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WIRELESS COMMUNICATIONS AND MOBILE COMPUTINGWirel. 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: ASurvey

Kun Zhu1, Dusit Niyato1, Ping Wang1, Ekram Hossain2,†∗, and Dong In Kim3

1School of Computer Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore2Department of Electrical and Computer Engineering, University of Manitoba, Winnipeg, Manitoba R3T 5V6,Canada3 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 varietyof intelligent transportation system (ITS) applications. The traditional mobility management schemes for Internetand mobile ad hoc network (MANET) cannot meet the requirements of vehicular networks, and the performancedegrades severely due to the unique characteristics of vehicular networks (e.g., high mobility). Therefore, mobilitymanagement solutions developed specifically for vehicular networks would be required. This article presentsa comprehensive survey on mobility management for vehicular networks. First, the requirements of mobilitymanagement for vehicular networks are identified. Then, classified based on two communication scenarios invehicular networks, namely, vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications, theexisting mobility management schemes are reviewed. The differences between host-based and network-basedmobility management are discussed. To this end, several open research issues in mobility management for vehicularnetworks 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 interestand progress in the field of intelligent transportationsystem (ITS) from both industry and academia. Typ-ical ITS applications can be categorized into safety,transport efficiency, and information/entertainmentapplications (i.e., infortainment) [1]. Vehicular ad hocnetworks (VANETs) are emerging ITS technologies

∗Correspondence to: Ekram Hossain, Department of Electricaland Computer Engineering at University of Manitoba, Winnipeg,Manitoba R3T 2N2, Canada†E-mail: [email protected]

integrating wireless communications to vehicles.Different Consortia (e.g., Car-to-Car CommunicationsConsortium (C2C-CC) [2]) and standardization orga-nization (e.g., IETF) have been working on variousissues in VANETs. C2C-CC aims to develop an openindustrial standard for inter-vehicle communicationusing wireless LAN (WLAN) technology. Forexample, IEEE 802.11p or dedicated short rangecommunications (DSRC) is an extension of 802.11standard for inter-vehicle communication by IEEEworking group. IETF has standardized NEtworkMObility Basic Support (NEMO BS) [3] for networkmobility 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) andvehicle-to-infrastructure (V2I) communications. V2Vrefers to the direct or multihop communicationsamong vehicles in VANET. V2V is efficient andcost effective due to its short range bandwidthadvantage and ad hoc nature. V2I refers to thecommunication between vehicles and infrastructureof roadside unit (RSU), e.g., base station and accesspoint connected with Internet. V2I communicationscan be used for Internet access. A typical VANETscenario is shown in Fig. 1. VANET is a specialtype of mobile ad hoc networks (MANETs) [4] withunique characteristics. Due to the high mobility ofvehicles, topologies of VANETs are highly dynamic.Also, the density of VANETs varies dramatically.Another major difference between VANETs andtraditional MANETs is that power consumption is nota major concern in VANETs. Instead, the efficiency ofVANETs protocols is paramount.

Originating from cellular networks, mobility man-agement has been an important and challengingissue to support seamless communication. Mobilitymanagement includes location management andhandoff management [5]. Location management hasthe functions of tracking and updating current locationof mobile node (MN). Handoff management aims tomaintain the active connections when MN changes itspoint of attachment.

Mobility management is essential for providinghigh-speed and seamless services for vehicularnetworks since MNs change their points of attachmentfrequently and network topology can be changedabruptly. Due to the differences between V2I andV2V communications, their mobility managementschemes can be designed differently to achieveoptimal performance. Since V2I communication needsdata exchange with Internet, for compatibility andinteroperability reasons, most mobility managementsolutions for V2I communication are designed basedon Internet mobility management protocols (e.g.,Mobile IPv6). For V2V communication, mobilitymanagement mainly focuses on route discovery,maintenance, and recovery which are similar to thosein MANETs [6]. A review of the current state of the arton mobility management for both V2V and V2I-basedVANETs will be provided in this paper.

The rest of this paper is organized as follows.In Section 2 we provide an overview of mobilitymanagement schemes for both V2I and V2Vcommunications. Host mobility and network mobilitysolutions designed at different OSI layers areintroduced in Sections 3 and 4, respectively. Then,

mobility management for VANET with heterogeneousaccess and simultaneous movement scenario isdiscussed in Section 5. The open issues for mobilitymanagement in vehicular networks are outlined inSection 6. Conclusion is given in Section 7. Inthis paper, the related terminologies of mobilitymanagement are consistent with those in RFC3753 [7]and RFC4885 [8].

Fig. 1. General model of vehicular networks.

2. Overview of Mobility Management inVehicular Networks

In this section, we discuss the mobility managementissues in vehicular networks for V2I and V2Vcommunications scenarios.

2.1. Mobility Management for V2ICommunications

In a V2I communications scenario, some ITSapplications require Internet access [9] throughan infrastructure or Internet gateway. The Internetgateway can provide global addressability andbidirectional Internet connectivity to the mobile nodesin a VANET [10]. In a VANET, mobile nodes can befar away from an Internet gateway, and their traffic canbe relayed through intermediate mobile nodes. Thisis 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 mobilitymanagement solutions cannot be applied directly sincethey require a direct connection between a mobilenode and infrastructure. Therefore, when integratingMIPv6-based solutions into VANETs, many issuesarise (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 areanetwork (PAN) attached to the mobile routers locatedin the vehicle). In this scenario, network mobility(NEMO) basic support protocol [3] was introducedto support mobility in a VAN. NEMO is an efficientand scalable scheme since the mobility management istransparent to the mobile devices (i.e., mobile devicedoes not send and receive signalling message directly).However, route optimization was not considered inNEMO BS. Triangular routing in MIPv4 becomesquadrangular routing in NEMO BS. Much work hasbeen done to address the route optimization problem.

In addition, the vehicular network can be het-erogeneous in which different wireless technologiesare integrated into one service. This will enableseamless and high-speed connection, since the mobilenode can select the most suitable network for datatransmission [11].

Mobility management should guarantee the reacha-bility to correspondent nodes (CN) in the Internet aswell as the global reachability to mobile nodes in avehicular network. Therefore, the mobility manage-ment has to meet the following requirements [12, 13]:

(i) Seamless mobility: Mobility of vehicles shouldbe seamless. Accessability and service continu-ity should be guaranteed regardless of vehicle’slocation and wireless technology.

(ii) Fast and vertical handover: Fast handoveris needed for delay-sensitive ITS applications(e.g., safety-related). Fast handover is also acrucial requirement for wireless networks withsmall coverage area (e.g., WiFi network), sincethe vehicle spends only short period of time ateach point of attachment (e.g., access point). Ina heterogeneous wireless environment, verticalhandover of the mobile users’ connectionsamong different wireless technologies must besupported to achieve seamless service.

(iii) IPv6 support: The global reachability requires apermanent globally routable IP address for eachmobile node. With large address space, IPv6can support a unique address for each sensoror mobile device in the vehicles. In addition to

the advantage in address space, IPv6 also hasbetter support of security and quality of service(QoS) which are the essential requirements ofITS applications.

(iv) Multihop communication support: Multihopcommunication can extend the transmissionrange of the mobile nodes to reach thedestination. Mobility management schemesfor vehicular networks need to consider themultihop communications requirements, andtherefore, need to be optimized accordingly.

(v) Scalability and efficiency: VANETs may belarge in size which can consist of hundredsof vehicles and thousands of devices inone network. Furthermore, due to the highfrequency of change of the point of attachment,the mobility management scheme must bescalable and efficient to support different typesof traffic.

