Mm wcmc v12

WIRELESS COMMUNICATIONS AND MOBILE COMPUTING
Wirel. Commun. Mob. Comput. 00: 1–20 (2009)
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/wcm.0000
Mobility and Handoff Management in Vehicular Networks: A
Survey
Kun Zhu1
, Dusit Niyato1
, Ping Wang1
, Ekram Hossain2,†∗
, and Dong In Kim3
1
School of Computer Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
2
Department of Electrical and Computer Engineering, University of Manitoba, Winnipeg, Manitoba R3T 5V6,
Canada
3
School of Information and Communication Engineering, Sungkyunkwan University (SKKU) 23528 Suwon, Korea
Summary
Mobility management is one of the most challenging research issues for vehicular networks to support a variety
of intelligent transportation system (ITS) applications. The traditional mobility management schemes for Internet
and mobile ad hoc network (MANET) cannot meet the requirements of vehicular networks, and the performance
degrades severely due to the unique characteristics of vehicular networks (e.g., high mobility). Therefore, mobility
management solutions developed specifically for vehicular networks would be required. This article presents
a comprehensive survey on mobility management for vehicular networks. First, the requirements of mobility
management for vehicular networks are identified. Then, classified based on two communication scenarios in
vehicular networks, namely, vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications, the
existing mobility management schemes are reviewed. The differences between host-based and network-based
mobility management are discussed. To this end, several open research issues in mobility management for vehicular
networks are outlined.
Copyright c 2009 John Wiley & Sons, Ltd.
KEY WORDS: Mobility management; vehicular networks; host mobility; network mobility
1. Introduction
In recent years, there have been significant interest
and progress in the field of intelligent transportation
system (ITS) from both industry and academia. Typ-
ical ITS applications can be categorized into safety,
transport efficiency, and information/entertainment
applications (i.e., infortainment) [1]. Vehicular ad hoc
networks (VANETs) are emerging ITS technologies
∗Correspondence to: Ekram Hossain, Department of Electrical
and Computer Engineering at University of Manitoba, Winnipeg,
Manitoba R3T 2N2, Canada
†E-mail: ekram@ee.umanitoba.ca
integrating wireless communications to vehicles.
Different Consortia (e.g., Car-to-Car Communications
Consortium (C2C-CC) [2]) and standardization orga-
nization (e.g., IETF) have been working on various
issues in VANETs. C2C-CC aims to develop an open
industrial standard for inter-vehicle communication
using wireless LAN (WLAN) technology. For
example, IEEE 802.11p or dedicated short range
communications (DSRC) is an extension of 802.11
standard for inter-vehicle communication by IEEE
working group. IETF has standardized NEtwork
MObility Basic Support (NEMO BS) [3] for network
mobility in VANETs.
Copyright c 2009 John Wiley & Sons, Ltd.

2 K. ZHU, D. NIYATO, P. WANG, E. HOSSAIN, D. I. KIM
VANET involves vehicle-to-vehicle (V2V) and
vehicle-to-infrastructure (V2I) communications. V2V
refers to the direct or multihop communications
among vehicles in VANET. V2V is efficient and
cost effective due to its short range bandwidth
advantage and ad hoc nature. V2I refers to the
communication between vehicles and infrastructure
of roadside unit (RSU), e.g., base station and access
point connected with Internet. V2I communications
can be used for Internet access. A typical VANET
scenario is shown in Fig. 1. VANET is a special
type of mobile ad hoc networks (MANETs) [4] with
unique characteristics. Due to the high mobility of
vehicles, topologies of VANETs are highly dynamic.
Also, the density of VANETs varies dramatically.
Another major difference between VANETs and
traditional MANETs is that power consumption is not
a major concern in VANETs. Instead, the efficiency of
VANETs protocols is paramount.
Originating from cellular networks, mobility man-
agement has been an important and challenging
issue to support seamless communication. Mobility
management includes location management and
handoff management [5]. Location management has
the functions of tracking and updating current location
of mobile node (MN). Handoff management aims to
maintain the active connections when MN changes its
point of attachment.
Mobility management is essential for providing
high-speed and seamless services for vehicular
networks since MNs change their points of attachment
frequently and network topology can be changed
abruptly. Due to the differences between V2I and
V2V communications, their mobility management
schemes can be designed differently to achieve
optimal performance. Since V2I communication needs
data exchange with Internet, for compatibility and
interoperability reasons, most mobility management
solutions for V2I communication are designed based
on Internet mobility management protocols (e.g.,
Mobile IPv6). For V2V communication, mobility
management mainly focuses on route discovery,
maintenance, and recovery which are similar to those
in MANETs [6]. A review of the current state of the art
on mobility management for both V2V and V2I-based
VANETs will be provided in this paper.
The rest of this paper is organized as follows.
In Section 2 we provide an overview of mobility
management schemes for both V2I and V2V
communications. Host mobility and network mobility
solutions designed at different OSI layers are
introduced in Sections 3 and 4, respectively. Then,
mobility management for VANET with heterogeneous
access and simultaneous movement scenario is
discussed in Section 5. The open issues for mobility
management in vehicular networks are outlined in
Section 6. Conclusion is given in Section 7. In
this paper, the related terminologies of mobility
management are consistent with those in RFC3753 [7]
and RFC4885 [8].
Fig. 1. General model of vehicular networks.
2. Overview of Mobility Management in
Vehicular Networks
In this section, we discuss the mobility management
issues in vehicular networks for V2I and V2V
communications scenarios.
2.1. Mobility Management for V2I
Communications
In a V2I communications scenario, some ITS
applications require Internet access [9] through
an infrastructure or Internet gateway. The Internet
gateway can provide global addressability and
bidirectional Internet connectivity to the mobile nodes
in a VANET [10]. In a VANET, mobile nodes can be
far away from an Internet gateway, and their traffic can
be relayed through intermediate mobile nodes. This
is referred to as multihop communications. However,
Copyright c 2009 John Wiley & Sons, Ltd. Wirel. Commun. Mob. Comput. 00: 1–20 (2009)
DOI: 10.1002/wcm

MOBILITY AND HANDOFF MANAGEMENT IN VEHICULAR NETWORKS 3
in such a scenario, traditional MIPv6-based mobility
management solutions cannot be applied directly since
they require a direct connection between a mobile
node and infrastructure. Therefore, when integrating
MIPv6-based solutions into VANETs, many issues
arise (e.g., movement detection and handoff decision).
To support ITS applications, vehicular area network
(VAN) can be established (e.g., fixed vehicle sensors,
passengers’ mobile devices or even personal area
network (PAN) attached to the mobile routers located
in the vehicle). In this scenario, network mobility
(NEMO) basic support protocol [3] was introduced
to support mobility in a VAN. NEMO is an efficient
and scalable scheme since the mobility management is
transparent to the mobile devices (i.e., mobile device
does not send and receive signalling message directly).
However, route optimization was not considered in
NEMO BS. Triangular routing in MIPv4 becomes
quadrangular routing in NEMO BS. Much work has
been done to address the route optimization problem.
In addition, the vehicular network can be het-
erogeneous in which different wireless technologies
are integrated into one service. This will enable
seamless and high-speed connection, since the mobile
node can select the most suitable network for data
transmission [11].
Mobility management should guarantee the reacha-
bility to correspondent nodes (CN) in the Internet as
well as the global reachability to mobile nodes in a
vehicular network. Therefore, the mobility manage-
ment has to meet the following requirements [12, 13]:
(i) Seamless mobility: Mobility of vehicles should
be seamless. Accessability and service continu-
ity should be guaranteed regardless of vehicle’s
location and wireless technology.
(ii) Fast and vertical handover: Fast handover
is needed for delay-sensitive ITS applications
(e.g., safety-related). Fast handover is also a
crucial requirement for wireless networks with
small coverage area (e.g., WiFi network), since
the vehicle spends only short period of time at
each point of attachment (e.g., access point). In
a heterogeneous wireless environment, vertical
handover of the mobile users’ connections
among different wireless technologies must be
supported to achieve seamless service.
