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An overview of vertical handover techniques: Algorithms, protocols and tools

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An overview of vertical handover techniques: Algorithms, protocols and tools
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyright  Author's personal copy Review An overview of vertical handover techniques: Algorithms, protocols and tools  Johann Márquez-Barja ⇑ , Carlos T. Calafate, Juan-Carlos Cano, Pietro Manzoni Universidad Politécnica de Valencia, Camino de Vera, S/N, 46022 Valencia, Spain a r t i c l e i n f o  Article history: Received 16 February 2010Received in revised form 15 September2010Accepted 26 November 2010Available online 4 December 2010 Keywords: Vertical handoverVehicular networksIEEE 802.21Decision algorithms a b s t r a c t Wireless technologies, under the ‘‘  Anywhere ,  Anytime ’’ paradigm, offer users the promise of being alwaysattached to the network. Mobile devices enabled with multiple wireless technologies make possible tomaintain seamless connectivity in highly dynamic scenarios such as vehicular networks (VNs), switchingfrom one wireless network to another by using vertical handover techniques (VHO). In this paper wepresent an overview of VHO techniques, along with the main algorithms, protocols and tools proposedin the literature. In addition we suggest the most appropriate VHO techniques to efficiently communicatein VN environments considering the particular characteristics of this type of networks.   2010 Elsevier B.V. All rights reserved. 1. Introduction In developed countries the user demand for mobile services isincreasing due to the need to access information anywhere, any-time. The ever-growing communications infrastructure allowsconnectivity through diverse wired and wireless technologies indifferent environments.The adoption of wireless technologies is increasing at a very fastrate. This trend is basically due to factors such as: (i) the miniaturi-zation of devices such as laptops, Personal Digital Assistants (PDA),and netbooks, (ii) the multiple networking interfaces available inmost devices, (iii) the availability of several wireless technologiessuch as Wireless Fidelity (Wi-Fi), Worldwide interoperability forMicrowaveAccess(WiMAX),andUniversalMobileTelecommunica-tions System (UMTS), and (iv) the emerging mobile applications,such as those based on the Web 2.0 paradigm, car navigation, andlocation based services.The integration of different wireless network technologies isneeded to provide a ‘‘seamless’’ interoperability, integration andconvergence among these heterogeneous technologies and, there-fore, the use of   vertical handover (VHO ) techniques are required.Nowadays, the automobile industry is taking advantage of thewireless technologies to improve security and comfort offering tousers diverse services such as alert messages, toll payments, info-tainment, etc. In order to avoid any underperformance and to guar-anteetheQualityofService(QoS)oftheservices,theOn-BoardUnit(OBU)inthecarmustbeabletoperformVHOinaseamlessmanner.Handover, also know as Handoff, is an event taking place when-ever a mobile node moves from one wireless cell to another, aban-doning the connection with the first base station and gettingattached to the second one. When a handover occurs within thedomain of a single wireless access technology the process is knownas  horizontal handover  ; in contrast,  vertical handover   is a term thatrefers to handover among heterogeneous wireless access networktechnologies. Fig. 1 illustrates both horizontal and vertical hand-over events.Handover techniques have been widely studied in the cellularcommunications domain, and their popularity is increasing amongIP-based wireless networks [1]. Handover is considered ‘‘seamless’’when it is able to maintain the connectivity of all applications run-ning on the mobile device, providing a continuous end-to-end dataservice within the same session during the switchover, offeringboth low latency and minimal packet loss.One of the first approaches in order to provide seamless connec-tivity among heterogeneous networks appeared during the latenineties. Stemm and Katz [2] presented an implementation thatgathered Mobile IP and routing aspects, together in an applicationdeveloped to manage the handover process. This implementationwas based on the technology available in those days that consid-ered the IBM Infrared Wireless LAN, the AT&T WaveLAN and theMetricom Ricochet Network as in-building, campus, and wide areaunderlaying wireless technologies, respectively. These first studiesevidenced that vertical handover is a must when dealing with avariety of technologies.More recently, various works appeared covering the verticalhandover (VHO) among heterogeneous technologies including: 0140-3664/$ - see front matter    2010 Elsevier B.V. All rights reserved.doi:10.1016/j.comcom.2010.11.010 ⇑ Corresponding author. Tel.: +34 963 877 000x85720; fax: +34 96 387 7579. E-mail addresses:  jomarba@disca.upv.es (J. Márquez-Barja), calafate@dis-ca.upv.es (C.T. Calafate), jucano@disca.upv.es (J.