In traditional infrastructure-based mobile networks(e.g., cellular system), mobility management can beclassified according to the following criteria:

(i) Network structure: Mobility management canbe classified into mobility management inhomogeneous networks [5] and in heteroge-neous networks [14].

(ii) Users’ roaming area: Mobility managementcan be classified into macro-mobility andmicro-mobility management solutions whichprovide global and local mobility management,respectively. Due to the hierarchical designof global and local management, perfor-mance of mobile users can be improved. Formacro-mobility management, mobile IPv4 [15]and mobile IPv6 [16] were introduced. Formicro-mobility management, fast handover forMIPv6 (FMIPv6) [17], Hierarchical MIPv6(HMIPv6) [18], Cellular IP [19], HAWAII [20],and Proxy MIPv6 (PMIPv6) [21] were pro-posed.

(iii) Mobile host signalling: Mobility managementcan be classified into host mobility andnetwork mobility management depending onwhether or not the mobile host is involvedin signalling for mobility management. If thesignalling of mobility management is sent orreceived by the mobile host, it is called hostmobility management, and network mobilitymanagement, otherwise.

(iv) OSI layers: The type of mobility managementcan be identified by the OSI layer which



the mobility management belongs to. Mobilitymanagement can be implemented in data link,network, transport, application layer, or in crosslayer fashion.

For the review of mobility management forvehicular networks, more than one criterion will becombined to better characterize the schemes. In thiscase, the mobility management schemes for vehicularnetworks are first categorized into host mobility andnetwork mobility. Then, the protocol layer criterion isapplied for further classification.

2.2. Mobility Management for V2VCommunications

For vehicular ad hoc networks (VANETs), mobilityis managed through route discovery, maintenance,and recovery [6]. Efficient management of vehicularmobility is composed of topology control, locationmanagement, and handoff management.

(i) Topology management: Topology managementcan be proactive and reactive. Proactive schemesperiodically send signalling messages to explorethe topology information. On the other hand,reactive schemes obtain the topology informa-tion only when it is needed (e.g., when there isa new mobile node to join the network).Since VANETs can be very large, purelyhost-based topology control does not scalewell in such networks. The cluster-basedtopology control can solve this limitation. Inthis cluster-based topology control, vehiclesare grouped into multiple clusters. Head ofeach cluster is responsible for intra-topologymanagement. These cluster heads coordinateamong each other to manage the entire ad hocnetwork topology. However, due to the highspeed and constrained mobility (e.g., movingalong a straight road) of vehicles, currentclustering schemes developed for MANETscannot achieve the optimal performance inVANETs and the clusters could be unstable. Toaddress this problem, clustering for open inter-vehicle communication (IVC) networks (COIN)was proposed [22]. The cluster head electionis based on mobility information and driverintentions. Besides, COIN can accommodatethe oscillatory nature of inter-vehicle distances.In [23], a prediction-based reactive topologycontrol was proposed. The basic concept of thisscheme is to increase the topology maintenance

interval and to reduce the periodic beaconingprocess by mobility prediction. Updates areonly needed when the predicted topologyinformation is incorrect. A location-awareframework, i.e., kinetic graph, was introducedto support the use of standard ad hoc networkprotocols. With kinetic graph, the standardad hoc protocols can perform efficiently inVANETs.

(ii) Location management: With unique mobilitycharacteristics of VANETs, basic ad hocrouting protocols cannot be directly appliedto VANETs due to the large latency andoverhead [23]. However, geographic routingwas shown to be efficient and effectivefor VANETs. Using geographic routing (e.g.,greedy perimeter stateless routing (GPSR) [24],geographical routing algorithm (GRA) [25]),communicating nodes are required to have thelocation information of each other. Therefore,location management scheme, which dealswith the storage, maintenance, and retrieval ofmobile node location information, is neededin VANETs [26]. It is worth noting that, thelocation here refers to geographical locationwhich is not the same as the addressing locationin Internet [6].Location management in VANET can beclassified into flooding-based and rendezvous-based approaches [27]. Using a flooding-based approach, the source floods the locationquery to the entire network which incurs hugeoverhead. On the other hand, in a rendezvous-based approach, location servers are responsiblefor location management. Nodes update theirlocation and query the location of destinationfrom location servers. Many schemes wereproposed for location management in MANETs.For example, region-based location servicemanagement protocol (RLSMP) which supportsboth scalability and locality awareness wasproposed for VANETs [28]. In RLSMP,message aggregation with the enhancementfrom geographical clustering was used for bothlocation updating and querying to improvescalability. For locality awareness, local searchwas used to locate the destination node.

(iii) Handoff management: Handoff managementin vehicular ad hoc networks is performedby rerouting to construct a new path to thedestination. When a mobile node moves, agroup of neighbors changes and hence the new



route of data transfer needs to be establishedquickly for better handoff performance. Handoffmanagement can be proactive and reactivewhich uses the same concepts as those in mobilead hoc network routing. A survey of routingschemes in VANETs can be found in [29].

A simple taxonomy of mobility managementsolutions for vehicular network is shown in Fig. 2.

Fig. 2. Taxonomy of mobility management solutions forvehicular network.

3. Host Mobility Solutions for VehicularNetworks

Host mobility management is the scheme in which themobility of each mobile node is managed individually.Host mobility management can be designed andimplemented at different OSI layers [30] [31].In the following, mobility management schemesimplemented in different layers are reviewed andtheir suitability to vehicular networks (i.e., for V2Icommunications) is discussed.

Link layer: When the mobile node moves betweenaccess points (APs) within a common subnet, mobilityis managed by link layer protocol [32]. With WLAN asan example, the movement of mobile nodes betweentwo APs connecting to the same access router ishandled by WLAN specific link layer handoff scheme.In [33], an enhanced link-layer handover scheme inwhich mobile node can receive real-time downlinkservice from base station during handover process wasproposed. MAC management message, i.e., Fast DL-MAP-IE, was also defined in [33] to support downlinktraffic reception during handoff process and to reducethe downlink transmission delay.

Network layer: When a mobile node moves toa different subnet, the original home IPv6 address

will be topologically invalid. As a result, mobilitymanagement scheme in network layer is required.MIPv6 is the fundamental network layer protocol forhost mobility support standardized by IETF. MIPv6is independent of lower layer and also transparentto upper layer protocols. However, the shortcomingsof MIPv6 are the long handoff latency and highpacket loss. Besides, MIPv6 is not scalable. With theincreasing number of mobile nodes, the signallingoverhead increases dramatically. In this case, MIPv6can be used as the location and path update protocolrather 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 permanenthome address (PHA) and a care-of-address (CoA).PHA is used to identify, while CoA is used tolocate the mobile node. When a mobile nodeis attached to its home network, conventional IProuting mechanisms can be used to forward packetsto mobile nodes. If a mobile node moves to aforeign network, movement detection is performed byreceiving periodical router advertisement from a newaccess router (AR). A new CoA is obtained accordingto the advertised foreign subnet prefix through statefulor stateless IPv6 autoconfiguration mechanisms. Then,duplication address detection (DAD) is performed toensure the uniqueness of the mobile node’s local linkaddress as well as its new CoA. Once the addressconfiguration is done, the mobile node sends a bindingupdate (BU) message to its home agent to register thenew address. Then, packets from correspond nodes(CNs) with the destination to the mobile node’s homeaddress will be intercepted by its home agent and thentunneled to its current CoA. The process of MIPv6 isillustrated in Fig. 3. The BU message can also be sentto the MIPv6-enabled CN for direct communicationwith mobile node without involving the home agent.This is referred to as route optimization.