(iii) IPv6 support: The global reachability requires a
permanent globally routable IP address for each
mobile node. With large address space, IPv6
can support a unique address for each sensor
or mobile device in the vehicles. In addition to
the advantage in address space, IPv6 also has
better support of security and quality of service
(QoS) which are the essential requirements of
ITS applications.
(iv) Multihop communication support: Multihop
communication can extend the transmission
range of the mobile nodes to reach the
destination. Mobility management schemes
for vehicular networks need to consider the
multihop communications requirements, and
therefore, need to be optimized accordingly.
(v) Scalability and efficiency: VANETs may be
large in size which can consist of hundreds
of vehicles and thousands of devices in
one network. Furthermore, due to the high
frequency of change of the point of attachment,
the mobility management scheme must be
scalable and efficient to support different types
of traffic.
In traditional infrastructure-based mobile networks
(e.g., cellular system), mobility management can be
classified according to the following criteria:
(i) Network structure: Mobility management can
be classified into mobility management in
homogeneous networks [5] and in heteroge-
neous networks [14].
(ii) Users’ roaming area: Mobility management
can be classified into macro-mobility and
micro-mobility management solutions which
provide global and local mobility management,
respectively. Due to the hierarchical design
of global and local management, perfor-
mance of mobile users can be improved. For
macro-mobility management, mobile IPv4 [15]
and mobile IPv6 [16] were introduced. For
micro-mobility management, fast handover for
MIPv6 (FMIPv6) [17], Hierarchical MIPv6
(HMIPv6) [18], Cellular IP [19], HAWAII [20],
and Proxy MIPv6 (PMIPv6) [21] were pro-
posed.
(iii) Mobile host signalling: Mobility management
can be classified into host mobility and
network mobility management depending on
whether or not the mobile host is involved
in signalling for mobility management. If the
signalling of mobility management is sent or
received by the mobile host, it is called host
mobility management, and network mobility
management, otherwise.
(iv) OSI layers: The type of mobility management
can be identified by the OSI layer which
DOI: 10.1002/wcm

the mobility management belongs to. Mobility
management can be implemented in data link,
network, transport, application layer, or in cross
layer fashion.
For the review of mobility management for
vehicular networks, more than one criterion will be
combined to better characterize the schemes. In this
case, the mobility management schemes for vehicular
networks are first categorized into host mobility and
network mobility. Then, the protocol layer criterion is
applied for further classification.
2.2. Mobility Management for V2V
Communications
For vehicular ad hoc networks (VANETs), mobility
is managed through route discovery, maintenance,
and recovery [6]. Efficient management of vehicular
mobility is composed of topology control, location
management, and handoff management.
(i) Topology management: Topology management
can be proactive and reactive. Proactive schemes
periodically send signalling messages to explore
the topology information. On the other hand,
reactive schemes obtain the topology informa-
tion only when it is needed (e.g., when there is
a new mobile node to join the network).
Since VANETs can be very large, purely
host-based topology control does not scale
well in such networks. The cluster-based
topology control can solve this limitation. In
this cluster-based topology control, vehicles
are grouped into multiple clusters. Head of
each cluster is responsible for intra-topology
management. These cluster heads coordinate
among each other to manage the entire ad hoc
network topology. However, due to the high
speed and constrained mobility (e.g., moving
along a straight road) of vehicles, current
clustering schemes developed for MANETs
cannot achieve the optimal performance in
VANETs and the clusters could be unstable. To
address this problem, clustering for open inter-
vehicle communication (IVC) networks (COIN)
was proposed [22]. The cluster head election
is based on mobility information and driver
intentions. Besides, COIN can accommodate
the oscillatory nature of inter-vehicle distances.
In [23], a prediction-based reactive topology
control was proposed. The basic concept of this
scheme is to increase the topology maintenance
interval and to reduce the periodic beaconing
process by mobility prediction. Updates are
only needed when the predicted topology
information is incorrect. A location-aware
framework, i.e., kinetic graph, was introduced
to support the use of standard ad hoc network
protocols. With kinetic graph, the standard
ad hoc protocols can perform efficiently in
VANETs.
(ii) Location management: With unique mobility
characteristics of VANETs, basic ad hoc
routing protocols cannot be directly applied
to VANETs due to the large latency and
overhead [23]. However, geographic routing
was shown to be efficient and effective
for VANETs. Using geographic routing (e.g.,
greedy perimeter stateless routing (GPSR) [24],
geographical routing algorithm (GRA) [25]),
communicating nodes are required to have the
location information of each other. Therefore,
location management scheme, which deals
with the storage, maintenance, and retrieval of
mobile node location information, is needed
in VANETs [26]. It is worth noting that, the
location here refers to geographical location
which is not the same as the addressing location
in Internet [6].
Location management in VANET can be
classified into flooding-based and rendezvous-
based approaches [27]. Using a flooding-
based approach, the source floods the location
query to the entire network which incurs huge
overhead. On the other hand, in a rendezvous-
based approach, location servers are responsible
for location management. Nodes update their
location and query the location of destination
from location servers. Many schemes were
proposed for location management in MANETs.
For example, region-based location service
management protocol (RLSMP) which supports
both scalability and locality awareness was
proposed for VANETs [28]. In RLSMP,
message aggregation with the enhancement
from geographical clustering was used for both
location updating and querying to improve
scalability. For locality awareness, local search
was used to locate the destination node.
(iii) Handoff management: Handoff management
in vehicular ad hoc networks is performed
by rerouting to construct a new path to the
destination. When a mobile node moves, a
group of neighbors changes and hence the new
DOI: 10.1002/wcm

route of data transfer needs to be established
quickly for better handoff performance. Handoff
management can be proactive and reactive
which uses the same concepts as those in mobile
ad hoc network routing. A survey of routing
schemes in VANETs can be found in [29].
A simple taxonomy of mobility management
solutions for vehicular network is shown in Fig. 2.
Fig. 2. Taxonomy of mobility management solutions for
vehicular network.
3. Host Mobility Solutions for Vehicular
Networks
Host mobility management is the scheme in which the
mobility of each mobile node is managed individually.
Host mobility management can be designed and
implemented at different OSI layers [30] [31].
In the following, mobility management schemes
implemented in different layers are reviewed and
their suitability to vehicular networks (i.e., for V2I
communications) is discussed.
Link layer: When the mobile node moves between
access points (APs) within a common subnet, mobility
is managed by link layer protocol [32]. With WLAN as
an example, the movement of mobile nodes between
two APs connecting to the same access router is
handled by WLAN specific link layer handoff scheme.
In [33], an enhanced link-layer handover scheme in
which mobile node can receive real-time downlink
service from base station during handover process was
proposed. MAC management message, i.e., Fast DL-
MAP-IE, was also defined in [33] to support downlink
traffic reception during handoff process and to reduce
the downlink transmission delay.
Network layer: When a mobile node moves to
a different subnet, the original home IPv6 address
will be topologically invalid. As a result, mobility
management scheme in network layer is required.
MIPv6 is the fundamental network layer protocol for
host mobility support standardized by IETF. MIPv6
is independent of lower layer and also transparent
to upper layer protocols. However, the shortcomings
of MIPv6 are the long handoff latency and high
packet loss. Besides, MIPv6 is not scalable. With the
increasing number of mobile nodes, the signalling
overhead increases dramatically. In this case, MIPv6
can be used as the location and path update protocol
rather than a handover management protocol [34].
To address these limitations, extensions of MIPv6
(e.g., Fast handover for Mobile IPv6 (FMIPv6)
and Hierarchical Mobile IPv6 Mobility Management
(HMIPv6)) were proposed.
The mobile node with MIPv6 has a permanent
home address (PHA) and a care-of-address (CoA).
