal-to-Noise Ratio (SNR) is.

For the mobile node to get connected to a Foreign Mesh

Router (FMR) with good signal, it first needs to be

authenticated for access by the FMR to which the node is

trying to get connected to. The time taken for the mobile client

to request the FMR and in turn, for the FMR to authenticate

the mobile node, creates latency within the network, thus

causing overheads and security issues. According to [30],

there are two types of handoff authentication techniques that

ensure faster authentication of the mobile node by the foreign

mesh router and they are public key based and symmetric key

based. Handoff latency is reduced by issuing tickets for

authentication. Keys are generated for faster authentication

between the servers and the clients. A master key is created

between the mesh server and the mesh client. A group master

key is generated by selected mesh routers which will be used

by all mesh routers. Tickets generated by authentication

servers for the respective mesh routers based on the group

master key are distributed to all routers. The roaming client

requests for the authentication server whenever there is a

transfer from the home mesh router to the foreign mesh router

and if there is a match in the keys, then handoff is initiated.

j. Neighboring vehicle-assisted handoff

In [31], the method performs handover process with

assistance from a vehicle that happens to be a neighboring

node of the node trying to switch in an inter-communicating

Vehicular Ad-hoc network scenario. It is a cross-layer method

which makes use of neighboring vehicular nodes in fog

communications scenario. Fog communication is a type of

network in which the infrastructure resides close to the source.

The cross-layer vertical fast handover or CVFH is a method in

which the vehicle makes use of a neighboring vehicle’s help to

identify a qualified vehicle in the network as the target access

point and acquire necessary information before the node

moved into the area of the target access point.

The method takes into consideration, an

infrastructure that is based on vehicular fog communications

network. In this network, access points are distributed along

the roadside and every access point has two attributes

associated to it:

a) Getting connected to the internet

b) Storing and forwarding IP addresses to vehicular

nodes.

In this infrastructure, every vehicular node has a

built-in GPS which aids in obtaining the speed and geo-

location data [31]. The vehicular nodes which fall under the

subnet of an access point are able communicate amongst

themselves. According to this system model, vehicles are of

three types and access points are of two types. Current

Vehicle, Neighbor Vehicle, and Neighbor Aided Vehicle are

the types of vehicles while Serving Access Point and Target

Access Point form the different types of access points. The

serving access point continuously monitors the received signal

strength indicator from the vehicle. Once this signal goes

lower than the allowed threshold, it starts to check if packets

have been lost. If the packet loss ratio is high, the current

vehicle starts looking for a target access point by sending out

request messages to all of the neighboring vehicular nodes.

Once the message has been received by the neighbors, the

nodes that reply must satisfy the following constraints.

a) The serving access point of the neighboring node and

the current vehicle must not be the same, in a single hop

situation.

b) The neighbor that will be selected must be in front of

the current vehicle. This way, it is ensured that the current

vehicle moves closer to the target access point for handoff.

c) The neighbor does not receive any neighbor reply

message from the current vehicle. If this condition is not

satisfied, then it means that there are other nodes that can be

preferred.

d) The time interval in which the reply is sent is

calculated as soon as the above conditions are satisfied. The

time interval is calculated as,

Tx

Slot = { ((int) (PLR*10) •mySlotTime IsAP = 0

0 IsAP = 1 [19]

The value of AP is Boolean. If the AP value is 1, it means that

it is an access point. If the value is 0, it means that the

neighbor is a vehicle.

Once the reply that is forwarded by the neighboring node

is received by the current vehicle, the service access point of

the neighboring node becomes the target access point of the

current node and the neighboring node becomes the neighbor

aided vehicle. Then, the neighbor aided vehicle starts

authentication and shares information about the connection

parameters and the IP addresses of the target access points. As

the required information about the current vehicle has already

been authenticated by the target access point, the handoff

process takes place quickly. If the current vehicle does not get

a neighbor reply back any of the one hop neighbors, then the

coverage of the service access point is given to the roadside

units or standing infrastructures. Then, the current vehicle

continues to broadcast messages until the next neighbor aided

vehicle is found until a NAV replies with a neighbor reply, by

which time, the information about the vehicle node is shared

with the target access point for a faster handover process [31].