Based on the above mechanism of MIPv6, handofflatency is composed of link layer latency and networklayer latency [35]. Link layer latency is the delaydue to air link migration from current AP to newAP. Network layer latency is the delay due tomovement detection, network authentication, CoAconfiguration, DAD, and BU messaging. Packets sentto the mobile node during the handoff period will belost. Due to large handoff latency and high packetloss, MIPv6 is not suitable for V2I communicationespecially for real-time services. Furthermore, due



Fig. 3. Illustration of MIPv6

to heavy signalling overhead caused by fast movingvehicles, MIPv6 is also not sufficiently scalable forV2I communications.

To reduce the packet loss and the handoff latencyin MIPv6, FMIPv6 (i.e., fast handover for MobileIPv6) was proposed. This FMIPv6 addresses thefollowing problems: how to allow a mobile nodeto send packets as soon as it detects a new subnetlink, and how to deliver packets to a mobile node assoon as its attachment is detected by the new AR.FMIPv6 uses link layer triggers to predict networklayer handoff [17]. FMIPv6 relies on the predictionwhose accurate result is however difficult to obtainfor fast and randomly moving mobile nodes. Manyefforts have been devoted to improve reliability ofthe handoff prediction. In FMIPv6, when a mobilenode receives a link layer trigger, several messageswill be exchanged among mobile node, old AR, andnew AR. However, fast moving nodes, i.e., vehicles,may cross the boundary of adjacent cells quicklysuch that signalling message may not be completelyexchanged. To address this problem, an early bindingfast handover (EBFH) scheme was proposed for high-speed mobile nodes [36]. In EBFH, a fast movingnode can detect the new network by monitoring routeradvertisement and initiate early binding update withits current access router. This scheme can improvethe reliability of the prediction for high speed mobilenodes at the cost of larger overhead.

To reduce the amount of signalling among themobile node, CNs, and home agent, hierarchicalmobile IPv6 mobility management (HMIPv6) wasintroduced. In HMIPv6, mobility anchor point (MAP)located in the new network is used as a local home

agent (HA) for mobile nodes. With HMIPv6, amobile node has two CoAs i.e., MIPv6 CoA andregional CoA. A regional care-of-address (RCoA)has a similar role to home address. RCoA has thesame subnet prefix as MAP. If mobile node movesacross subnetworks but within a MAP domain, mobilenode only needs to register its new CoA with MAPwhile RCoA does not change. HMIPv6 can alsoreduce handoff latency due to smaller signalling andshorter path. Besides, as a simple extension to MIPv6,FMIPv6 can be combined with HMIPv6 (FHMIPv6)to further minimize or eliminate the intra MAPdomain handover latency. However, FMIPv6 does notsupport inter MAP domain movement. Therefore, itis not suitable for real-time services in fast movingvehicles. To address this problem, improved fasthandover protocol using HMIPv6 (IFHMIPv6) basedon IEEE 802.16e was proposed [37]. Layer 3 handovermessages of the FHMIPv6 are embedded into thelayer 2 handover messages so that multiple handoverprocedures can be performed simultaneously.

Upper layers: To avoid the change of currentInternet architecture, much work has been donetowards supporting mobility management in upperlayers. For example, at transport layer, mobile StreamControl Transmission Protocol (mSCTP) [38] wasproposed. Due to the multi-homing feature, mSCTPcan be used for Internet mobility support withoutchanging Internet architecture. However, interfacesbetween transport layer and application layer needto be modified and transport layer of CNs needs tobe changed. Another approach is to manage mobilityat application layer, for example, Session InitiationProtocol (SIP) [39] and its extensions [40]. However,due to large overhead and long latency, these upperlayer mobility management schemes are not suitablefor vehicular networks.

Cross-layer: Information from multiple layers canbe effectively exchanged to improve performance ofmobility management schemes. FMIPv6 is such across-layer design which uses link layer informationfor handover in network layer. In [41], a newcross-layering design for fast IPv6 handover supportover IEEE 802.16e was proposed. The predictionin FMIPv6 utilizes the information from link layerand physical layer protocols. The proposed schemeprovides an interaction between the IP layer and theMAC layer which can improve the performance ofFMIPv6 in IEEE 802.16e environment.

A summary of the host mobility solutions for V2Icommunications is shown in Table I. The aboveschemes are used to manage the mobility of a single



Table I. Host mobility solutions.

Schemein [27]

MIPVv6 FMIPv6 EBFH FHMIPv6 IFHMIPv6 mSCTP SIP Scheme in[35]

Protocol layers L2 L3 L3 L3 L3 L3 L4 L5 Crosslayer

Route optimizationsupport

NO YES YES NO NO NO YES YES NO

Signalling overheads LOW HIGH HIGH HIGH LOW LOW LOW HIGH HIGHHandover latency andpacket loss

LOW LARGE LOW LOW LOW LOW LARGE LARGE LOW

Change to currentarchitecture

NO HA HA 802.21GenericLink layer

HA andMAP

MAP NO NO HA

Change to currentprotocol stack

YES YES YES YES YES YES YES NO YES

Technique specific YES NO NO NO NO YES NO NO YESCross 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 communicatewith the Internet gateway directly. However, whenvehicles form a VANET connecting to the Internet,the issues related to the integration of MIPv6-based mobility management schemes into VANETbecome challenging. In VANET, some vehicles mayneed multihop communication to the gateway, e.g.,vehicles B and C shown in Fig. 4. MIPv6 cannotbe used directly in such a scenario since a directconnection between gateway and vehicles is notalways available [12]. The architecture of Internet andVANET is also different in terms of topology, routingprotocols, and mobility management. For example,

Internet uses Mobile IP to handle host mobility whilein VANET the mobility is managed by ad hoc routingprotocols.

Some research work have been done for theintegration of MANETs and Internet. A survey ofthis topic can be found in [42]. Since VANET is aspecial type of MANET, we review the solutions foran integration of MIPv6 with MANETs. The relatedissues are as follows [43].

(i) Internet gateway discovery: A mobile nodedetects a foreign network by monitoring routeradvertisement (RA) from the new Internet gate-way. In IPv4 networks, RA message containingforeign subnet prefix is broadcast to the mobilenodes 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 MANETrouting protocols, intermediate nodes cannotforward RA messages to the mobile nodeswhich have more than one hop to reach AP.As a result, these mobile nodes cannot detecttheir movement and cannot construct CoA. Anexample is shown in Fig. 4. When vehicle Dmoves from VANET2 to VANET1, the previouslink with IGW2 is broken. In this case, vehicleD should be aware of the service from IGW1and establish a new CoA according to the subnetprefix advertised by IGW1. However, vehicleA cannot forward the RA message to othervehicles. Therefore, vehicle D cannot find theservice of IGW1 and cannot obtain correct IPaddress.



(ii) Routes to CoA in MANETs: In MIPv6, afteracquiring a new CoA, a mobile node (e.g.,vehicle D in Fig. 4) sends BU message to homeagent which returns a binding acknowledgementmessage to mobile node. A bidirectional routebetween vehicle D and home agent is thenestablished. The BU message can be routedfrom vehicle D to gateway IGW1 using standardad hoc routing protocol, e.g., optimized linkstate routing (OLSR) [44], and subsequentlyrouted to home agent via IP routing. However,as the CoA assigned by IGW1 is not aroutable address in MANET, for routing bindingacknowledgement from IGW1 to the CoA ofvehicle D, no standard method can be used.