PHA is used to identify, while CoA is used to
locate the mobile node. When a mobile node
is attached to its home network, conventional IP
routing mechanisms can be used to forward packets
to mobile nodes. If a mobile node moves to a
foreign network, movement detection is performed by
receiving periodical router advertisement from a new
access router (AR). A new CoA is obtained according
to the advertised foreign subnet prefix through stateful
or stateless IPv6 autoconfiguration mechanisms. Then,
duplication address detection (DAD) is performed to
ensure the uniqueness of the mobile node’s local link
address as well as its new CoA. Once the address
configuration is done, the mobile node sends a binding
update (BU) message to its home agent to register the
new address. Then, packets from correspond nodes
(CNs) with the destination to the mobile node’s home
address will be intercepted by its home agent and then
tunneled to its current CoA. The process of MIPv6 is
illustrated in Fig. 3. The BU message can also be sent
to the MIPv6-enabled CN for direct communication
with mobile node without involving the home agent.
This is referred to as route optimization.
Based on the above mechanism of MIPv6, handoff
latency is composed of link layer latency and network
layer latency [35]. Link layer latency is the delay
due to air link migration from current AP to new
AP. Network layer latency is the delay due to
movement detection, network authentication, CoA
configuration, DAD, and BU messaging. Packets sent
to the mobile node during the handoff period will be
lost. Due to large handoff latency and high packet
loss, MIPv6 is not suitable for V2I communication
especially for real-time services. Furthermore, due
DOI: 10.1002/wcm

Fig. 3. Illustration of MIPv6
to heavy signalling overhead caused by fast moving
vehicles, MIPv6 is also not sufficiently scalable for
V2I communications.
To reduce the packet loss and the handoff latency
in MIPv6, FMIPv6 (i.e., fast handover for Mobile
IPv6) was proposed. This FMIPv6 addresses the
following problems: how to allow a mobile node
to send packets as soon as it detects a new subnet
link, and how to deliver packets to a mobile node as
soon as its attachment is detected by the new AR.
FMIPv6 uses link layer triggers to predict network
layer handoff [17]. FMIPv6 relies on the prediction
whose accurate result is however difficult to obtain
for fast and randomly moving mobile nodes. Many
efforts have been devoted to improve reliability of
the handoff prediction. In FMIPv6, when a mobile
node receives a link layer trigger, several messages
will be exchanged among mobile node, old AR, and
new AR. However, fast moving nodes, i.e., vehicles,
may cross the boundary of adjacent cells quickly
such that signalling message may not be completely
exchanged. To address this problem, an early binding
fast handover (EBFH) scheme was proposed for high-
speed mobile nodes [36]. In EBFH, a fast moving
node can detect the new network by monitoring router
advertisement and initiate early binding update with
its current access router. This scheme can improve
the reliability of the prediction for high speed mobile
nodes at the cost of larger overhead.
To reduce the amount of signalling among the
mobile node, CNs, and home agent, hierarchical
mobile IPv6 mobility management (HMIPv6) was
introduced. In HMIPv6, mobility anchor point (MAP)
located in the new network is used as a local home
agent (HA) for mobile nodes. With HMIPv6, a
mobile node has two CoAs i.e., MIPv6 CoA and
regional CoA. A regional care-of-address (RCoA)
has a similar role to home address. RCoA has the
same subnet prefix as MAP. If mobile node moves
across subnetworks but within a MAP domain, mobile
node only needs to register its new CoA with MAP
while RCoA does not change. HMIPv6 can also
reduce handoff latency due to smaller signalling and
shorter path. Besides, as a simple extension to MIPv6,
FMIPv6 can be combined with HMIPv6 (FHMIPv6)
to further minimize or eliminate the intra MAP
domain handover latency. However, FMIPv6 does not
support inter MAP domain movement. Therefore, it
is not suitable for real-time services in fast moving
vehicles. To address this problem, improved fast
handover protocol using HMIPv6 (IFHMIPv6) based
on IEEE 802.16e was proposed [37]. Layer 3 handover
messages of the FHMIPv6 are embedded into the
layer 2 handover messages so that multiple handover
procedures can be performed simultaneously.
Upper layers: To avoid the change of current
Internet architecture, much work has been done
towards supporting mobility management in upper
layers. For example, at transport layer, mobile Stream
Control Transmission Protocol (mSCTP) [38] was
proposed. Due to the multi-homing feature, mSCTP
can be used for Internet mobility support without
changing Internet architecture. However, interfaces
between transport layer and application layer need
to be modified and transport layer of CNs needs to
be changed. Another approach is to manage mobility
at application layer, for example, Session Initiation
Protocol (SIP) [39] and its extensions [40]. However,
due to large overhead and long latency, these upper
layer mobility management schemes are not suitable
for vehicular networks.
Cross-layer: Information from multiple layers can
be effectively exchanged to improve performance of
mobility management schemes. FMIPv6 is such a
cross-layer design which uses link layer information
for handover in network layer. In [41], a new
cross-layering design for fast IPv6 handover support
over IEEE 802.16e was proposed. The prediction
in FMIPv6 utilizes the information from link layer
and physical layer protocols. The proposed scheme
provides an interaction between the IP layer and the
MAC layer which can improve the performance of
FMIPv6 in IEEE 802.16e environment.
A summary of the host mobility solutions for V2I
communications is shown in Table I. The above
schemes are used to manage the mobility of a single
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Table I. Host mobility solutions.
Scheme
in [27]
MIPVv6 FMIPv6 EBFH FHMIPv6 IFHMIPv6 mSCTP SIP Scheme in
[35]
Protocol layers L2 L3 L3 L3 L3 L3 L4 L5 Cross
layer
Route optimization
support
NO YES YES NO NO NO YES YES NO
Signalling overheads LOW HIGH HIGH HIGH LOW LOW LOW HIGH HIGH
Handover latency and
packet loss
LOW LARGE LOW LOW LOW LOW LARGE LARGE LOW
Change to current
architecture
NO HA HA 802.21
Generic
Link layer
HA and
MAP
MAP NO NO HA
Change to current
protocol stack
YES YES YES YES YES YES YES NO YES
Technique specific YES NO NO NO NO YES NO NO YES
Cross layer informa-
tion need
NO NO YES YES YES YES NO NO YES
H A: Home Agent MAP: Mobility Anchor Point
Fig. 4. Multihop vehicular ad hoc networks (VANETs).
mobile node (i.e., vehicle) that can communicate
with the Internet gateway directly. However, when
vehicles form a VANET connecting to the Internet,
the issues related to the integration of MIPv6-
based mobility management schemes into VANET
become challenging. In VANET, some vehicles may
need multihop communication to the gateway, e.g.,
vehicles B and C shown in Fig. 4. MIPv6 cannot
be used directly in such a scenario since a direct
connection between gateway and vehicles is not
always available [12]. The architecture of Internet and
VANET is also different in terms of topology, routing
protocols, and mobility management. For example,
Internet uses Mobile IP to handle host mobility while
in VANET the mobility is managed by ad hoc routing
protocols.
Some research work have been done for the
integration of MANETs and Internet. A survey of
this topic can be found in [42]. Since VANET is a
special type of MANET, we review the solutions for
an integration of MIPv6 with MANETs. The related
issues are as follows [43].
(i) Internet gateway discovery: A mobile node
detects a foreign network by monitoring router
advertisement (RA) from the new Internet gate-
way. In IPv4 networks, RA message containing
foreign subnet prefix is broadcast to the mobile
nodes in coverage range of access point (AP).
Since IPv6 does not support broadcasting, all-
nodes multicast address will be used instead.
However, due to the unicast nature of MANET
routing protocols, intermediate nodes cannot
forward RA messages to the mobile nodes
which have more than one hop to reach AP.
As a result, these mobile nodes cannot detect
their movement and cannot construct CoA. An
example is shown in Fig. 4. When vehicle D
moves from VANET2 to VANET1, the previous
link with IGW2 is broken. In this case, vehicle
D should be aware of the service from IGW1
and establish a new CoA according to the subnet
prefix advertised by IGW1. However, vehicle
A cannot forward the RA message to other
vehicles. Therefore, vehicle D cannot find the
service of IGW1 and cannot obtain correct IP
address.