-C. Cano), pmanzoni@disca.upv.es (P. Manzoni).Computer Communications 34 (2011) 985–997 Contents lists available at ScienceDirect Computer Communications journal homepage: www.elsevier.com/locate/comcom  Author's personal copy UMTS and Wi-Fi [3], Wi-Fi and Wireless Broadband (WiBro) [4], Wi-Fi and WiMAX [5], WiMAX and UMTS [6], WiBro and UMTS [7], WiMAX, Wi-Fi and UMTS [8], Bluetooth (BT) and Wi-Fi [9], broadcast communication technologies like Digital Video Broad-casting (DVB) and Multi media Broadcast/Multicast Service(MBMS) [10], or even UMTS and Low Earth Orbit (LEO) satellite[11]. Considering networking technologies, Table 1 shows the tech- nologies used by the different proposals. As observed, most pro-posals (72.9%) evaluate the VHO using only two technologies,being that the remaining 27.1% of the proposals have consideredthree technologies instead. In addition, about 53.6% of the propos-als focus on evaluating the VHO viability between Wi-Fi and UMTS.The main drawback of this broad variety of solutions stands in thefact that none proposes a unique homogeneous approach that canbe adapted to all the available wireless technologies.The research community has been making significant effortstowards the convergence of the different wireless networkingtechnologies. As a consequence, there are different proposalsaddressing heterogeneous scenarios, protocols, handover tech-niques and algorithms, network technologies, metrics, and proce-dures. In addition, since 2004, the IEEE 802.21 Working Grouphas been working in the Media Independent Handover ServicesProtocol [46] whose purpose is to provide an homogeneousfunction-interface between heterogeneous network technologies,offering standard handover services between lower and upper lay-ers. The IEEE 802.21 standard was finally approved in November2008. Nevertheless, there have been proposals and studiesaddressing the performance of 802.21 and offering improvementsin terms of VHO effectiveness [47,37,48].In this work we survey the most significant proposals found inthe literature concerning VHO techniques, including both particu-lar proposals and standards. In addition, we emphasize on the VHOprocess from a vehicular networks (VNs) perspective, highlightingthose techniques and algorithms that fit better to this type of net-works. This survey can be useful to the research community sincemost proposals in the literature, merely evaluate specific VHOtechniques  per se . Notice that we use the acronym VNs referringto wide area networks where vehicles are interconnected througha base station, and not referring to Vehicular  Ad-hoc   Networks (VA-NETs) where the vehicles are connected among them using a short-range wireless technology.The rest of this paper is organized as follows. Sections 2 and 3present a general overview of VNs and the VHO process, respec-tively.In Section 4 the handover informationgatheringphase is de-scribed. The handover decision phase and the diverse handoveralgorithms are presented in Section 5. Section 6 refers to the hand- over execution phase and the different processes involved in thehandover execution. The main testbeds, simulators and tools forVHO evaluation found in the literature are collected and presentedin Section 7. Section 8 presents some guidelines and open research issues for the development of efficient VHO systems in vehicularenvironments. Finally, the main conclusions are presented inSection 9. 2. Vehicular network overview VNs offer many application branches which can be classified as:(i)  safety  such as emergency warning systems for vehicles, transitor emergency vehicle priority signaling, and (ii)  non-safety  applica-tions. As examples of the latter we have cooperative adaptivecruise control, electronic parking and toll payments, infotainment UMTS (3GPP)WiMAX (802.16)Wi-Fi (802.11)Vertical Handover Horizontal Handover  Fig. 1.  Illustration of the horizontal and vertical handover processes.  