(iii) Redundant routes: Using MIPv6, packets withthe destination to a mobile node outside homenetwork are intercepted by its home agentand then forwarded/tunneled to the currentCoA. However, though suitable paths betweentwo mobile nodes within a MANET exist,mobile nodes still communicate through thehome agents which causes redundant routeand latency. To reduce latency and bandwidth,the mobile nodes can communicate with eachother directly without the involvement of homeagents.

To address the Internet gateway discovery problemin MANET, several approaches were proposed.

(i) Ad hoc routing extensions: The ad hoc routingprotocol was extended to support mobile IPin MANET. There are two solutions. Onesolution is to modify the routing protocol tosupport IP broadcast. The main idea is to useIP broadcast to discover the Internet gatewayinstead of local link range broadcast used inMobile IP. However, such extension is notsuitable for VANET since it cannot supportMIPv6. Also, this extension is not scalabledue to the broadcasting of agent advertisementsand agent solicitations. The other solution isto extend ad hoc routing protocol to supportmulticast. The main idea is to use IP multicastin ad hoc network to discover Internet gateway.The problem is that the use of multicast in adhoc networks is less efficient and scalability islimited [12, 45].

(ii) Service discovery: Service discovery protocolscan be used for mobile nodes within anad hoc network to identify and register toInternet gateway. However, since vehicles move

at high speed, the performance of servicediscovery degrades severely in VANETs. Also,this solution is not scalable [12, 45].

Due to the limitations of the existing solutionsfor MANET, Internet gateway discovery solutions forVANETs were proposed with many improvements.DiscoveRy of Internet gateways from VEhicles(DRIVE) based on Service Location Protocol (SLP)was developed [45] with scalability and efficiencyenhancement. In addition, it has the ability to selectthe most suitable Internet gateways among multipleavailable choices. In [43], the OLSR control packetsare used to carry the foreign subnet prefix (i.e.,RA/OLSR) and to distribute them to all mobile nodesin the VANET. OLSR control packets are used sincethey can be flooded to all VANET nodes. The ability ofOLSR to construct routes to CoAs was also exploitedin [43]. With OLSR, a mobile node can have its ownmultiple routable addresses. Once acquiring a newCoA, a mobile node advertises its CoA as multipleroutable addresses. In this way, a route to CoA inVANET can be established using OLSR.

To solve the redundant route problem statedabove, [43] proposed a new route optimizationscheme. The general idea is to add the home ofaddress (HoA) of the mobile node into routing tableas a host route. Similar to the route constructionmechanism to CoAs, the work in [43] uses OLSR toenable the routability of HoA in MANET. After that,packets to HoA in the same MANET are transmittedto correspondent node (CN) directly. Although thisscheme was primarily designed for MANET, it is alsoapplicable to VANET.

VANET mobility management scheme (i.e.,MMIP6) was proposed in [12] for integration withInternet. This MMIP6 was optimized to be scalableand efficient. The key idea is to combine a proactiveservice discovery protocol with an optimized mobilitymanagement protocol. Although it was designedto support IPv6, MMIP6 is based on the principlesof MIPv4 in terms of using both home agent andforeign agent. An important feature in MMIP6 is thatit only uses a permanent and global IPv6 addressrather than CoA when a mobile node moves intoa foreign network. Using Fig. 4 as an example,the communication based on MMIP6 works asfollows. CN wants to communicate with vehicle Din VANET1. Packets from CN to vehicle D’s homeaddress are received by D’s home agent. The homeagent then tunnels these packets to IGW1 which actsas vehicle D’s foreign agent. Finally, IGW1 delivers



the decapsulated packets to vehicle D using VANETrouting protocol.

4. Network Mobility Solutions for VehicularNetworks

With the proliferation of embedded and portablecommunication devices, a mobile network can beestablished in which the vehicles move as a group.Network mobility is referred to as the situation inwhich the mobile network dynamically changes itspoint of attachment to the Internet. Compared withhost mobility, network mobility (NEMO) is moreeffective and efficient for vehicular networks dueto reduced handoff and complexity [46]. Therefore,network mobility management is important forvehicular networks. NEMO Basic Support (NEMOBS) protocol was standardized by IETF to providebasic network mobility support. In this section,scenarios and characteristics of network mobilityare firstly introduced. Then, the requirements andadvantages of NEMO are discussed. NEMO BS andsome extended NEMO solutions are also reviewed.In addition, we will illustrate two NEMO routeoptimization solutions and discuss the problem ofintegrating NEMO with VANET.

4.1. Scenarios and Characteristics of NetworkMobility

A mobile network consists of one or more mobileIP-subsets formed by one or more mobile routers(MR). This MR, which can change its point ofattachment, provides Internet connectivity to mobilenetwork nodes (MNNs) within the network. IETFdefines three types of MNNs, i.e., local fixed node(LFN), local mobile node (LMN), and visiting mobilenode (VMN). LFN is a fixed node belonging to mobilenetwork without mobility support. LMN and VMNare MNNs with mobility support. The home link ofLMN belongs to the mobile network while that ofVMN does not. An MNN can be either a host or arouter and either fixed or mobile. A mobile networkis called nested if there is another attached mobilenetwork inside. The nested mobility is unique fornetwork mobility [47]. For network mobility, there isno size limitation for mobile networks. The simplestnetwork may only consist of a mobile router andMNNs. A mobile network can also consist of hundredsof mobile routers and several nested mobile networks.The common mobile network scenarios in networkmobility are as follows [48].

(i) Personal area network (PAN): As shown inFig. 5(a), the personal area network (PAN)consists of a mobile phone with cellular andbluetooth interfaces. Using bluetooth, a PDAand a laptop connect to mobile phone whichacts as the mobile router to provide Internetconnectivity.

(ii) Public transportation mobile network: Mobilehotspots deployed in public transportation (e.g.bus or train) can provide Internet connectivity toIP devices (e.g., PDA and laptop). Besides, sucha mobile network may be nested if passengers’PANs are attached. As shown in Fig. 5(b), onthe bus, a passenger with PAN is attached tothe mobile router (MR) via WLAN interfaceof the laptop. In this case, the laptop performsas the sub mobile router for PAN which is thenested mobile network. The MR is called theroot mobile router.

(iii) Intra-vehicle embedded mobile network: Tosupport ITS applications, vehicles are usuallyequipped with sensors, GPS, and embeddeddevices as shown in Fig. 5(c). These devices canconnect to mobile router located in the vehiclefor Internet connectivity.

The characteristics of mobile networks in networkmobility are as follows:

(i) Group of nodes move as a unit: From Internetperspective, the entire mobile network changesits reachability in relation to the fixed Internettopology as a group or unit [3].

(ii) Various sizes and moving speeds: Mobilenetworks have various sizes and moving speeds.For example, pedestrian with PAN may walkat speed of 5km/h while access networksaccommodating hundreds of devices in a trainmay move at speed of 100km/h.

(iii) Various mobile network nodes: Mobile networknodes have various types, i.e., mobile hostand mobile router, local nodes and visitingnodes, 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 canbe nested with arbitrary number of levels.

(v) Mobility transparency to mobile network nodes:In most cases, the internal topology of mobilenetwork is relatively stable [48]. For example,a laptop attached to a mobile router in amoving bus will not change its point ofattachment frequently. Therefore, the link layer



connection of the laptop and the mobile routercan be maintained even when the mobile routerchanges its point of attachment to the Internet.The mobile network nodes do not need to beaware of location change with respect to theInternet.

Fig. 5. Three scenarios of mobile networks.