DOI: 10.1002/wcm

(ii) Routes to CoA in MANETs: In MIPv6, after
acquiring a new CoA, a mobile node (e.g.,
vehicle D in Fig. 4) sends BU message to home
agent which returns a binding acknowledgement
message to mobile node. A bidirectional route
between vehicle D and home agent is then
established. The BU message can be routed
from vehicle D to gateway IGW1 using standard
ad hoc routing protocol, e.g., optimized link
state routing (OLSR) [44], and subsequently
routed to home agent via IP routing. However,
as the CoA assigned by IGW1 is not a
routable address in MANET, for routing binding
acknowledgement from IGW1 to the CoA of
vehicle D, no standard method can be used.
(iii) Redundant routes: Using MIPv6, packets with
the destination to a mobile node outside home
network are intercepted by its home agent
and then forwarded/tunneled to the current
CoA. However, though suitable paths between
two mobile nodes within a MANET exist,
mobile nodes still communicate through the
home agents which causes redundant route
and latency. To reduce latency and bandwidth,
the mobile nodes can communicate with each
other directly without the involvement of home
agents.
To address the Internet gateway discovery problem
in MANET, several approaches were proposed.
(i) Ad hoc routing extensions: The ad hoc routing
protocol was extended to support mobile IP
in MANET. There are two solutions. One
solution is to modify the routing protocol to
support IP broadcast. The main idea is to use
IP broadcast to discover the Internet gateway
instead of local link range broadcast used in
Mobile IP. However, such extension is not
suitable for VANET since it cannot support
MIPv6. Also, this extension is not scalable
due to the broadcasting of agent advertisements
and agent solicitations. The other solution is
to extend ad hoc routing protocol to support
multicast. The main idea is to use IP multicast
in ad hoc network to discover Internet gateway.
The problem is that the use of multicast in ad
hoc networks is less efficient and scalability is
limited [12, 45].
(ii) Service discovery: Service discovery protocols
can be used for mobile nodes within an
ad hoc network to identify and register to
Internet gateway. However, since vehicles move
at high speed, the performance of service
discovery degrades severely in VANETs. Also,
this solution is not scalable [12, 45].
Due to the limitations of the existing solutions
for MANET, Internet gateway discovery solutions for
VANETs were proposed with many improvements.
DiscoveRy of Internet gateways from VEhicles
(DRIVE) based on Service Location Protocol (SLP)
was developed [45] with scalability and efficiency
enhancement. In addition, it has the ability to select
the most suitable Internet gateways among multiple
available choices. In [43], the OLSR control packets
are used to carry the foreign subnet prefix (i.e.,
RA/OLSR) and to distribute them to all mobile nodes
in the VANET. OLSR control packets are used since
they can be flooded to all VANET nodes. The ability of
OLSR to construct routes to CoAs was also exploited
in [43]. With OLSR, a mobile node can have its own
multiple routable addresses. Once acquiring a new
CoA, a mobile node advertises its CoA as multiple
routable addresses. In this way, a route to CoA in
VANET can be established using OLSR.
To solve the redundant route problem stated
above, [43] proposed a new route optimization
scheme. The general idea is to add the home of
address (HoA) of the mobile node into routing table
as a host route. Similar to the route construction
mechanism to CoAs, the work in [43] uses OLSR to
enable the routability of HoA in MANET. After that,
packets to HoA in the same MANET are transmitted
to correspondent node (CN) directly. Although this
scheme was primarily designed for MANET, it is also
applicable to VANET.
VANET mobility management scheme (i.e.,
MMIP6) was proposed in [12] for integration with
Internet. This MMIP6 was optimized to be scalable
and efficient. The key idea is to combine a proactive
service discovery protocol with an optimized mobility
management protocol. Although it was designed
to support IPv6, MMIP6 is based on the principles
of MIPv4 in terms of using both home agent and
foreign agent. An important feature in MMIP6 is that
it only uses a permanent and global IPv6 address
rather than CoA when a mobile node moves into
a foreign network. Using Fig. 4 as an example,
the communication based on MMIP6 works as
follows. CN wants to communicate with vehicle D
in VANET1. Packets from CN to vehicle D’s home
address are received by D’s home agent. The home
agent then tunnels these packets to IGW1 which acts
as vehicle D’s foreign agent. Finally, IGW1 delivers
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the decapsulated packets to vehicle D using VANET
routing protocol.
4. Network Mobility Solutions for Vehicular
Networks
With the proliferation of embedded and portable
communication devices, a mobile network can be
established in which the vehicles move as a group.
Network mobility is referred to as the situation in
which the mobile network dynamically changes its
point of attachment to the Internet. Compared with
host mobility, network mobility (NEMO) is more
effective and efficient for vehicular networks due
to reduced handoff and complexity [46]. Therefore,
network mobility management is important for
vehicular networks. NEMO Basic Support (NEMO
BS) protocol was standardized by IETF to provide
basic network mobility support. In this section,
scenarios and characteristics of network mobility
are firstly introduced. Then, the requirements and
advantages of NEMO are discussed. NEMO BS and
some extended NEMO solutions are also reviewed.
In addition, we will illustrate two NEMO route
optimization solutions and discuss the problem of
integrating NEMO with VANET.
4.1. Scenarios and Characteristics of Network
Mobility
A mobile network consists of one or more mobile
IP-subsets formed by one or more mobile routers
(MR). This MR, which can change its point of
attachment, provides Internet connectivity to mobile
network nodes (MNNs) within the network. IETF
defines three types of MNNs, i.e., local fixed node
(LFN), local mobile node (LMN), and visiting mobile
node (VMN). LFN is a fixed node belonging to mobile
network without mobility support. LMN and VMN
are MNNs with mobility support. The home link of
LMN belongs to the mobile network while that of
VMN does not. An MNN can be either a host or a
router and either fixed or mobile. A mobile network
is called nested if there is another attached mobile
network inside. The nested mobility is unique for
network mobility [47]. For network mobility, there is
no size limitation for mobile networks. The simplest
network may only consist of a mobile router and
MNNs. A mobile network can also consist of hundreds
of mobile routers and several nested mobile networks.
The common mobile network scenarios in network
mobility are as follows [48].
(i) Personal area network (PAN): As shown in
Fig. 5(a), the personal area network (PAN)
consists of a mobile phone with cellular and
bluetooth interfaces. Using bluetooth, a PDA
and a laptop connect to mobile phone which
acts as the mobile router to provide Internet
connectivity.
(ii) Public transportation mobile network: Mobile
hotspots deployed in public transportation (e.g.
bus or train) can provide Internet connectivity to
IP devices (e.g., PDA and laptop). Besides, such
a mobile network may be nested if passengers’
PANs are attached. As shown in Fig. 5(b), on
the bus, a passenger with PAN is attached to
the mobile router (MR) via WLAN interface
of the laptop. In this case, the laptop performs
as the sub mobile router for PAN which is the
nested mobile network. The MR is called the
root mobile router.
(iii) Intra-vehicle embedded mobile network: To
support ITS applications, vehicles are usually
equipped with sensors, GPS, and embedded
devices as shown in Fig. 5(c). These devices can
connect to mobile router located in the vehicle
for Internet connectivity.
The characteristics of mobile networks in network
mobility are as follows:
(i) Group of nodes move as a unit: From Internet
perspective, the entire mobile network changes
its reachability in relation to the fixed Internet
topology as a group or unit [3].
(ii) Various sizes and moving speeds: Mobile
networks have various sizes and moving speeds.
For example, pedestrian with PAN may walk
at speed of 5km/h while access networks
accommodating hundreds of devices in a train
may move at speed of 100km/h.
(iii) Various mobile network nodes: Mobile network
nodes have various types, i.e., mobile host
and mobile router, local nodes and visiting
nodes, mobility aware nodes (e.g. MIPv6-
enabled nodes) and mobility unaware nodes
(e.g., standard IPv6 nodes) [49].