E. Analysis and Comparison of Methods

Numerous handoff methods are discussed in this study.

Each one of these methods employ a different approach to

perform the handoff operation. Some of the methods focus on

attributes such as reducing latency, reducing the PLR,

increasing throughput while others help in deciding which

network can be selected for optimal handoff and data

transmission. In table 1, the different approaches which are

followed by the various handoff methods can be seen.

Advantages and Disadvantages of the different methods are

also identified in table 1 such that, understanding how these

methods work becomes easier. Handoff methods that were

suggested are numerous. Each handoff method focuses on a

different problem and identifies a solution for it. Some

handoff schemes focus on selecting the best network from a

spectrum while others may focus on selecting the best access

point within the same network. There are methods that do

both, that is, select an optimal access point across

heterogeneous networks. As we have already discussed

handoff based on the type of selection of networks, table 1

will help us to understand the concepts behind the methods in

a simpler manner.

TABLE I. FEATURES OF THE DIFFERENT METHODS DISCUSSED IN THE

PAPER. (TABLE ATTACHED AT THE BOTTOM)

F. CONCLUSION

The study aims at bringing together various handoff

methods and understanding their various characteristics, thus

helping the reader in understanding what each of these

methods propose and how they work. The handoff methods

have been categorized based on the type of network selection

that they perform. Handoff has become an integral part of ad-

hoc networks that are mobile in nature as the connection links

need to be maintained at all times for optimum QoS. Various

methods try to perform handoff by taking into consideration

features such as packet delay, Packet Loss Ratio, latency all

while maintaining seamless communication links between the

mobile nodes and the access points. Some methods make use

of unconventional methods such as using RFID tags, making

use of a neighboring node etc., to perform handoff in a simpler

and faster manner. A few methods try to perform handoff

across various networks and these face a lot of complex

problems that intra-network handoff. The future scope of the

paper is to compare more handoff schemes, so that the

literature coverage is much more extensive and can help any

researcher to easily identify the method they are trying to

modify or work upon.

ACKNOWLEDGMENT

The authors would like to thank the Dept. of Computer Science at Christ University, Bengaluru, India for their whole- hearted support.

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Table 1. Features of the different methods discussed in the paper.

Features of the different methods discussed in the paper

Name Approach Advantages Disadvantages

Early Binding Handover on IP

Anticipatory method by

making use of early

binding

Reduced binding latency.

Improved QoS for an

anticipatory method of

handoff.

Does not reduce

overhead. Consumes

more bandwidth of

wireless link due to the

storage of adjacent router

information.

Vertical Handoff in

Heterogeneous Wireless

Networks

Vertical handoff for

selecting optimal networks

amongst various types by

implementing vertical

handoff decision making

algorithm across various

vertical handoff decision

making controllers.

Optimizes a combined cost

function that includes

battery life and load

balancing over the access

points. DSR protocol is used

for routing which enables

piggybacking once the

downlink channel goes

below a certain threshold

Handoff Delay and

Latency period is not

covered.

Handoff Mechanism using CoA

in VANET

Incessant Handoff. Makes

use of pre-allocation

method to allocate the

address to each mobile

node. Care of Address is

used to identify the mobile

node whenever it requests

for handoff to a particular

access point

Incessant Handoff has a

reduced amount of

registration delay and packet

loss.

Does not ensure load

balancing.

Handover for IPv6-based

VANETs

Angle of Arrival is used by

a static infrastructure to

identify and predict

approaching vehicles.

There is no requirement for

CoA. Connection is not

terminated at any point.

Relies on neighboring

vehicle. Heavy network

overhead If there is a lot

of traffic.

