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, oppo