4.2. Advantages and Requirements of NetworkMobility

With NEMO, once attached to a mobile network,the mobility management for the MNNs is fullyperformed by the mobile router. In particular, themobility management is transparent to the mobilenetwork nodes. The advantages can be summarized asfollows [49]:

(i) Scalability: A mobile network may consistof hundreds of MNNs. Without a networkmobility solution, these MNNs have to handlemobility independently. For one node, severalsignalling messages need to be exchanged withthe point of attachment. On the other hand,using basic network mobility solutions, mobilityis handled only by the mobile router andhence the signalling overhead can be reducedsignificantly.

(ii) Reduced handoff: Due to the relatively stableinternal topology of mobile network (e.g.,topology among mobile router and MNNs),mobile network nodes do not change their pointsof attachment and hence can avoid link layerhandoff.

(iii) Reduced complexity: Mobile network canprovide mobility support to standard IPv6.The IP addresses of MNNs will not changeeven if the mobile router changes its pointof attachment. Therefore, the complexity ofsoftwares and hardware used in MNNs can bereduced.

The requirements of NEMO solution can besummarized as follows [48].

(i) Global reachability and session continuity ofMNN: This is the fundamental requirement fornetwork mobility. Mobile network nodes mustbe globally reachable given a permanent IPaddress. During the movement of mobile router,ongoing sessions of MNNs must be maintained.

(ii) Minimum changes: For basic network mobilitysupport, no modifications should be requiredto any entities other than mobile router and itshome agent.

(iii) Support for different nodes: Basic networkmobility solutions must support all types ofmobile network nodes mentioned above.

(vi) Compatibility: The solutions must be com-patible with existing Internet standards. Forexample, it should not affect the operation ofMIPv6 or standard IP addressing and routingschemes.

(v) Nested mobility support: The solutions shouldsupport mobile network nodes which are locatedin nested mobile networks at different levels.

(vi) Internal configuration transparency: The inter-nal configurations (e.g., topology) should betransparent to the solutions. In other words, thesolutions can be applied to mobile networkswith arbitrary internal topologies.

(vii) Scalability: To support large mobile networks,the solution needs to be scalable.

(viii) Security: The solutions must have sufficientprotection from the attack.

4.3. Network Mobility Solutions

Similar to host mobility, solutions for networkmobility can be designed and implemented in differentlayers. In the following, we mainly focus on networklayer and application layer solutions.

Network layer solutions: Network Mobility BasicSupport (NEMO BS) protocol was proposed byIETF to provide basic network mobility support. Tominimize the change to existing architecture and tomaintain backward compatibility, NEMO BS was



designed based on MIPv6 with minimal extensions.Similar to mobile host in MIPv6, mobile router hashome address and home agent (i.e., HA-MR). NEMOBS specifies operation of mobile router and homeagent, while the details of mobile network nodes arethe same as that in MIPv6 [50].

In NEMO BS, when a visiting MNN connects tothe mobile network using with MIPv6, the MNN willreceive a subnet prefix (i.e., network prefix (MNP))advertised by the mobile router. Then, the MNNestablishes new care-of-address (CoA) based on MNP.Once the address configuration is done, the MNNsends a binding update (BU) message to its homeagent. The home agent sends binding acknowledgeback to finish the location update procedure.

When the mobile router changes its point ofattachment, it also acquires a CoA from the visitingnetwork and updates the binding cache of its homeagent. Since the CoAs of MNNs remain unchanged,location update messages do not need to be sent to thehome agent of the MNNs. Once the binding procedureis completed, a bi-directional tunnel between mobilerouter and home agent is established based on IP-in-IPencapsulation [3].

However, route optimization is not considered inNEMO BS due to the security and incompatibilityissues. All packets to and from mobile network nodesneed to be tunneled by the home agent of the mobilerouter. Packets from the MNNs to the correspondentnodes (CNs) are encapsulated by mobile routerand then tunneled to home agent of mobile router.Then the home agent decapsulates these packets andforwards them to the destinations. In the oppositedirection, packets from CNs to MNNs will be firstreceived by the home agent of the mobile router.The home agent tunnels these packets to mobilerouter which then forward them to mobile networknodes. However, the binding cache of home agentonly contains home address of mobile router. Theaddresses of mobile network nodes are not binded withcurrent CoA of mobile router. As a result, packetscannot be tunneled to the mobile router correctly.To solve this problem, prefix scope binding update(PSBU) [51] was proposed. Using PSBU, the mobilerouter sends binding update message to home agentassociating with mobile network prefix rather than thehome address with current CoA. Having the prefixinformation, the home agent can tunnel packets to thecorrect mobile router.

The handoff performance, signalling and routingoverhead of NEMO BS were analyzed in [52]. Theresults show that NEMO BS itself is not sufficient for

seamless handover, and optimization of the protocol isnecessary.

Fig. 6. Handoff components.

The components to support handoff in NEMO BSare shown in Fig. 6. These components are similar tothose in MIPv6. To reduce handoff delay in networkattachment process, in [52], fast RA mechanism [53]was adopted to remove the random delay. Optimisticduplication address detection (ODAD) [54] wasused to reduce DAD delay. However, the handoffperformance of NEMO BS with above optimizationis still not sufficient for QoS-sensitive applications.Latency of link layer handoff and NEMO signallingoverhead affect the overall performance of mobilitymanagement significantly. Novel make-before-break(MBB) handoff scheme was proposed in [52] to reducehandoff delay and packet loss. To enable MBB, twointerfaces are needed for simultaneously listening tomultiple APs. Besides, a method to minimize theoverhead in route optimization was proposed forfurther performance enhancement [55]. However, thisextended scheme can only be applied to the mobilenetwork nodes with host mobility support.

A reactive handoff optimization was proposedin [56]. Compared with proactive schemes whichdetect and predict the movement before the currentlink is broken, reactive solutions are simpler toimplement and more robust. Many limitations areeliminated in reactive solutions (i.e., possible erro-neous movement, moving speed limitation, and highsignalling overhead). A new cross-layer optimizedmovement detection procedure and a new DADscheme were also proposed. A novel reactive handoffprocedure combining the above two new schemeswas designed in which the movement detection andDAD are performed simultaneously. Compared withexisting reactive handoff solutions, the solution in [56]does not rely on prediction information, buffering,bicasting, and soft handover.



In NEMO BS, the binding update traffics areminimized at the cost of tunneling overhead. Toreduce the tunneling overhead, an adaptive NEMOsupport protocol based on hierarchical mobile IPv6(HMIPv6) was proposed in [57]. The general ideais to tradeoff between binding update traffic andtunneling overhead adaptively. An adaptive bindingupdate strategy was deployed based on the session-to-mobility ratio (SMR). This SMR with a predefinedthreshold was compared with different binding updateprocedures. An optimal threshold for adaptive bindingupdates was also derived in [57].

Application layer solutions: To reduce the deploy-ment cost and avoid the suboptimal routing problemsof NEMO BS, SIP-based network mobility (SIP-NEMO) was proposed for network mobility manage-ment in application layer. The system architectureof SIP-NEMO is shown in Fig. 7. Three types ofSIP entities are employed in SIP-NEMO, i.e., SIPnetwork mobility server (SIP-NMS), SIP home server(SIP-HS), and SIP foreign server (SIP-FS). Similar tomobile router in NEMO BS, SIP-NMS is used as agateway between mobile network and Internet. ThisSIP-NMS also manages the mobility of entire mobilenetwork. In SIP-NEMO, the mobile network nodescan be both SIP clients and SIP-NMS. Therefore, theconcept of nested mobile network in SIP-NEMO issimilar to that in NEMO BS. SIP-HS plays a similarrole to home agent in NEMO BS. The SIP-FS is usedfor handoff management. SIP-FS will send requeststo the corresponding nodes according to UniversalResource Identifier (URI) list. This list is maintainedby SIP-NMS when the mobile network changes thepoint of attachment.