Table 1 Networking technologies used in VHO studies. Proposals % Networking technologiesWi-Fi WiMAX WiBro UMTS GPRS Bluetooth Ethernet Two technologies [12–31,3,32] 53.6  w w [33] 2.5  w w [9] 2.5  w w [6,8] 4.9  w w [34] 2.5  w w [4] 2.5  w w [35] 2.5  w w [36] 2.5  w w Three technologies [37–40,5,41,42] 14.7  w w w [43,44] 4.9  w w w [1] 2.5  w w w [7] 2.5  w w w [45] 2.5  w w w 986  J. Márquez-Barja et al./Computer Communications 34 (2011) 985–997   Author's personal copy services, and content delivery, among others. Within VNs a vehicleis considered as a node of the network, being equipped with multi-ple interfaces that provide access to different technologies such asGlobal Positioning System (GPS), Wi-Fi, WiMAX and UMTS. Vehi-cles are able to communicate among them and with their base sta-tions (access points or point of attachments) under the  Ad-hoc   orthe infrastructure mode [49], respectively. Notice that we focus our study on access-point-based networks (VNs) and not on thead hoc mode (VANETs). Further information about VANETs couldbe found in [50].The main specific characteristics of VNs can be summarized in:(i)  topology restrictions , (ii)  mobility patterns , (iii)  power consump-tion , (iv)  scalability , (v)  node reliability  and (vi)  speed  [49]. VNs aregeographically constrained to the roadmap layout; consequently,the topology is heavily dependent on the roadmap or the vehicularscenario. Nevertheless, this restriction allows to improve networkperformance or VHO decisions allowing to predict mobility.Concerning power consumption, wireless devices are traditionallylimited by the battery autonomy. However, in vehicular network(VN) environments, the devices are continuously powered by thevehicle’s energy system; therefore, there are no energy constrainslimiting their performance. It is important to mention that VNscan reach a high number of nodes, which requires them to be veryscalable. Finally, node reliability must be taken into account since anode may quickly join or leave the network coverage area depend-ing on the speed and the destination of the vehicle.Several worldwide projects are being developed and promotedin order to provide solutions to reinforce safety and to extendthe use of non-safety applications on cars and highways throughthe use of wireless technologies. Industry, researchers and govern-ments are working towards a standardization for those solutions:Europe through projects and consortiums such as Car2Car Commu-nication Consortium, Network on Wheels, and Safespot; USAthrough the Vehicle Safety Consortium (VSC), Vehicular Infrastruc-ture Integration (VII) and Intelligent Vehicle Highway Systems(IVHS); finally, Japanese projects Advanced Safety Vehicle (ASV)and Advanced Highway Systems (AHS) focus their efforts on set-ting common issues such as frequency allocation, protocol defini-tion and infrastructure deployment [51].Wireless Access in Vehicular Environments (WAVE) [52] is astandard developed for VNs, to satisfy the communication needsof a large class of applications using the Dedicated Short RangeCommunication (DSRC) spectrum [53]. The WAVE protocol archi-tecture encompasses the IEEE P802.11p standard [54] for thePHY/MAC layers and the IEEE P1609 [52] family of protocols forthe application layers. These protocols must be efficient and reli-able in order to provide safety and comfort services to passengersvia context sensitive applications [55,56], as well as low latencies to deliver contents within acceptable time bounds.The International Organization for Standardization (ISO) is alsoworking towards a seamless connectivity solution for a wide vari-ety of wireless access technologies. This proposal, called Continu-ous Air Interface for Long and Medium Range (CALM M5), hasbeen specifically designed to reduce the end-to-end latency com-munication in mobile environments such as VNs [57], and also works on top of IEEE 802.11p.Referring to VN environments, we consider that VHO strategiesmust be designed and applied depending on the underlyingnetwork technologies geographically available and the expectedlifetime of each possible connection. Therefore, at least twoscenarios might be evaluated: (i)  Urban scenarios  and (ii)  Highwayscenarios . Concerning urban scenarios, multiple short and widerange wireless technologies such as BT, Wi-Fi, General PacketRadio System (GPRS), UMTS and WiMAX may be considered inthe design of the VHO strategy. In highway scenarios the predom-inant wireless technologies are those that cover a wide range, andso VHO strategies should focus mostly on technologies such asUMTS, WiMAX and WiBro. 