mobility models for next generation wireless networks (ad hoc, vehicular and mesh networks) ||...

17Opportunistic Networks

In this chapter, we will briefly describe the main features of an emerging classof next generation wireless networks, namely, opportunistic networks. Afterpresenting the state of the art in opportunistic networking and related tech-nologies, we will describe representative use cases. Finally, we will presenta short overview of the main envisioned technological evolutions, and of thechallenges to be faced by opportunistic network designers.

17.1 Opportunistic Networks: State of the Art

An opportunistic network is a short-range wireless network characterized bya very sparse network topology – see Figure 17.1: if one takes a snapshot ofthe network at an arbitrary time instant, what is typically observed is a largefraction of isolated nodes, and a small fraction of nodes having active linkswith a few other nodes in the network. From a networking perspective, whatis lacking in the network topology is connectivity , that is, the possibility fora node in the network to establish a (possibly multi-hop) communicationpath with all the other nodes. However, network connectivity – or at least,as we will see, a weak form of connectivity – can be achieved by exploitingthe temporal dimension and node mobility: since nodes in an opportunisticnetwork move, isolated nodes can get in touch with other nodes as timegoes by. Similarly, a node A which is currently in touch with node B mightlater get in touch with another node C, and so on – see Figure 17.1. So, ifmessages circulating in the network are stored in the node buffers for a longenough time interval, it is indeed possible to establish multi-hop communica-tion paths between nodes in the network by exploiting these relatively seldomcommunication opportunities – whence the name of the network – achievingnetwork-wide communications.

Mobility Models for Next Generation Wireless Networks: Ad Hoc, Vehicular and Mesh Networks,First Edition. Paolo Santi. 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

218 Mobility Models for Next Generation Wireless Networks

wireless link

A B

C

time 0 time t

A

B

C

Figure 17.1 Typical opportunistic network architecture.

Since the communication mechanism described above – called store,carry, and forward , see Chapter 18 – relies on node mobility to physicallycarry around and forward messages within the network, and human/animal/vehicular mobility is several orders of magnitude slower than the speed ofradio signal propagation in the air, which is comparable to the speed oflight, it is clear that the delays in communicating a message from source todestination are much higher than those typical of other types of networks.Hence, applications running in opportunistic networks must be able to tol-erate very large delays – in the order of minutes or hours – which explainswhy opportunistic networks are also called delay-tolerant networks (DTNs).

Unlike the other types of next generation wireless networks considered sofar in this book, opportunistic networks are not related to a specific wire-less technology, but rather are a class of mobile networks characterized bythe above-described specific topological property of being extremely sparse.This means that, depending on the scenario, the radio technology used toestablish wireless links between nodes can be different: it can be a WLANtechnology such as WiFi, a PAN technology such as Bluetooth, or a vehic-ular or wireless sensor communication technology if some of the membersof the opportunistic network are vehicles and/or wireless sensor nodes. It isindeed the case that in some scenarios different technologies coexist, andare alternately used to establish peer-to-peer wireless links: the most typicalcase in this respect is that of a smart phone with both WiFi and a Bluetoothinterface and which can use either interface to establish a wireless link withanother cell phone.

One type of opportunistic network that has recently attracted a lot of atten-tion in the research community is the one which is dynamically formed byindividuals carrying advanced personal communication devices (PCDs) capa-ble of establishing direct wireless links, such as smart phones, PDAs, etc.

Opportunistic Networks 219

Table 17.1 Short-range wireless technologies for opportunistic networkcommunications

Name Domain Typical range (m) Max data rate (Mbps)

WiFi WLAN 30–250 600 (802.11n)Bluetooth PAN 10–100 24 (v. 3.0)ZigBee WSN 10–75 0.25DSRC Vehicular 150–1000 54

This type of opportunistic network is known as a pocket-switched network(PSN) in the literature (Hui et al. 2005), and is considered very interestingunder both the mobility analysis/characterization viewpoint and the potentialapplication viewpoint. In fact, since members of a PSN are people, char-acterization of human mobility patterns can be used to improve the PSNperformance. Furthermore, PSNs can potentially enable novel participatoryand social networking applications based on opportunistic communications,as described in the next section.