(vi) Arbitrary nested level: The mobile network can
be nested with arbitrary number of levels.
(v) Mobility transparency to mobile network nodes:
In most cases, the internal topology of mobile
network is relatively stable [48]. For example,
a laptop attached to a mobile router in a
moving bus will not change its point of
attachment frequently. Therefore, the link layer
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connection of the laptop and the mobile router
can be maintained even when the mobile router
changes its point of attachment to the Internet.
The mobile network nodes do not need to be
aware of location change with respect to the
Internet.
Fig. 5. Three scenarios of mobile networks.
4.2. Advantages and Requirements of Network
Mobility
With NEMO, once attached to a mobile network,
the mobility management for the MNNs is fully
performed by the mobile router. In particular, the
mobility management is transparent to the mobile
network nodes. The advantages can be summarized as
follows [49]:
(i) Scalability: A mobile network may consist
of hundreds of MNNs. Without a network
mobility solution, these MNNs have to handle
mobility independently. For one node, several
signalling messages need to be exchanged with
the point of attachment. On the other hand,
using basic network mobility solutions, mobility
is handled only by the mobile router and
hence the signalling overhead can be reduced
significantly.
(ii) Reduced handoff: Due to the relatively stable
internal topology of mobile network (e.g.,
topology among mobile router and MNNs),
mobile network nodes do not change their points
of attachment and hence can avoid link layer
handoff.
(iii) Reduced complexity: Mobile network can
provide mobility support to standard IPv6.
The IP addresses of MNNs will not change
even if the mobile router changes its point
of attachment. Therefore, the complexity of
softwares and hardware used in MNNs can be
reduced.
The requirements of NEMO solution can be
summarized as follows [48].
(i) Global reachability and session continuity of
MNN: This is the fundamental requirement for
network mobility. Mobile network nodes must
be globally reachable given a permanent IP
address. During the movement of mobile router,
ongoing sessions of MNNs must be maintained.
(ii) Minimum changes: For basic network mobility
support, no modifications should be required
to any entities other than mobile router and its
home agent.
(iii) Support for different nodes: Basic network
mobility solutions must support all types of
mobile network nodes mentioned above.
(vi) Compatibility: The solutions must be com-
patible with existing Internet standards. For
example, it should not affect the operation of
MIPv6 or standard IP addressing and routing
schemes.
(v) Nested mobility support: The solutions should
support mobile network nodes which are located
in nested mobile networks at different levels.
(vi) Internal configuration transparency: The inter-
nal configurations (e.g., topology) should be
transparent to the solutions. In other words, the
solutions can be applied to mobile networks
with arbitrary internal topologies.
(vii) Scalability: To support large mobile networks,
the solution needs to be scalable.
(viii) Security: The solutions must have sufficient
protection from the attack.
4.3. Network Mobility Solutions
Similar to host mobility, solutions for network
mobility can be designed and implemented in different
layers. In the following, we mainly focus on network
layer and application layer solutions.
Network layer solutions: Network Mobility Basic
Support (NEMO BS) protocol was proposed by
IETF to provide basic network mobility support. To
minimize the change to existing architecture and to
maintain backward compatibility, NEMO BS was
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designed based on MIPv6 with minimal extensions.
Similar to mobile host in MIPv6, mobile router has
home address and home agent (i.e., HA-MR). NEMO
BS specifies operation of mobile router and home
agent, while the details of mobile network nodes are
the same as that in MIPv6 [50].
In NEMO BS, when a visiting MNN connects to
the mobile network using with MIPv6, the MNN will
receive a subnet prefix (i.e., network prefix (MNP))
advertised by the mobile router. Then, the MNN
establishes new care-of-address (CoA) based on MNP.
Once the address configuration is done, the MNN
sends a binding update (BU) message to its home
agent. The home agent sends binding acknowledge
back to finish the location update procedure.
When the mobile router changes its point of
attachment, it also acquires a CoA from the visiting
network and updates the binding cache of its home
agent. Since the CoAs of MNNs remain unchanged,
location update messages do not need to be sent to the
home agent of the MNNs. Once the binding procedure
is completed, a bi-directional tunnel between mobile
router and home agent is established based on IP-in-IP
encapsulation [3].
However, route optimization is not considered in
NEMO BS due to the security and incompatibility
issues. All packets to and from mobile network nodes
need to be tunneled by the home agent of the mobile
router. Packets from the MNNs to the correspondent
nodes (CNs) are encapsulated by mobile router
and then tunneled to home agent of mobile router.
Then the home agent decapsulates these packets and
forwards them to the destinations. In the opposite
direction, packets from CNs to MNNs will be first
received by the home agent of the mobile router.
The home agent tunnels these packets to mobile
router which then forward them to mobile network
nodes. However, the binding cache of home agent
only contains home address of mobile router. The
addresses of mobile network nodes are not binded with
current CoA of mobile router. As a result, packets
cannot be tunneled to the mobile router correctly.
To solve this problem, prefix scope binding update
(PSBU) [51] was proposed. Using PSBU, the mobile
router sends binding update message to home agent
associating with mobile network prefix rather than the
home address with current CoA. Having the prefix
information, the home agent can tunnel packets to the
correct mobile router.
The handoff performance, signalling and routing
overhead of NEMO BS were analyzed in [52]. The
results show that NEMO BS itself is not sufficient for
seamless handover, and optimization of the protocol is
necessary.
Fig. 6. Handoff components.
The components to support handoff in NEMO BS
are shown in Fig. 6. These components are similar to
those in MIPv6. To reduce handoff delay in network
attachment process, in [52], fast RA mechanism [53]
was adopted to remove the random delay. Optimistic
duplication address detection (ODAD) [54] was
used to reduce DAD delay. However, the handoff
performance of NEMO BS with above optimization
is still not sufficient for QoS-sensitive applications.
Latency of link layer handoff and NEMO signalling
overhead affect the overall performance of mobility
management significantly. Novel make-before-break
(MBB) handoff scheme was proposed in [52] to reduce
handoff delay and packet loss. To enable MBB, two
interfaces are needed for simultaneously listening to
multiple APs. Besides, a method to minimize the
overhead in route optimization was proposed for
further performance enhancement [55]. However, this
extended scheme can only be applied to the mobile
network nodes with host mobility support.
A reactive handoff optimization was proposed
in [56]. Compared with proactive schemes which
detect and predict the movement before the current
link is broken, reactive solutions are simpler to
implement and more robust. Many limitations are
eliminated in reactive solutions (i.e., possible erro-
neous movement, moving speed limitation, and high
signalling overhead). A new cross-layer optimized
movement detection procedure and a new DAD
scheme were also proposed. A novel reactive handoff
procedure combining the above two new schemes
was designed in which the movement detection and
DAD are performed simultaneously. Compared with
existing reactive handoff solutions, the solution in [56]
does not rely on prediction information, buffering,
bicasting, and soft handover.
DOI: 10.1002/wcm

In NEMO BS, the binding update traffics are
minimized at the cost of tunneling overhead. To
reduce the tunneling overhead, an adaptive NEMO
support protocol based on hierarchical mobile IPv6
(HMIPv6) was proposed in [57]. The general idea
is to tradeoff between binding update traffic and
tunneling overhead adaptively. An adaptive binding
update strategy was deployed based on the session-
to-mobility ratio (SMR). This SMR with a predefined
threshold was compared with different binding update
procedures. An optimal threshold for adaptive binding
updates was also derived in [57].
Application layer solutions: To reduce the deploy-
ment cost and avoid the suboptimal routing problems
of NEMO BS, SIP-based network mobility (SIP-
NEMO) was proposed for network mobility manage-
ment in application layer. The system architecture
of SIP-NEMO is shown in Fig. 7. Three types of
SIP entities are employed in SIP-NEMO, i.e., SIP
network mobility server (SIP-NMS), SIP home server
(SIP-HS), and SIP foreign server (SIP-FS). Similar to
mobile router in NEMO BS, SIP-NMS is used as a
gateway between mobile network and Internet. This
SIP-NMS also manages the mobility of entire mobile
network. In SIP-NEMO, the mobile network nodes
can be both SIP clients and SIP-NMS. Therefore, the
concept of nested mobile network in SIP-NEMO is
similar to that in NEMO BS. SIP-HS plays a similar
role to home agent in NEMO BS. The SIP-FS is used
for handoff management. SIP-FS will send requests
to the corresponding nodes according to Universal
Resource Identifier (URI) list. This list is maintained
by SIP-NMS when the mobile network changes the
point of attachment.