3. Vertical handover overview An accurate VHO process should take into account and careabout the service continuity, network discovery, network selection,security, device’s power-management and QoS issues [9,33,36],focusing mostly on the latter. Several proposals [25,26,58] splitthe VHO process into three parts: (i) Handover information gather-ing, (ii) Handover decision, and (iii) Handover execution. Fig. 2shows the interactions among the three phases required to imple-ment handover in heterogeneous networks.  3.1. Handover information gathering  The handover information gathering phase collects not onlynetwork information, but also information about the rest of thecomponents of the system such as network properties, mobile de-vices, access points, and user preferences. For that reason thisphase receives different names:  handover information gathering  [24], system discovery  [59], system detection  [60], handover initiation [61,18] or simply  network discovery  [62,48,43]. In this phase, theinformation is collected to be used and processed for making deci-sions in the handover decision phase. The information typicallycollected is the following:   Availability of neighboring network links by offering informa-tion such as throughput, cost, packet loss ratio, handoff rate,Received Signal Strength (RSS), Noise Signal Ratio (NSR), Carrierto Interference Ratio (CIR), Signal to Interference Ratio (SIR), BitError Ratio (BER), distance, location, and QoS parameters.   The Mobile device’s state by gathering information about bat-tery status, resources, speed, and service class.   User preferences information such as budget and servicesrequired. Fig. 2.  Handover management procedure.  J. Márquez-Barja et al./Computer Communications 34 (2011) 985–997   987  Author's personal copy Section 4 describes this phase along with the techniques usedby the different proposals to perform the data gathering task.  3.2. Handover decision The handover decision phase is one of the most critical pro-cesses during the handover. This phase is also know as system selec-tion  [26],  network selection  [25,43] or  handover preparation  [61].Based on the gathered information, this phase is in charge of decid-ing  When and  Where  to trigger the handover.The  When  decisionre-fers to the precise instant in time to make an optimal handover,while the  Where  refers to selecting the best network fulfilling ourrequirements for the switching.In a homogeneous network environment deciding  When  tohandover usually depends on RSS values, while the  Where  is notan issue since we use the same networking technology (horizontalhandover). In heterogeneous networks the answer to these ques-tions is quite complex. To make the best decisions the informationgathered must be evaluated taking into account many parametersobtained from the different information sources, i.e. network, mo-bile devices, and user preferences. Vertical Handover DecisionAlgorithm (VHA) are used to weight up and evaluate the parame-ters involved under each specific criteria. As examples of algo-rithms allowing to evaluate cross-layer multi-parameters wehave techniques such as fuzzy logic, neural networks, and patternrecognition, among others. Section 5 presents a brief descriptionand a taxonomy of the VHA found in the literature.  3.3. Handover execution This phase performs the handover itself; besides performing thehandover, the phase should also guarantee a smooth session tran-sition process. In order to perform the VHO different handoverstrategies cooperate with control signaling, and the IP manage-ment protocols. This phase is usually known as  Handover execution [59,24,58], but it also receives the name of   VHO assessment   [18] or Handoff implementation  [62]. A detailed description of the execu-tion phase is presented in Section 6.Concerning VNs, the performance of each phase must be fo-cused on the distinctive characteristics and features of such typeof networks. The information gathering phase must consider thedynamism of the available information at the devices and the net-work. Making decisions based on highly dynamic information witha given degree of the device’s mobility requires a quick and reliabledecision algorithm. Finally, the execution of the VHO must be care-fully controlled to achieve accuracy by considering the geographi-cal location, the selected network and the precise time. 4. Handover information gathering phase In order to perform an ‘‘ always best connected ’’ handover [63], a