It is important to observe that, while most nodes in an opportunistic net-work are mobile, there might exist application scenarios in which some ofthe nodes in the network are fixed and act as data collection and/or messagerelay points. An important example of this class of opportunistic networks isDieselNet, an opportunistic network composed of 40 radio-equipped publicbuses in the city of Amherst, Massachusetts (Group 2007). In DieselNet,some of the bus stops are equipped with wireless collection points calledthrowboxes that act as message relays, thus speeding up the message prop-agation process in the network. Another example is that of a very sparsewireless sensor network composed of mobile sensor nodes (e.g., sensor nodesattached to animals), where fixed data gathering stations can be installed instrategic locations (e.g., at a water source).

From a technological viewpoint, some of the short-range wirelesstechnologies that can be used in opportunistic networks are mature (e.g.,WiFi and Bluetooth for PSN applications), some are close to fully ripening(e.g., ZigBee for WSN-related applications), while others are in an advanceddevelopment stage (e.g., IEEE 802.11p/DSRC (Dedicated Short-RangeCommunications) for vehicular-related applications). A summary of the mainfeatures of wireless technologies for opportunistic network communicationsis given in Table 17.1.

17.2 Opportunistic Networks: User Scenarios

In this section, we report some representative user scenarios for opportunisticnetworks in the domain of PSN, WSN, and vehicular network applications.


17.2.1 User Scenarios in PSNs

Suppose traveler Bob has just arrived at an airport in a foreign country, andhe would like to take a taxi to the Hotel Wonderland where he is staying. Ifpossible, Bob would like to share the ride with other travelers staying at thesame hotel, and thus to share the costs of the taxi ride. With PSNs, Bob coulddisseminate a message expressing his interest in going to the Hotel Wonder-land by taxi. Due to the very dense population of PCD-equipped individualstypically found in an airport – highlighting that, in some cases, a part of anopportunistic network can indeed be relatively dense – Bob’s expression ofinterest is likely to be disseminated quickly in the airport. So, in a few min-utes since issuing the message, Bob could get a reply from other travelersinterested in taking a taxi to the Hotel Wonderland, and who maybe are cur-rently still waiting to claim their baggage. Another possible reply to Bob’sexpression of interest message might be a notification from a local shuttleservice that a shared shuttle service to the Hotel Wonderland is available,together with directions on how to reach the shuttle stop. After getting thereplies, Bob could decide to wait for the other travelers, or inform them thathe was going instead to take the taxi by himself or the cheaper shuttle service.

Note that a smart transportation service like this one could be realizedalso through the Internet, for instance, by accessing a web server collectingtravelers’ posts about their transportation needs. However, this would requireusers to have Internet access enabled on their cell phones, which comes ata cost, especially considering that many subscription plans have rather highrates for accessing the Internet when roaming. If PSNs are used instead, noconnection to the Internet is required, and also relatively cheap cell phoneswith a Bluetooth interface and no Internet connection can be used to accessthe smart transportation service.

Another possible use of PSNs is in the aftermath of disasters. Sup-pose an earthquake hit the city of Futuria, and the communicationinfrastructure – including the cellular network – is compromised. PSNscan be used during the hours/days needed to restore the communicationinfrastructure to enable exchange of information between survivors, membersof the rescue teams, etc. For instance, a person living in Woodland Streetwho survived the earthquake could take some pictures with her cell phoneto show the situation concerning the buildings and roads in the vicinity, anddisseminate this information on the PSN. In a few minutes, the informationcould be delivered to a member of a rescue team, who would then share thisinformation with other members of the team and local authorities to gaina better understanding of the situation. Alternatively, another person livingin Woodland Street currently located in Central Avenue, where he works,might disseminate on the PSN an enquiry for any information relating to thesituation in Woodland Street. After a few minutes, he might start to receive


text messages and pictures reporting the situation in Woodland Street, fromwhich he can see that his house survived the earthquake.