When the user agent client (UAC) of SIP moves
into a foreign network, a new care-of-address (CoA)
will be constructed. Then, this CoA registers to the
SIP-NMS to obtain a new contact address according
to the domain name of SIP-NMS. To perform location
update, UAC sends a REGISTER message with its
new contact address to its SIP home server. This
REGISTER message is translated by SIP-NMS and
then forwarded to the SIP home server of UAC.
Similar to the binding update of mobile router in
NEMO BS, when SIP-NMS changes its point of
attachment, SIP-NMS sends a REGISTER message
with its new CoA in the contact field to its SIP home
server.
Since mobility management schemes in network
layer and higher layers have their own advantages
and limitations, an alternative way is to deploy
them together in a proper way. HarMoNy, a scheme
Fig. 7. The system architecture of SIP-NEMO.
integrating the host identity protocol (HIP) with
NEMO was proposed in [58]. HIP introduces a public
keys-based host identity name.
Since the control signalling and data delivery are
separated in SIP. An explicit session establishment
procedure is required in SIP-NEMO. Using Fig. 7 as
an example, if SIP client UA1 in a mobile network
wants to communicate with a corresponding SIP user
agent client UA2, it first sends an INVITE message to
the SIP-NMS. After translation, the SIP-NMS sends
the INVITE message to the SIP home server of
UA2 which finally forwards the message to UA2.
The dash line in Fig. 7 shows the outgoing session
establishment. If the session is initiated by UA2, it
sends an INVITE message to the SIP home server of
UA1. Since the SIP-HS of UA1 registers its current
location, the INVITE message is then redirected to the
SIP home server of SIP-NMS. Also, SIP home server
of SIP-NMS forwards the message to the current CoA
of SIP-NMS which forwards the message to UA1.
The dotted line in Fig. 7 shows this incoming session
establishment.
Unlike NEMO BS, SIP-NEMO routes the packet
directly between SIP clients [59]. In addition, as
an application layer solution, SIP-NEMO has the
advantage since SIP-NEMO can be deployed without
modifications to the Internet architecture. However,
the handoff delay in SIP-NEMO can be large due to
longer message length. A comparative study of NEMO
BS and SIP-NEMO was presented in [50]. A summary
of network mobility solutions is shown in Table II.
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Table II. Network mobility solutions.
NEMO BS Scheme in [45] Scheme in [49] Scheme in [50] SIP-NEMO
Protocol layers L3 L3 L3 L3 L5
Route optimization support NO YES YES YES YES
Signalling overheads HIGH LOW LOW Adaptive HIGH
Handover latency and packet loss HIGH LOW LOW LOW HIGH
Change to current architecture HA HA HA and MAP HA and MAP NO
Change to current protocol stack YES YES YES YES NO
Technique specific NO NO NO NO NO
Cross layer information need NO NO YES NO NO
H A: Home Agent MAP: Mobility Anchor Point
4.4. Solutions for Route Optimization
Since basic support protocol of NEMO does not
consider route optimization, all packets need to be
tunneled by the home agent of mobile router even
when a shorter path exists. This scheme causes
suboptimal route problems especially for multilevel
nested mobile network [60]. NEMO BS uses IP-in-
IP tunneling. In particular, packets are encapsulated
by all upper level mobile routers and then tunneled to
their home agents. Therefore, the delay and overhead
become large with increasing number of nested levels.
Besides, since all packets of mobile network nodes
must go through home agents of upper mobile routers,
this home agent may be congested and become the
bottleneck. Also, if the home agent is unavailable, the
total path will be cut, and the communication becomes
unreachable.
Using three level nested mobile networks shown
in Fig. 8 as an example, MNN is a visiting mobile
node with MIPv6 support. This node is attached to
MR3 and the correspondent node is a standard IPv6
node. MR1, the parent-MR of MR2, is the top level
mobile router of the mobile network. MR3 is attached
to MR2. The packets from CN to MNN are first
sent to home agent of MNN (HA-MNN). HA-MNN
encapsulates the packets and then tunnels them to the
home agent of MR3 (i.e., HA-MR3). HA-MR3, HA-
MR2, and HA-MR1 handle the packets in a similar
way. While receiving these multilevel encapsulated
packets, MR1, MR2, and MR3 decapsulate them
sequentially. Finally, MR3 forwards the decapsulated
packets to MNN.
The main purposes of route optimization for NEMO
are to avoid data packets passing through the home
agent of mobile router and to reduce the number of
additional IPv6 headers added to the original packets.
MIPv6 route optimization does not work in mobile
networks due to the compulsory MR-HA tunneling.
A simple extension of MIPv6 route optimization for
NEMO is to use NEMO prefix option to inform the
Fig. 8. Nested mobile network.
CN about the location of the mobile network prefix
(MNP) [61]. However, security is a major problem
and the modification to the correspondent nodes is
required.
In the following, two NEMO route optimization
solutions are reviewed. It is worth noting that,
although route optimization schemes can yield some
benefits, the tradeoffs (i.e., additional signalling over-
head, increased protocol complexity, and processing
load) need to be taken into account [62].
Mobile IPv6 route optimization for NEMO
(MIRON) was proposed in [61]. MIRON combines
two different operation modes applied to different
types of mobile network nodes. Specifically, for the
nodes without host mobility support (e.g., standard
IPv6 nodes), the mobile router which works as
Proxy-MR is responsible for all the mobility and
route optimization management. While for the nodes
with standard MIPv6 support, MIRON uses a PANA
(Protocol for carrying Authentication for Network
Access) and Dynamic Host Configuration Protocol
(DHCP) based address delegation mechanism to
enable self management of mobility and route
optimization. Such combination guarantees route
optimization for all types of nodes and network
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topology [61]. MIRON has a deployment advantage
in that modification is needed only in mobile routers.
Evaluation results show that MIRON can significantly
improve the performance over NEMO BS in terms of
larger TCP throughput and smaller overhead.
The Route Optimization solution for nested mobile
networks using Tree Information Option (ROTIO) was
proposed in [63]. ROTIO extends the NEMO BS by
modifying binding update and router advertisement
messages. Two binding update messages are used
by nested mobile router. One message sent to the
Top Level Mobile Router (TLMR) contains routing
information of TLMR, and the other sent to the
home agent of nested mobile router contains the
home address of TLMR. Therefore, only two extra
entities, i.e., home agents of the mobile router and
TLMR, are required in the path from correspondent
node (CN) to the MNNs. Locations of MNN and CN
are incorporated in ROTIO. Basic ROTIO scheme is
used to optimize the route between MNN and CN
which are not located in the same mobile network.
The extended ROTIO scheme is used for intra-NEMO
routing optimization. Besides, location privacy and
mobility transparency are guaranteed in ROTIO.
Many other related works were introduced for
NEMO route optimization [64, 65, 66, 67, 68, 69, 70,
71, 72]. An analytical framework with performance
metrics (e.g. transmission latency, memory usage,
and BU’s occurrence number) was proposed in [73].
Detailed classification, evaluation, and analysis can be
found in [74].
4.5. VANETs with Network Mobility
Similar to MIPv6, NEMO BS is designed for mobile
network with direct communication link with an
Internet access infrastructure. However, multihop
communication is not supported in this scheme.
Mobile routers in vehicles may form VANETs. To
guarantee the consistent reachability to Internet from
mobile networks via both direct and indirect links,
it is necessary to integrate VANETs with NEMO.
