Mobile ad hoc networking: milestones, challenges, and new research directions
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IEEE Communications Magazine January 2014 850163-6804/14/$25.00 2014 IEEE
INTRODUCTIONThe multihop (mobile) ad hoc networkingparadigm emerged, in the civilian field, in the1990s with the availability of off-the-shelf wirelesstechnologies able to provide direct network con-nections among users devices: Bluetooth (IEEE802.15.1) for personal area networks, and the802.11 standards family for high-speed wirelessLAN (see Chapters 2 and 3 in [1, 2]). Specifical-ly, these wireless standards allow direct commu-nications among network devices within thetransmission range of their wireless interfaces,thus making the single-hop ad hoc network areality, that is, infrastructureless WLAN/WPANwhere devices communicate without the need forany network infrastructure (Fig. 1).
The multihop paradigm was then conceived toextend the possibility to communicate with anycouple of network nodes, without the need todevelop any ubiquitous network infrastructure. In
the 90s, we assisted in the usage of the multihopparadigm in mobile ad hoc networks (MANETs),where nearby users directly communicate (byexploiting the wireless-network interfaces of theirdevices in ad hoc mode) not only to exchangetheir own data but also to relay the traffic ofother network nodes that cannot directly commu-nicate, thus operating as routers do in the legacyInternet. For this reason, in a MANET, theusers devices cooperatively provide the Internetservices, usually provided by the network infra-structure (e.g., routers, switches, servers).
At its birth, the MANET was seen as one ofthe most innovative and challenging wireless net-working paradigms , and was promising tobecome one of the major technologies, increasing-ly present in the everyday life of everybody. Thepotentialities of this networking paradigm madead hoc networking an attractive option for build-ing fourth-generation (4G) wireless networks, andhence MANET immediately gained momentum,and this produced tremendous research efforts inthe mobile network community [1, 2, 4].
The Internet model was central to theMANET Internet Engineering Task Force(IETF) working group, which, inheriting theTCP/IP protocols stack layering, assumed an IP-centric view of a MANET; see Mobile Ad HocNetworks (MANETs) by J. P. Macker and M.S. Scott Corson in . The MANET researchcommunity focused on what we call pure general-purpose MANETs, where pure indicates that noinfrastructure is assumed to implement the net-work functions, and no authority is in charge ofmanaging and controlling the network. General-purpose denotes that these networks are notdesigned with any specific application in mind,but rather to support any legacy TCP/IP applica-tion, as shown in Fig. 2.
Following this view, the research focused onenhancing and extending the IP-layer routingand forwarding functionalities in order to sup-port the legacy Internet services in a networkwithout any infrastructure. At the network layer,we observed a proliferation of routing protocolproposals as legacy Internet routing protocolsdeveloped for wired networks are clearly notsuitable for the unpredictable and dynamic
ABSTRACTIn this article we discuss the state of the art
of (mobile) multihop ad hoc networking. Thisparadigm has often been identified with thesolutions developed inside the IETF MANETworking group, and for this reason it is calledthe MANET paradigm. However, they do notcoincide, and in the last decade they clearlydiverged. In this article, we start from the rea-sons why the MANET paradigm did not have amajor impact on computer communications, andwe discuss the evolution of the multihop ad hocnetworking paradigm by building on the lessonslearned from the MANET research. Specifically,we analyze four successful networkingparadigms, mesh, sensor, opportunistic, andvehicular networks, that emerged from theMANET world as a more pragmatic applicationof the multihop ad hoc networking paradigm.We also present the new research directions inthe multihop ad hoc networking field: people-centric networking, triggered by the increasingpenetration of the smartphones in everyday life,which is generating a people-centric revolutionin computing and communications.
AD HOC AND SENSOR NETWORKS
Marco Conti, Italian National Research Council
Silvia Giordano, University of Applied Technology of Southern Switzerland
Mobile Ad Hoc Networking: Milestones, Challenges, and New Research Directions
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nature of MANET topology . Extensive effortshave been dedicated to building a set of stan-dard protocols. However, the released standardsprotocols, Ad Hoc On-Demand Distance Vector(AODV), Optimized Link State Routing(OLSR), Dynamic Source Routing (DSR), andTopology Broadcast Based on Reverse Path For-warding (TBRPF) (see the IETF MANET webpage1) have their pros and cons, and none ofthem is superior to the others in all contexts.Therefore, they are still under discussion asexperimental RFCs. Currently, the group is pur-suing a reactive (DYMO, i.e., AODV version 2)and a proactive protocol (OLSRv2).