17.2.2 User Scenarios in WSNs

Suppose a WSN is used to monitor the health status and movement of wildanimals. A small fraction of the monitored animals are equipped with radios.Furthermore, a set of fixed base stations with satellite Internet connection islocated in selected feeding grounds and near water sources. When two radio-equipped animals move into each other’s transmission range, they exchangesummaries of their movement/health status in the last time period. This way,the data generated by an animal can reach the remote user monitoring theanimals through the Internet in two ways: directly, when the tracked animalvisits one of the regions covered by a base station; or through a multi-hoppath, if the animal’s movement/health status summary is delivered to a basestation by another, recently met animal.

Note that this scenario, similar, for example, to that of the well-knownZebraNet experiment (Martonosi et al. 2004), is well within the realm ofopportunistic networks when the density of radio-equipped animals is rela-tively low, and eventual communication of tracked data to the base stationsis achieved through exploitation of animal movements.

17.2.3 User Scenarios in Vehicular Networks

Consider an advanced urban traffic monitoring system whose purpose is togive local authorities and vehicle drivers a real-time picture of the currenttraffic status in a city. Traffic status is tracked in real time both throughfixed cameras at selected points and through traffic reports generated bythe vehicles themselves. In particular, a set of traffic checkpoints is definedwithin the road network by the traffic authorities, and communicated to thetraveling vehicles through the communication infrastructure – composed, forinstance, of a set of sparsely deployed IEEE 802.11p RSUs. When traveling,vehicles automatically detect whether they have just passed a checkpointand, if so, they compute the time elapsed since the passing time at theprevious checkpoint. The computed traveling time, together with the ID of thetwo checkpoints, constitutes a traffic report, which is delivered to the trafficmonitoring service through the communication infrastructure, for example,when the vehicle gets in touch with one of the RSUs. If the density of RSUsis not too sparse, or in case multi-hop forwarding of traffic reports is allowed,traffic reports reach the traffic monitoring service center in a few seconds,thus enabling a real-time assessment of the situation. A traffic monitoringservice similar to the one described above has been recently proposed in theIPERMOB project (Consortium 2010).


The previously described scenario, at least the part concerning com-munication of traffic reports to the monitoring service center, displays allthe features of opportunistic networks: a network composed of relativelysparse mobile nodes (we can expect that DSRC-equipped vehicles will be aminority of all circulating vehicles for several years to come), complementedwith a few fixed collection points (RSUs); data generated by the vehicles iseventually delivered to the monitoring service center through a RSU, witha possibly faster data delivery process in case vehicle-to-vehicle forwardingof traffic reports is realized.

17.3 Opportunistic Networks: Perspectives

While opportunistic networks are considered a very promising type of nextgeneration wireless network, technological and network design challenges arestill to be satisfactorily addressed before these networks can be considered afeasible and practical solution.

In terms of technology, we are currently in a situation where some of theshort-range wireless technologies exploited for opportunistic communicationsare mature, while others – especially in vehicular applications – are still tomature. However, where the radio technology is mature, the problem ofdesigning energy-efficient solutions is still to be faced. Curiously, in thosesituations where energy consumption is not an issue, such as in vehicularapplications, the communication technology is relatively less mature.

For instance, considering PSNs, it is well known that the energy drain ofa smart phone when the WiFi or Bluetooth interfaces are active is very high,leading to a very short battery life if these interfaces are continuously keptactive. Thus, applications for opportunistic networks should be designed tak-ing into account that short-range radio interfaces are not necessarily active,possibly leading to missed communication opportunity detection. This meansthat energy efficiency should be carefully traded off against a lower likelihoodof detecting communication opportunities, which are the fundamental build-ing blocks of any opportunistic networking protocol. Another possible wayof addressing the problem of energy efficiency is through improved hardwaredesign of the short-range radio interfaces, possibly integrating them betterwith the other interfaces present in the smart phone.