In addition, such integration can also be used for
route optimization [75]. The works in [76] [77]
realized route optimization in terms of delay and
bandwidth by switching from NEMO to MANET.
In [75], MANET with NEMO was evaluated and the
results show performance improvement. From security
perspective, [78] proposed VARON, a Vehicular Ad
hoc Route Optimization solution for NEMO using
MIPv6 security concept to provide the same level of
security of current Internet.
MANEMO is the concept to integrate the MANET
with NEMO [79], which combines the advantages of
both the schemes. Since the schemes for integrating
VANET with NEMO are inherited from MANEMO,
it is worth reviewing MANEMO solutions. The
MANEMO solutions can be designed based on
MANET or NEMO, i.e., MANET-centric approaches
and NEMO-centric approaches, respectively. Accord-
ing to the definitions in [9], in MANET-centric
solution, NEMO techniques (e.g., NEMO BS)
is applied directly to MANETs. Similar to the
aforementioned multihop communication methods
in the former section, foreign subnetwork prefix
advertising problems were addressed by specific
extensions of MANET routing protocols. The main
idea of this NEMO-centric solution is to use at
least one intermediate mobile router along the
multihop path between the mobile router and the
infrastructure for relaying packets. In NEMO-centric
solutions, NEMO techniques are used to provide
and maintain Internet connectivity while MANET
protocols are used to optimize the routing within a
mobile network [9]. NEMO-centric solutions can only
be used in networks with hierarchical topologies (e.g.,
nested mobile networks).
Considering a VANET scenario, a comparison of
MANET-centric and NEMO-centric approaches with
respect to VANET specific requirements was also
performed in [9] based on economic, functional
and performance criteria. The conclusion is that the
MANET-centric approach outperforms the NEMO-
centric approach for VANET in terms of complexity,
routing performance, and cost. In the following, two
MANET-centric solutions are reviewed.
(i) Unified MANEMO architecture (UMA) is a
protocol combining the functionality of the
optimized link state routing (OLSR) protocol
and NEMO BS [79]. In UMA, every UMA-
enabled mobile router with direct connection
to the Internet is required to establish a MR-
HA bidirectional tunnel and acts as a gateway
mobile router in the MANET. Then, the gateway
mobile router advertises its reachability to the
Internet via OLSR host and network association
(HNA) messages. Therefore, once receiving
such HNA messages, the mobile router can start
binding update process via the gateway mobile
router.
(ii) A solution applying NEMO BS to VANETs
was proposed in [80]. The network layer is
divided into two sublayers. In this solution,
topology-based routing or geographical routing
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protocol is used as VANET routing protocol
while NEMO BS runs on top of this protocol
to support mobility without any modification.
This solution was designed specifically for
IEEE 802.11-based VANET based on Car2Car
Communication Consortium (i.e., C2C-CC [2])
system architecture. The laboratory measure-
ments show the effectiveness of the solution for
highly dynamic vehicular networks.
The above solutions are based on NEMO BS.
Terminal mobility with network mobility support
(PTEN), a terminal-assisted network mobility man-
agement scheme proposed in [81], does not use
MIPv6-based NEMO BS schemes. Instead, PTEN
uses Uniform Resource Identifier (URI) to locate a
mobile network node. Directory service is used to
map URI to IP address. An IP-IP address mapping
scheme, an IPv6 header extension on IP packets, and
an IP address redirection scheme on the MRs were
implemented in [81].
5. Mobility Management for Heterogeneous
Wireless Access
Current mobile nodes or mobile routers in vehicles
can be equipped with multiple radio access interfaces
for different wireless networks (e.g., 3G, WiMAX,
and WiFi). This is referred to as heterogeneous
access. For session continuity and better wireless
access performance, seamless vertical handoff (i.e.,
handoff among different wireless technologies) should
be performed. Besides, for load balancing purpose,
mobile vehicular nodes should be able to access
multiple networks simultaneously. To achieve the
optimal performance, efficient mobility management
schemes are required for vehicular networks in
presence of heterogeneous wireless access. Recent
research has taken advantages of multihoming, which
enables a mobile node to use multiple access networks
simultaneously to perform smooth vertical handoff.
An analysis of multihoming in network mobility
support can be found in [82].
Many works were done for host mobility in
heterogeneous wireless networks. However, little
attention has been paid on network mobility. The
main challenge is due to the heterogeneity of access
scenarios in mobile networks. A mobile network may
have one mobile router with multiple access interfaces,
or the heterogeneity may arise from several mobile
routers in a mobile network each with a different
access interface.
A solution for multiple mobile routers was
proposed in [83]. This solution consists of mobile
DHCPv6 agents and a handoff management center
(HMC). Location management and forward loss
recovery were implemented based on mobility
prediction. Cooperative mobile router-based handover
(CoMoRoHo) was proposed in [84]. CoMoRoHo uses
multihoming techniques to reduce handoff latency
and packet loss for long-vehicular multihomed mobile
networks. Multiple mobile routers connected to
different access networks can also cooperate during
handoff to reduce packet loss due to handoff latency
and overlapped reception. In [85], Mobile IP based
mobility management architecture for highly mobile
users and vehicular networks was proposed. This
architecture focuses on network selection and timely
handoff. The handoff decisions are based on network
layer metrics and the frequencies of BU messages are
dynamically adjusted according to the speed of the
mobile node.
The simultaneous mobility is referred to the
situation when both mobile nodes move to other
networks simultaneously [86]. Due to the high handoff
rate caused by highly mobile vehicles, simultaneous
mobility will occur frequently in vehicular networks.
The mobility management schemes with route
optimization that sends location binding updates to
correspondent nodes (e.g., MIPv6, SIP-NEMO) are
vulnerable to such simultaneous mobility problem.
In particular, when two communicating mobile nodes
change their points of attachment at the same time,
they both send binding update messages to each other.
However, these two binding update messages are sent
to their outdated addresses and the messages will be
lost.
An analytical framework and solution of simul-
taneous mobility were proposed in [86]. Besides,
[86] also proposed and compared different solutions
to support simultaneous mobility for MIPv6, SIP
based mobility management, and MIP location
registration. The solutions for simultaneous mobility
are broadly divided into receiver-side and sender-side
mechanisms based on the entity which is responsible
for sending a particular binding update message.
Both receiver-side and sender-side mechanisms can be
further categorized into timer-based retransmissions,
forwarding, pro-active forwarding, redirecting, and
pro-active redirecting. Details can be found in [86].
Network mobility solutions also have to take
simultaneous mobility issue into account. A proxy-
aided simultaneous handover (PASH) mechanism for
mobile networks in vehicles was proposed in [87].
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This PASH mechanism aims to solve addressing
problem resulting from simultaneous handover in
SIP-NEMO. Besides, a Fast Route/local routE re-
Establishment (FREE) algorithm was developed.
The basic concept is to improve the speed of
reestablishment process of the optimized routing path
and to ensure that the signalling messages will be
successfully received.
6. Open Research Issues
Future vehicular networks will provide seamless
services to the mobile users. Despite the existing
research efforts, there are still many open research
issues related to mobility management for vehicular
networks.
(i) Quality of service (QoS) issues: QoS require-
ments of vehicular applications pose great chal-
lenges to mobility management design. Safety
applications have higher priority than non-
safety applications, and such priority should
be guaranteed even if handoff is performed.
For multimedia applications, handoff latency
should be minimized. For vertical handoff, a
QoS mapping scheme for different wireless
technologies may be required. Besides, for
both horizontal and vertical handoff, a resource
allocation mechanism for handed over sessions
is needed to meet QoS requirements. Scalability
and resource utilization are important factors
when designing such resource allocation mech-
anisms.
(ii) Access selection issues: Vehicular mobile nodes
with multiple access interfaces need to perform
access selection in heterogeneous environment.
Many factors (e.g., cost, bandwidth, and delay)
and their weights for decision need to be
defined. Access selection is also related to
handoff decision. If multiple access networks
are selected simultaneously, an efficient load
balancing scheme is desirable. Besides, when
integrating VANETs with Internet, multiple
Internet gateways (e.g., a direct Internet gateway
and a indirect Internet gateway) may be
available for some nodes. Internet gateway
selection is also required for the vehicular
nodes.
