Handoff Schemes in Mobile Environments: A
Comparative Study
Libin Thomas
Dept. of Computer Science
Christ University
Bengaluru, India
J Sandeep
Dept. of Computer Science
Christ University
Bengaluru, India
Bhargavi Goswami Joy Paulose
Dept. of Computer Science Dept. of Computer Science
Christ University Christ University
Bengaluru, India Bengaluru, India
[email protected] [email protected]
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.