Handover Management in IP

based mobile networks

Vehicular Route

Solicitation is shared as a

proxy message so that it

announces its readiness for

handover to the access

points and it contains

handover assist

information that contains

the various addresses that

help during handoff.

Connection is not lost

during handoff. Packets are

buffered at switching nodes

which ensures that the

packets are not lost.

Buffering of packets

require additional storage

support. Data

transmission can also take

time due to traversing

multiple nodes.

Proactive Approach based

Handoff

The method makes use of

Access Point graph for a

proactive approach that

scans only the selected AP

based on association

patterns.

Handoff latency reduced due

to re-association relations.

Works with multiple

vehicular nodes.

Higher signaling

overhead.

QoS-Enabled Handoff Using

CoMP

eNodeB to eNodeB

Handoff method that

makes use of Coordinated

Multiple Point

transmission for femtocell

The link between the outside

transceivers and the

eNodeBs are maintained

intact with the help of

CoMP. The selection of

Two signals are taken in

by the nodes

simultaneously and

outage can occur if both

are bad signals.

Features of the different methods discussed in the paper

Name Approach Advantages Disadvantages

networks. target eNodeB is made

simpler thus improving the

speed of handoff. SINR is

significantly low. Outage

probability is reduced with

the use of CoMP.

Proxy MIPv6 Handoff Scheme

in VANETs

The mobile access gateway

maintains a pool of IP

addresses of all the nodes.

Previous mobile access

gateway identifies the next

mobile access gateway

from the table of gateways

available. Also makes use

of GPS to identify the

node's path.

The mobile node is always

connected to the Mobile

Access Gateway. Handoff is

performed before the vehicle

leaves a network and there is

a noticeable reduction in

packet loss.

Handoff Delay which is

caused due to the use of

CoA which requires

Duplicate Address

Detection to verify the

uniqueness.

Handoff Mechanism using RFID

Tags

RFID tags are used in this

method which are

embedded into the

vehicular nodes and these

tags contain MAC

addresses of the vehicles.

RFID scanner strips along

the roadsides are used to

scan these RFID tags and

these scanners further

forward the MAC

addresses to the access

gateways for faster handoff

process.

The access router will be

able to identify any vehicle

that is approaching the

subnet beforehand so that

there is no handoff delay.

Although there is a

potential for the use of

RFID tags on moving

nodes, the use of other

methods must be

implemented to reduce

latency, SINR, packet

overhead etc.

Vertical Handoff based on MIH

standards

Media Independent

Handover is considered in

this method with added

parameters.

Handoff latency is reduced.

Performance improvement

in terms of throughput and

PLR.

Data storage overhead

maybe large due to the

number of parameters

used.

Handoff in Wireless Mesh

Network (FHT for WMN)

The use of Keys are

predominant here where

public keys and symmetric

keys can improve the

efficiency in handoff.

The use of a single group

master key for a group of

selected mesh networks

reduces the storage

overhead. Because of the

use of public keys,

authentication becomes

faster, thus reducing latency.

Computational Latency

and Overhead are the

only factors that are

considered in this

method.

Neighboring vehicle-assisted

handoff

A cross layer and a

neighboring vehicle aided

handoff where a mobile

node makes extensive use

of its neighboring vehicle

to identify the apt target

access points in a vehicular

fog networks.

The handoff delay is

reduced and the throughput

is improved.

Overhead may be large.

Spectrum Handoff for Network

Selection in NEMO based

networks

Multiple Attributes

Decision Making methods

like Cost Based Method

and Grey Relational

Analysis Methods are

employed here which

considers various attributes

of networks for optimal

selection.

PLR, Data Rate, Price,

Delay are the attributes that

are considered in the cases

such as voice, video, best

effort service in various

combinations.

This can be considered in

a vehicle assisted

scenario

Fig.1 A Simple Handoff Procedure

Fig.2 Categorization of Handoff Based on the types of networks involved.