(iii) Issues related to mobility model: Performance
evaluation is required for both designing new
protocols and applying extensions of existing
protocols for vehicular networks. Accurate
mobility model is required for performance
evaluation of vehicular networking protocols.
Traditional mobility models (e.g., random way-
point) are not suitable for vehicular networks
since they assume a random direction selection
and random speed. However, mobility of vehi-
cles is constrained by pre-built roads, vehicle
speed, and driving regulations. A flexible but
realistic mobility model for vehicular networks
is needed.
(iv) Ad hoc routing issues: Mobility was not
considered in ad hoc routing protocols. For both
V2I and V2V mobility management solutions,
the handoff performance degrades severely with
increasing number of hops. Mobility-aware
vehicular ad hoc routing protocol is required to
facilitate fast handoff.
(v) Transport and application-layer performance
issues: Performance of transport and application
layer protocols (e.g., TCP, UDP) need to be
optimized for vehicular networks. Effects of
mobility management schemes on transport
and application layer performances are worth
investigation.
7. Conclusions
In this paper, we have presented a comprehensive
survey of mobility management solutions for vehicular
networks. We have classified the mobility man-
agement solutions for vehicular networks based on
vehicle-to-vehicle (V2V) or vehicle-to-infrastructure
(V2I) communications. The traditional Internet and
mobile ad hoc network mobility management tech-
niques and their suitability to vehicular networks have
been discussed. Existing works for both V2I and V2V
mobility management have been reviewed. Several
open research issues have been also outlined.
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Acknowledgment
This work was done in the Centre for Multimedia
and Network Technology (CeMNet) of the School
of Computer Engineering, Nanyang Technological
University, Singapore. This work was supported
in part by the AUTO21 NCE research grant for
the project F303-FVT, and the MKE (Ministry
of Knowledge Economy), Korea under the ITRC
(Information Technology Research Center) support
program supervised by the IITA (Institute of
Information Technology Assessment).
DOI: 10.1002/wcm

Authors’ Biographies
Kun Zhu is a Ph.D. student in School of
Computer Engineering, Nanyang Technological University,
Singapore. He received the B.Eng. and M.Eng. degrees both
in Computer Engineering from Beijing Jiaotong University,
Beijing, China, in 2005 and 2007, respectively. His research
interests are in the area of mobility management and network
selection in heterogeneous wireless networks.
Dusit Niyato is currently an assistant
professor in the School of Computer Engineering, at the
Nanyang Technological University, Singapore. He obtained
his Bachelor of Engineering in Computer Engineering
from King Mongkut’s Institute of Technology Ladkrabang
(KMITL), Bangkok, Thailand. He received Ph.D. in
Electrical and Computer Engineering from the University of
Manitoba, Canada. His research interests are in the area of
radio resource management in cognitive radio networks and
broadband wireless access networks.
Ping Wang (M’09) received the Ph.D.
degree in electrical engineering in 2008 from the University
of Waterloo, Canada. She is currently an assistant professor
at School of Computer Engineering, Nanyang Technological
University, Singapore. Her current research interests include
QoS provisioning and resource allocation in multimedia
wireless communications. She was a co-recipient of a
Best Paper Award from IEEE ICC 2007. She is an
Editor of EURASIP Journal on Wireless Communications
and Networking, International Journal of Communication
Systems, and International Journal of Ultra Wideband
Communications and Systems.
Ekram Hossain is currently an
Associate Professor in the Department of Electrical and
Computer Engineering at University of Manitoba, Winnipeg,
Canada. Dr. Hossain’s current research interests include
design, analysis, and optimization of wireless communi-
cation networks and cognitive radio systems. He is a co-
author/co-editor for the books “Dynamic Spectrum Access
and Management in Cognitive Radio Networks” (Cambridge
University Press, 2009), “Heterogeneous Wireless Access
Networks” (Springer, 2008), “Introduction to Network
Simulator NS2” (Springer, 2008), “Cognitive Wireless
Communication Networks” (Springer, 2007), and “Wireless
Mesh Networks: Architectures and Protocols” (Springer,
2007). Dr. Hossain serves as an Editor for the IEEE
Transactions on Mobile Computing, the IEEE Transactions
on Wireless Communications, the IEEE Transactions on
Vehicular Technology, IEEE Wireless Communications,
IEEE Communications Surveys and Tutorials and several
other international journals. He served as a guest editor
for the special issues of IEEE Communications Magazine
(Cross-Layer Protocol Engineering for Wireless Mobile
Networks, Advances in Mobile Multimedia) and IEEE
Wireless Communications (Radio Resource Management
and Protocol Engineering for IEEE 802.16). He served as
a technical program co-chair for the IEEE Globecom 2007,
IEEE WCNC 2008, IEEE VTC 2008-Fall, and QShine 2008:
International Conference on Heterogeneous Networking for
Quality, Reliability, Security, and Robustness. Dr. Hossain
served as the technical program chair for the workshops
on “Cognitive Wireless Networks” (CWNets 2007) and
“Wireless Networking for Intelligent Transportation Sys-
tems” (WiN-ITS 2007) held in conjunction with QShine
2007 during August 14-17, in Vancouver, Canada, and
the First IEEE International Workshop on Cognitive Radio
and Networks (CRNETS 2008) in conjunction with IEEE
International Symposium on Personal, Indoor and Mobile
Radio Communications (PIMRC 2008). He served as the
technical program co-chair for the Symposium on “Next
Generation Mobile Networks” (NGMN’06), NGMN’07,
NGMN08, NGMN09 held in conjunction with Interna-
tional Wireless Communications and Mobile Computing
Conference (IWCMC’06), IWCMC’07, IWCMC’08, and
IWCMC’09. Dr. Hossain has several research awards to
his credit which include Lucent Technologies Research
Award for contribution to IEEE International Conference on
Personal Wireless Communications (ICPWC’97), and the
Best Student-paper Award in IWCMC’06. He is a registered
Professional Engineer (P.Eng.) in the Province of Manitoba,
Canada. Dr. Hossain is a Senior Member of the IEEE.
DOI: 10.1002/wcm

Dong In Kim received the B.S.
and M.S. degrees in Electronics Engineering from Seoul
National University, Seoul, Korea, in 1980 and 1984,
respectively, and the M.S. and Ph.D. degrees in Electrical
Engineering from University of Southern California (USC),
Los Angeles, in 1987 and 1990, respectively.
From 1984 to 1985, he was a Researcher with Korea
Telecom Research Center, Seoul. From 1986 to 1988,
he was a Korean Government Graduate Fellow in the
Department of Electrical Engineering, USC. From 1991 to
2002, he was with the University of Seoul, Seoul, leading
the Wireless Communications Research Group. From 2002
to 2007, he was a tenured Full Professor in the School
of Engineering Science, Simon Fraser University, Burnaby,
BC, Canada. From 1999 to 2000, he was a Visiting Professor
at the University of Victoria, Victoria, BC. Since 2007, he
has been with Sungkyunkwan University (SKKU), Suwon,
Korea, where he is a Professor and SKKU Fellow in the
School of Information and Communication Engineering.
Since 1988, he is engaged in the research activities in
the areas of wideband wireless transmission and access.
His current research interests include cooperative relaying
and base station (BS) cooperation, multiuser cognitive
radio networks, advanced transceiver design, and cross-layer
design.
Dr. Kim was an Editor for the IEEE Journal on Selected
Areas in Communications: Wireless Communications Series
and also a Division Editor for the Journal of Commu-
nications and Networks. He is currently an Editor for
Spread Spectrum Transmission and Access for the IEEE
Transactions on Communications and an Area Editor for
Transmission Technology III for the IEEE Transactions on
Wireless Communications. He also serves as Co-Editor-in-
Chief for the Journal of Communications and Networks.
DOI: 10.1002/wcm

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