The research interest rapidly spread fromrouting to all layers of the Internet protocolstack; [1, 2] present a complete view of MANETresearch from the physical up to the applicationlayer. On top of the IP, MANET generallyassumes the use of the UDP and TCP transportprotocols. Unfortunately, TCP does not workproperly in this scenario, as extensively discussedin the literature . To improve the perfor-mance of TCP in a MANET, several proposalshave been presented. Most of these proposalsare modified versions of the legacy TCP used inthe Internet. However, TCP-based solutionsmight not be the best approach when operatingin MANET environments; hence, several authorshave proposed novel transport protocols tailoredto the MANET features. On top of that, middle-ware and applications constitute the less investi-gated areas in the MANET field. Indeed, in thedesign of general-purpose MANETs there wasnot a clear understanding of the applications forwhich multihop ad hoc networks are an opportu-nity. Lack of attention to the applications proba-bly constitutes one of the major causes for thenegligible MANET impact in the wireless net-working field. Lack of attention to the applica-tions also limited the interest to developmiddleware solutions tailored on MANETs .
In addition to an in-depth re-analysis of alllayers of the protocol stack, MANET researchalso focused on cross-layer research topics withspecial attention to energy efficiency, security,and cooperation .
After more than one decade of intenseresearch efforts, the MANET research field pro-duced profound theoretical results (e.g., perfor-mance bounds on MANET performance [5, 6]),or innovative protocol and architectural solu-tions (e.g., innovative cross-layer architecturesand protocols as discussed in [7, Chapter 1]), butin terms of real world implementations andindustrial deployments, the pure general-pur-pose MANET paradigm suffers from scarceexploitation and low interest in the industry andamong users . Why has this happened? Aninitial answer to this question was provided in2007, in two companion articles that made a crit-ical analysis of the MANET research activities[8, 9] pointing out that the main reasons forMANETs missed expectations are due to thelack of: Implementation, integration, and experimenta-
tion Simulation credibility Socio-economic motivations
In addition, that analysis also highlightedthat the mesh network, vehicular network,opportunistic network, and sensor networkparadigms were or ig inat ing f rom theMANET research field. These multihop adhoc networking paradigms, by learning fromthe MANET experience, emerged with thepromise to avo id MANETs mis takes .Indeed, these new MANET-bornparadigms distanced themselves from themain weaknesses of the MANET by follow-ing a more pragmatic development approach.Currently, six years after that analysis, mesh,sensor , opportunist ic , and vehicular net-works are a reality in the mobile ad hoc net-work ing f ie ld , and the i r success can besummarized by the following pragmatic devel-opment strategy: 1 Application oriented development, which (as
opposed to the general-purpose design ofMANET) first identifies the applicationscenarios to be addressed before startingthe development of the technical solutions.
2 Complexity reduction. Depending on the spe-cific application scenario, some MANETconstraints have been relaxed; for example,the assumption that the network is com-posed only of users devices, and/or that thecommunication model has to comply withthe Internet one.
3 The focused research approach addressesonly the research topics relevant to buildingrobust and effective networks for support-ing the specific application scenario(s), andnot pretending to replace the Internet.
4 The use of realistic simulation models inorder to base the protocol development oncredible simulation studies.
5 The development of real network testbedswith the users involvement, in the earlystages of the design of these new paradigms,in order to put the users in the loop of thenetwork design and experimentation.
Figure 1. Single-hop ad hoc network.
Figure 2. The pure general-purpose MANET approach.
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In this article, we review the current status ofthe (mobile) multihop ad hoc networkingresearch. Specifically, we discuss the milestonesand challenges in mesh, sensor, opportunistic,and vehicular networks. We also discuss what isforeseen in the future of multihop ad hoc net-working. In particular, we discuss the people-centric revolution. Thanks to the increasingdiffusion of smartphones, the people-centricparadigm combines wireless communicationsand sensor networks with the daily life andbehaviors of people to build computing andcommunication solutions that are tightly coupledwith people.