When the user agent client (UAC) of SIP movesinto a foreign network, a new care-of-address (CoA)will be constructed. Then, this CoA registers to theSIP-NMS to obtain a new contact address accordingto the domain name of SIP-NMS. To perform locationupdate, UAC sends a REGISTER message with itsnew contact address to its SIP home server. ThisREGISTER message is translated by SIP-NMS andthen forwarded to the SIP home server of UAC.Similar to the binding update of mobile router inNEMO BS, when SIP-NMS changes its point ofattachment, SIP-NMS sends a REGISTER messagewith its new CoA in the contact field to its SIP homeserver.

Since mobility management schemes in networklayer and higher layers have their own advantagesand limitations, an alternative way is to deploythem together in a proper way. HarMoNy, a scheme

Fig. 7. The system architecture of SIP-NEMO.

integrating the host identity protocol (HIP) withNEMO was proposed in [58]. HIP introduces a publickeys-based host identity name.

Since the control signalling and data delivery areseparated in SIP. An explicit session establishmentprocedure is required in SIP-NEMO. Using Fig. 7 asan example, if SIP client UA1 in a mobile networkwants to communicate with a corresponding SIP useragent client UA2, it first sends an INVITE message tothe SIP-NMS. After translation, the SIP-NMS sendsthe INVITE message to the SIP home server ofUA2 which finally forwards the message to UA2.The dash line in Fig. 7 shows the outgoing sessionestablishment. If the session is initiated by UA2, itsends an INVITE message to the SIP home server ofUA1. Since the SIP-HS of UA1 registers its currentlocation, the INVITE message is then redirected to theSIP home server of SIP-NMS. Also, SIP home serverof SIP-NMS forwards the message to the current CoAof SIP-NMS which forwards the message to UA1.The dotted line in Fig. 7 shows this incoming sessionestablishment.

Unlike NEMO BS, SIP-NEMO routes the packetdirectly between SIP clients [59]. In addition, asan application layer solution, SIP-NEMO has theadvantage since SIP-NEMO can be deployed withoutmodifications to the Internet architecture. However,the handoff delay in SIP-NEMO can be large due tolonger message length. A comparative study of NEMOBS and SIP-NEMO was presented in [50]. A summaryof network mobility solutions is shown in Table II.



Table II. Network mobility solutions.

NEMO BS Scheme in [45] Scheme in [49] Scheme in [50] SIP-NEMOProtocol layers L3 L3 L3 L3 L5Route optimization support NO YES YES YES YESSignalling overheads HIGH LOW LOW Adaptive HIGHHandover latency and packet loss HIGH LOW LOW LOW HIGHChange to current architecture HA HA HA and MAP HA and MAP NOChange to current protocol stack YES YES YES YES NOTechnique specific NO NO NO NO NOCross layer information need NO NO YES NO NOH A: Home Agent MAP: Mobility Anchor Point

4.4. Solutions for Route Optimization

Since basic support protocol of NEMO does notconsider route optimization, all packets need to betunneled by the home agent of mobile router evenwhen a shorter path exists. This scheme causessuboptimal route problems especially for multilevelnested mobile network [60]. NEMO BS uses IP-in-IP tunneling. In particular, packets are encapsulatedby all upper level mobile routers and then tunneled totheir home agents. Therefore, the delay and overheadbecome large with increasing number of nested levels.Besides, since all packets of mobile network nodesmust go through home agents of upper mobile routers,this home agent may be congested and become thebottleneck. Also, if the home agent is unavailable, thetotal path will be cut, and the communication becomesunreachable.

Using three level nested mobile networks shownin Fig. 8 as an example, MNN is a visiting mobilenode with MIPv6 support. This node is attached toMR3 and the correspondent node is a standard IPv6node. MR1, the parent-MR of MR2, is the top levelmobile router of the mobile network. MR3 is attachedto MR2. The packets from CN to MNN are firstsent to home agent of MNN (HA-MNN). HA-MNNencapsulates the packets and then tunnels them to thehome agent of MR3 (i.e., HA-MR3). HA-MR3, HA-MR2, and HA-MR1 handle the packets in a similarway. While receiving these multilevel encapsulatedpackets, MR1, MR2, and MR3 decapsulate themsequentially. Finally, MR3 forwards the decapsulatedpackets to MNN.

The main purposes of route optimization for NEMOare to avoid data packets passing through the homeagent of mobile router and to reduce the number ofadditional IPv6 headers added to the original packets.MIPv6 route optimization does not work in mobilenetworks due to the compulsory MR-HA tunneling.A simple extension of MIPv6 route optimization forNEMO 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 problemand the modification to the correspondent nodes isrequired.

In the following, two NEMO route optimizationsolutions are reviewed. It is worth noting that,although route optimization schemes can yield somebenefits, the tradeoffs (i.e., additional signalling over-head, increased protocol complexity, and processingload) need to be taken into account [62].

Mobile IPv6 route optimization for NEMO(MIRON) was proposed in [61]. MIRON combinestwo different operation modes applied to differenttypes of mobile network nodes. Specifically, for thenodes without host mobility support (e.g., standardIPv6 nodes), the mobile router which works asProxy-MR is responsible for all the mobility androute optimization management. While for the nodeswith standard MIPv6 support, MIRON uses a PANA(Protocol for carrying Authentication for NetworkAccess) and Dynamic Host Configuration Protocol(DHCP) based address delegation mechanism toenable self management of mobility and routeoptimization. Such combination guarantees routeoptimization for all types of nodes and network



topology [61]. MIRON has a deployment advantagein that modification is needed only in mobile routers.Evaluation results show that MIRON can significantlyimprove the performance over NEMO BS in terms oflarger TCP throughput and smaller overhead.

The Route Optimization solution for nested mobilenetworks using Tree Information Option (ROTIO) wasproposed in [63]. ROTIO extends the NEMO BS bymodifying binding update and router advertisementmessages. Two binding update messages are usedby nested mobile router. One message sent to theTop Level Mobile Router (TLMR) contains routinginformation of TLMR, and the other sent to thehome agent of nested mobile router contains thehome address of TLMR. Therefore, only two extraentities, i.e., home agents of the mobile router andTLMR, are required in the path from correspondentnode (CN) to the MNNs. Locations of MNN and CNare incorporated in ROTIO. Basic ROTIO scheme isused to optimize the route between MNN and CNwhich are not located in the same mobile network.The extended ROTIO scheme is used for intra-NEMOrouting optimization. Besides, location privacy andmobility transparency are guaranteed in ROTIO.

Many other related works were introduced forNEMO route optimization [64, 65, 66, 67, 68, 69, 70,71, 72]. An analytical framework with performancemetrics (e.g. transmission latency, memory usage,and BU’s occurrence number) was proposed in [73].Detailed classification, evaluation, and analysis can befound in [74].

4.5. VANETs with Network Mobility

Similar to MIPv6, NEMO BS is designed for mobilenetwork with direct communication link with anInternet access infrastructure. However, multihopcommunication is not supported in this scheme.Mobile routers in vehicles may form VANETs. Toguarantee the consistent reachability to Internet frommobile networks via both direct and indirect links,it is necessary to integrate VANETs with NEMO.In addition, such integration can also be used forroute optimization [75]. The works in [76] [77]realized route optimization in terms of delay andbandwidth by switching from NEMO to MANET.In [75], MANET with NEMO was evaluated and theresults show performance improvement. From securityperspective, [78] proposed VARON, a Vehicular Adhoc Route Optimization solution for NEMO usingMIPv6 security concept to provide the same level ofsecurity of current Internet.