full set of information is gathered and provided to the decisionphase. To collect the available information from different sources,the mobile device surveys the surrounding networks in order todiscover services, data rates, and power consumption. As a comple-ment to the information gathered through scanning, networks mayalso advertise their supported services and QoS parameters, whilethe device information is also collected, i.e., speed, battery status,features, and so on. Finally, information concerning user prefer-ences is also a relevant element to the decision-making process,mostly due to its impact on the end user’s satisfaction.Gathering information reliably is crucial for the VHO processsince the decision-making procedure relies on that data. Table 2presentsthe informationthatshouldbe takeninto accountin orderto maximize the benefits of decision-making. It clearly shows thatinformation should be collected at each and every layer of the pro-tocol stack in order to cover all the possible information sources.Moreover, Table 3 presents diverse parameters used by differentauthors in their proposals and works.Different proposals addressing this phase are based not only inmonitoring different layers, but also by implementing events andtriggered notifications. Attaullah et al. [8] present a trigger man-agementsystemthat monitors and collects multi-level parameters.Similarly, works [16,4] present modules called ‘‘Link Layer State’’and ‘‘Network Connection Module’’, respectively, where both per-form the same monitoring task. An xml-based process is used bya CORBA communication middleware to gather information andto notify events in [64]. Seigneur et al. [65] use different operating system’s Application Programming Interface (API) to monitor thedevice and the network in order to evaluate the electrosmog expo-sure to trigger the vertical handover (VHO).Several proposals [48,58,21,66–72] rely on the Media Indepen-dent Information Service (MIIS) mechanism offered by the IEEE802.21 standard for this phase, which allows interaction with awide set of wireless technologies in a unified manner. The 802.21addresses the optimization of both network detection and selec-tion by providing a source of extensible and semantically definedinformation to facilitate optimized handover decision making[61]. As shown in Fig. 3, the IEEE 802.21 offers a middleware pro- tocol called  Media Independent Handover Function (MIHF)  that isable to encapsulate the different underlaying network technologies(e.g. 802.3, 802.11, 802.16, 3GPP, and 3GPP2) to the upper layers,allowing the handover management process to operate indepen-dently of the physical and data link layers.The MIHF protocol defines the messages exchanged betweenpeer MIH entities for handover, offering a common message pay-load across different media (802.3, 802.11, 802.16, Cellular). Thestandard refers as  lower layers  to the technology dependentcomponents, and as  upper layers  to the requesting modules. Theselower layers can be accessed by different functions to retrieveinformation to detect, prepare and execute the vertical handover(VHO), while the upper ones demand that information; therefore,they are also referred to as  Media Independent Handover User  ( MIHU  ). The MIIS allows the MIHF to discover its network environ-ment by gathering information that the upper layers use to makedecisions. Selected information refers to the list of available net-works, location of the point of attachment (PoA), operator ID,roaming partners, cost, security, QoS, PoA capabilities, and vendorspecific information, among others.In heterogeneous vehicular networks it is important to considerthe environmental conditions when performing the vertical hand-over (VHO). A high mobility degree as well as the mobility patternswill significantly affect the reliability of measurements and thedecision-time. Therefore, vehicular mobility patterns and speed  Table 2 Information parameters pertinent to the VHO process. Layers ParametersApplication User preferences (e.g. cost, provider)Context information (e.g. speed)QoS parameters (e.g. bw offered, delay, jitter)Security alerts (e.g. notifications)Transport Network load (e.g. bw available)Network Available foreign agentsNetwork pre-authenticationNetwork configurationNetwork topologyRouting informationData-link Radio access network conditionsLink parametersLink statusPhysical Available access media988  J. Márquez-Barja et al./Computer Communications 34 (2011) 985–997 
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