We present and discuss mesh, sensor, oppor-tunistic, and vehicular networks, respectively.For each paradigm, we show how the pure andgeneral-purpose MANET paradigm (and therelated research results) has been turned into anetworking paradigm that has gained usersand market acceptance by exploiting thelessons learned from MANETs. For this rea-son, the presentation of each paradigm is orga-nized to highlight the main elements of thepragmatic development strategy: application-ori-ented development, problem complexity reduc-tion, focused research, use of realisticsimulation models, and development of realnetwork testbeds with the users involvement.Finally, we conclude by summarizing theparadigms, followed by the introduction of themulti-paradigm vision.
MESH NETWORKING: AN EFFECTIVELOW-COST EXTENSION OF THE
The design of the wireless mesh network (WMN)paradigm started from a well defined set ofapplication scenarios that is summarized in :providing a flexible and low cost extension ofthe Internet. The initial WMN prototypes weremainly driven by the initiatives of communitiesof users with individual volunteers setting upIEEE 802-11-based long-distance point-to-pointlinks among their houses (Fig. 3) to build a com-munity network and offer a variety of services totheir participants, ranging from file sharing andcommunity-wide voice over IP (VoIP) to Inter-net access through community owned WMN-to-Internet gateways. Nowadays, metropolitan-scaleWMNs are a reality in many modern urban areassupported by municipalities and governmentorganizations2 offering a wide range of servicesranging from security surveillance to intelligenttransportation services.
COMPLEXITY REDUCTIONStarting from the well defined application sce-nario, it was immediately possible to reduce theMANETs complexities. Specifically, the WMNparadigm introduces an architectural shift, withrespect to MANETs, by adopting a two-tier net-work architecture based on multihop communi-cations. A multihop wireless backbone is formedby dedicated (and often fixed) wireless meshrouters, which run a multihop routing strategy to
interconnect with each other, as illustrated inFig. 4. Some of the mesh routers act as gate-ways, providing the WMN with a direct connec-tion to the Internet and other wired/wirelessnetworks. Finally, to support seamless datatransport services for users devices (also calledmesh clients), mesh access points are connectedto the mesh routers to offer connectivity to meshclients.
Consequently, topology changes due to mobil-ity or energy issues do not influence traffic for-warding in WMNs as in MANETs, and theimpact of the mobility is restricted to the lasthop (i.e., the connection between the users andthe mesh access points).
FOCUSED RESEARCHFollowing the above architectural organization,the main challenges in WMNs have been subdi-vided into three main areas: Exploiting the wireless mesh routers to
build a robust network backbone intercon-necting all mesh routers, and possibly somegateways to/from the Internet
Defining a set of routing protocols that, byexploiting the network backbone, can iden-tify the best path(s) for traffic forwardinginside the WMN and to/from the Internet
Supporting the users mobility among meshaccess pointsAll areas have been the subjects of intense
research activities providing effective solutionsto these problems. In particular, a large body ofresearch focused on building a robust wirelessmesh backbone by exploiting advanced multi-radio, multi-channel, and multi-rate capabili-ties, using heterogeneous wireless technologiesand types of antennas . This means that aWMN environment is characterized by a richlink and path diversity, which provides anunprecedented opportunity to find paths thatcan satisfy the application requirements. Effec-tive channel assignment solutions have beendeveloped to construct robust and efficientwireless mesh backbones ([12, 13]). However,the multifaceted characteristics of a WMNcomplicate the path selection process. Forexample, the interference in WMNs is a verycomplex phenomenon, and the routing process
Figure 3. Community mesh networks.
2 See http://www.muni-wireless.com
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must be aware of the interference existingbetween links and traffic flows to take advan-tage of the multi-channel and multi-radio capa-bilities . In addition, in most applicationscenarios the portion of traffic a mesh routerdelivers to other routers in the network (i.e.,intra-mesh traffic) will be minimal with respectto the traffic conveyed over connections estab-lished with external hosts (i.e., inter-mesh traf-fic). As a result, most WMN traffic is usuallybetween mesh clients and mesh gateway(s).This implies that traffic gets aggregated at themesh gatewa...