MANEMO is the concept to integrate the MANETwith NEMO [79], which combines the advantages ofboth the schemes. Since the schemes for integratingVANET with NEMO are inherited from MANEMO,it is worth reviewing MANEMO solutions. TheMANEMO solutions can be designed based onMANET or NEMO, i.e., MANET-centric approachesand NEMO-centric approaches, respectively. Accord-ing to the definitions in [9], in MANET-centricsolution, NEMO techniques (e.g., NEMO BS)is applied directly to MANETs. Similar to theaforementioned multihop communication methodsin the former section, foreign subnetwork prefixadvertising problems were addressed by specificextensions of MANET routing protocols. The mainidea of this NEMO-centric solution is to use atleast one intermediate mobile router along themultihop path between the mobile router and theinfrastructure for relaying packets. In NEMO-centricsolutions, NEMO techniques are used to provideand maintain Internet connectivity while MANETprotocols are used to optimize the routing within amobile network [9]. NEMO-centric solutions can onlybe used in networks with hierarchical topologies (e.g.,nested mobile networks).

Considering a VANET scenario, a comparison ofMANET-centric and NEMO-centric approaches withrespect to VANET specific requirements was alsoperformed in [9] based on economic, functionaland performance criteria. The conclusion is that theMANET-centric approach outperforms the NEMO-centric approach for VANET in terms of complexity,routing performance, and cost. In the following, twoMANET-centric solutions are reviewed.

(i) Unified MANEMO architecture (UMA) is aprotocol combining the functionality of theoptimized link state routing (OLSR) protocoland NEMO BS [79]. In UMA, every UMA-enabled mobile router with direct connectionto the Internet is required to establish a MR-HA bidirectional tunnel and acts as a gatewaymobile router in the MANET. Then, the gatewaymobile router advertises its reachability to theInternet via OLSR host and network association(HNA) messages. Therefore, once receivingsuch HNA messages, the mobile router can startbinding update process via the gateway mobilerouter.

(ii) A solution applying NEMO BS to VANETswas proposed in [80]. The network layer isdivided into two sublayers. In this solution,topology-based routing or geographical routing



protocol is used as VANET routing protocolwhile NEMO BS runs on top of this protocolto support mobility without any modification.This solution was designed specifically forIEEE 802.11-based VANET based on Car2CarCommunication Consortium (i.e., C2C-CC [2])system architecture. The laboratory measure-ments show the effectiveness of the solution forhighly 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 useMIPv6-based NEMO BS schemes. Instead, PTENuses Uniform Resource Identifier (URI) to locate amobile network node. Directory service is used tomap URI to IP address. An IP-IP address mappingscheme, an IPv6 header extension on IP packets, andan IP address redirection scheme on the MRs wereimplemented in [81].

5. Mobility Management for HeterogeneousWireless Access

Current mobile nodes or mobile routers in vehiclescan be equipped with multiple radio access interfacesfor different wireless networks (e.g., 3G, WiMAX,and WiFi). This is referred to as heterogeneousaccess. For session continuity and better wirelessaccess performance, seamless vertical handoff (i.e.,handoff among different wireless technologies) shouldbe performed. Besides, for load balancing purpose,mobile vehicular nodes should be able to accessmultiple networks simultaneously. To achieve theoptimal performance, efficient mobility managementschemes are required for vehicular networks inpresence of heterogeneous wireless access. Recentresearch has taken advantages of multihoming, whichenables a mobile node to use multiple access networkssimultaneously to perform smooth vertical handoff.An analysis of multihoming in network mobilitysupport can be found in [82].

Many works were done for host mobility inheterogeneous wireless networks. However, littleattention has been paid on network mobility. Themain challenge is due to the heterogeneity of accessscenarios in mobile networks. A mobile network mayhave one mobile router with multiple access interfaces,or the heterogeneity may arise from several mobilerouters in a mobile network each with a differentaccess interface.

A solution for multiple mobile routers wasproposed in [83]. This solution consists of mobileDHCPv6 agents and a handoff management center(HMC). Location management and forward lossrecovery were implemented based on mobilityprediction. Cooperative mobile router-based handover(CoMoRoHo) was proposed in [84]. CoMoRoHo usesmultihoming techniques to reduce handoff latencyand packet loss for long-vehicular multihomed mobilenetworks. Multiple mobile routers connected todifferent access networks can also cooperate duringhandoff to reduce packet loss due to handoff latencyand overlapped reception. In [85], Mobile IP basedmobility management architecture for highly mobileusers and vehicular networks was proposed. Thisarchitecture focuses on network selection and timelyhandoff. The handoff decisions are based on networklayer metrics and the frequencies of BU messages aredynamically adjusted according to the speed of themobile node.

The simultaneous mobility is referred to thesituation when both mobile nodes move to othernetworks simultaneously [86]. Due to the high handoffrate caused by highly mobile vehicles, simultaneousmobility will occur frequently in vehicular networks.The mobility management schemes with routeoptimization that sends location binding updates tocorrespondent nodes (e.g., MIPv6, SIP-NEMO) arevulnerable to such simultaneous mobility problem.In particular, when two communicating mobile nodeschange their points of attachment at the same time,they both send binding update messages to each other.However, these two binding update messages are sentto their outdated addresses and the messages will belost.

An analytical framework and solution of simul-taneous mobility were proposed in [86]. Besides,[86] also proposed and compared different solutionsto support simultaneous mobility for MIPv6, SIPbased mobility management, and MIP locationregistration. The solutions for simultaneous mobilityare broadly divided into receiver-side and sender-sidemechanisms based on the entity which is responsiblefor sending a particular binding update message.Both receiver-side and sender-side mechanisms can befurther categorized into timer-based retransmissions,forwarding, pro-active forwarding, redirecting, andpro-active redirecting. Details can be found in [86].

Network mobility solutions also have to takesimultaneous mobility issue into account. A proxy-aided simultaneous handover (PASH) mechanism formobile networks in vehicles was proposed in [87].



This PASH mechanism aims to solve addressingproblem resulting from simultaneous handover inSIP-NEMO. Besides, a Fast Route/local routE re-Establishment (FREE) algorithm was developed.The basic concept is to improve the speed ofreestablishment process of the optimized routing pathand to ensure that the signalling messages will besuccessfully received.

6. Open Research Issues

Future vehicular networks will provide seamlessservices to the mobile users. Despite the existingresearch efforts, there are still many open researchissues related to mobility management for vehicularnetworks.

(i) Quality of service (QoS) issues: QoS require-ments of vehicular applications pose great chal-lenges to mobility management design. Safetyapplications have higher priority than non-safety applications, and such priority shouldbe guaranteed even if handoff is performed.For multimedia applications, handoff latencyshould be minimized. For vertical handoff, aQoS mapping scheme for different wirelesstechnologies may be required. Besides, forboth horizontal and vertical handoff, a resourceallocation mechanism for handed over sessionsis needed to meet QoS requirements. Scalabilityand resource utilization are important factorswhen designing such resource allocation mech-anisms.

(ii) Access selection issues: Vehicular mobile nodeswith multiple access interfaces need to performaccess selection in heterogeneous environment.Many factors (e.g., cost, bandwidth, and delay)and their weights for decision need to bedefined. Access selection is also related tohandoff decision. If multiple access networksare selected simultaneously, an efficient loadbalancing scheme is desirable. Besides, whenintegrating VANETs with Internet, multipleInternet gateways (e.g., a direct Internet gatewayand a indirect Internet gateway) may beavailable for some nodes. Internet gatewayselection is also required for the vehicularnodes.

(iii) Issues related to mobility model: Performanceevaluation is required for both designing newprotocols and applying extensions of existingprotocols for vehicular networks. Accurate

mobility model is required for performanceevaluation of vehicular networking protocols.Traditional mobility models (e.g., random way-point) are not suitable for vehicular networkssince they assume a random direction selectionand random speed. However, mobility of vehi-cles is constrained by pre-built roads, vehiclespeed, and driving regulations. A flexible butrealistic mobility model for vehicular networksis needed.

(iv) Ad hoc routing issues: Mobility was notconsidered in ad hoc routing protocols. For bothV2I and V2V mobility management solutions,the handoff performance degrades severely withincreasing number of hops. Mobility-awarevehicular ad hoc routing protocol is required tofacilitate fast handoff.

(v) Transport and application-layer performanceissues: Performance of transport and applicationlayer protocols (e.g., TCP, UDP) need to beoptimized for vehicular networks. Effects ofmobility management schemes on transportand application layer performances are worthinvestigation.

7. Conclusions

In this paper, we have presented a comprehensivesurvey of mobility management solutions for vehicularnetworks. We have classified the mobility man-agement solutions for vehicular networks based onvehicle-to-vehicle (V2V) or vehicle-to-infrastructure(V2I) communications. The traditional Internet andmobile ad hoc network mobility management tech-niques and their suitability to vehicular networks havebeen discussed. Existing works for both V2I and V2Vmobility management have been reviewed. Severalopen research issues have been also outlined.

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Acknowledgment

This work was done in the Centre for Multimediaand Network Technology (CeMNet) of the Schoolof Computer Engineering, Nanyang TechnologicalUniversity, Singapore. This work was supportedin part by the AUTO21 NCE research grant forthe project F303-FVT, and the MKE (Ministryof Knowledge Economy), Korea under the ITRC(Information Technology Research Center) supportprogram supervised by the IITA (Institute ofInformation Technology Assessment).



Authors’ Biographies

Kun Zhu is a Ph.D. student in School ofComputer Engineering, Nanyang Technological University,Singapore. He received the B.Eng. and M.Eng. degrees bothin Computer Engineering from Beijing Jiaotong University,Beijing, China, in 2005 and 2007, respectively. His researchinterests are in the area of mobility management and networkselection in heterogeneous wireless networks.

Dusit Niyato is currently an assistantprofessor in the School of Computer Engineering, at theNanyang Technological University, Singapore. He obtainedhis Bachelor of Engineering in Computer Engineeringfrom King Mongkut’s Institute of Technology Ladkrabang(KMITL), Bangkok, Thailand. He received Ph.D. inElectrical and Computer Engineering from the University ofManitoba, Canada. His research interests are in the area ofradio resource management in cognitive radio networks andbroadband wireless access networks.

Ping Wang (M’09) received the Ph.D.degree in electrical engineering in 2008 from the Universityof Waterloo, Canada. She is currently an assistant professorat School of Computer Engineering, Nanyang TechnologicalUniversity, Singapore. Her current research interests includeQoS provisioning and resource allocation in multimediawireless communications. She was a co-recipient of aBest Paper Award from IEEE ICC 2007. She is anEditor of EURASIP Journal on Wireless Communicationsand Networking, International Journal of CommunicationSystems, and International Journal of Ultra WidebandCommunications and Systems.

Ekram Hossain is currently anAssociate Professor in the Department of Electrical andComputer Engineering at University of Manitoba, Winnipeg,Canada. Dr. Hossain’s current research interests includedesign, analysis, and optimization of wireless communi-cation networks and cognitive radio systems. He is a co-author/co-editor for the books “Dynamic Spectrum Accessand Management in Cognitive Radio Networks” (CambridgeUniversity Press, 2009), “Heterogeneous Wireless AccessNetworks” (Springer, 2008), “Introduction to NetworkSimulator NS2” (Springer, 2008), “Cognitive WirelessCommunication Networks” (Springer, 2007), and “WirelessMesh Networks: Architectures and Protocols” (Springer,2007). Dr. Hossain serves as an Editor for the IEEETransactions on Mobile Computing, the IEEE Transactionson Wireless Communications, the IEEE Transactions onVehicular Technology, IEEE Wireless Communications,IEEE Communications Surveys and Tutorials and severalother international journals. He served as a guest editorfor the special issues of IEEE Communications Magazine(Cross-Layer Protocol Engineering for Wireless MobileNetworks, Advances in Mobile Multimedia) and IEEEWireless Communications (Radio Resource Managementand Protocol Engineering for IEEE 802.16). He served asa technical program co-chair for the IEEE Globecom 2007,IEEE WCNC 2008, IEEE VTC 2008-Fall, and QShine 2008:International Conference on Heterogeneous Networking forQuality, Reliability, Security, and Robustness. Dr. Hossainserved as the technical program chair for the workshopson “Cognitive Wireless Networks” (CWNets 2007) and“Wireless Networking for Intelligent Transportation Sys-tems” (WiN-ITS 2007) held in conjunction with QShine2007 during August 14-17, in Vancouver, Canada, andthe First IEEE International Workshop on Cognitive Radioand Networks (CRNETS 2008) in conjunction with IEEEInternational Symposium on Personal, Indoor and MobileRadio Communications (PIMRC 2008). He served as thetechnical program co-chair for the Symposium on “NextGeneration Mobile Networks” (NGMN’06), NGMN’07,NGMN08, NGMN09 held in conjunction with Interna-tional Wireless Communications and Mobile ComputingConference (IWCMC’06), IWCMC’07, IWCMC’08, andIWCMC’09. Dr. Hossain has several research awards tohis credit which include Lucent Technologies ResearchAward for contribution to IEEE International Conference onPersonal Wireless Communications (ICPWC’97), and theBest Student-paper Award in IWCMC’06. He is a registeredProfessional Engineer (P.Eng.) in the Province of Manitoba,Canada. Dr. Hossain is a Senior Member of the IEEE.



Dong In Kim received the B.S.and M.S. degrees in Electronics Engineering from SeoulNational University, Seoul, Korea, in 1980 and 1984,respectively, and the M.S. and Ph.D. degrees in ElectricalEngineering from University of Southern California (USC),Los Angeles, in 1987 and 1990, respectively.

From 1984 to 1985, he was a Researcher with KoreaTelecom Research Center, Seoul. From 1986 to 1988,he was a Korean Government Graduate Fellow in theDepartment of Electrical Engineering, USC. From 1991 to2002, he was with the University of Seoul, Seoul, leadingthe Wireless Communications Research Group. From 2002to 2007, he was a tenured Full Professor in the Schoolof Engineering Science, Simon Fraser University, Burnaby,BC, Canada. From 1999 to 2000, he was a Visiting Professorat the University of Victoria, Victoria, BC. Since 2007, hehas been with Sungkyunkwan University (SKKU), Suwon,Korea, where he is a Professor and SKKU Fellow in theSchool of Information and Communication Engineering.Since 1988, he is engaged in the research activities inthe areas of wideband wireless transmission and access.His current research interests include cooperative relayingand base station (BS) cooperation, multiuser cognitiveradio networks, advanced transceiver design, and cross-layerdesign.

Dr. Kim was an Editor for the IEEE Journal on SelectedAreas in Communications: Wireless Communications Seriesand also a Division Editor for the Journal of Commu-nications and Networks. He is currently an Editor forSpread Spectrum Transmission and Access for the IEEETransactions on Communications and an Area Editor forTransmission Technology III for the IEEE Transactions onWireless Communications. He also serves as Co-Editor-in-Chief for the Journal of Communications and Networks.