It is important to observe that efficient use of the battery is a prerequisitefor the success of opportunistic network-based applications: a user is likelyto be willing to use these applications if the perceived “added value” theyprovide is not overwhelmed by the increase in energy consumption; if runningan opportunistic network application – even a very exciting one – drains thebattery in a few hours, it is likely that most users will end up not using it.

Another major challenge facing the opportunistic network designer isrelated to the design and realization of networking protocols explicitly


designed for opportunistic networks. So far, most research efforts havebeen devoted to optimizing the performance of well-known networkingprimitives such as unicast, broadcast, multicast, and so on, in opportunisticnetworks. However, these primitives appear to be unsuitable for an effectivedesign of opportunistic network applications such as those described inthe previous section. Consider for instance the airport scenario. Whatnetworking primitive should be used to propagate Bob’s message within theairport PSN? Unicast is not suitable, since there is no specific destinationthat Bob is aware of to which the message should be sent. Similarly forbroadcast, since it is not even clear who the members of the airport PSN areat a given time; furthermore, most of the airport PSN members are likelynot to be interested in Bob’s message. Multicast could be a reasonablechoice, but it is quite unlikely that a multicast group composed of “all theusers staying at the Hotel Wonderland” could be distributively built andefficiently maintained in such a dynamic setting.

The example above highlights the need to design radically differentnetworking primitives for opportunistic networks, which are designed, forinstance, to deliver a message to all users sharing similar interests (staying atthe Hotel Wonderland in the airport example) or to all users of a community(those living on Woodland Street in the earthquake example), and so on.

Finally, security and privacy issues are still to be addressed, especiallyin PSNs. Most existing opportunistic networking solutions exploit informa-tion about a user’s mobility pattern, social ties, and so on, to optimize thespreading of messages and information within the network. While these solu-tions achieve their goal in terms of improved network performance, it isvery unlikely that users will be willing to expose such sensitive informa-tion to potential strangers to run opportunistic network applications. Hence,techniques should be designed to enable a secure and privacy-preservingspreading of information within opportunistic networks.

17.4 Further Reading

This chapter is just a short introduction to opportunistic networks and relatedtechnologies. The reader interested in gaining a better understanding of thistopic is referred to the books and surveys available in the literature, such asZhang (2006), Farrell and Cahill (2006), Harras (n.d.), and Denko (2008).

ReferencesConsortium I 2010 http://www.ipermob.org .Denko M 2008 Mobile Opportunistic Networks: Architectures, Protocols and Applica-

tions . Auerbach Publications, Boca Raton, FL.Farrell S and Cahill V 2006 Delay- and Disruption-Tolerant Networking . Artech House,

London.


Group PR 2007 http://prisms.cs.umass.edu/dome/umassdieselnet .Harras K n.d. Challenged Networks: Protocol and Architectural Challenges in Delay

and Disruption Tolerant Networks . VDM, Saarbr.Hui P, Chaintreau A, Scott J, Gass R, Crowcroft J and Diot C 2005 Pocket-switched

networks and human mobility in conference environments. Proceeding of the ACMWorkshop on Delay-Tolerant Networks .

Martonosi M, Lyon S, Peh LS, Poor V, Rubenstein D, Sadler C, Juang P, Liu T,Wang Y and Zhang P 2004 http://www.princeton.edu/mrm/zebranet.html .

Zhang Z 2006 Routing in intermittently connected mobile ad hoc networks and delaytolerant networks: Overview and challenges. IEEE Communications Surveys & Tuto-rials 8, 24–37.

mobility models for next generation wireless networks (ad hoc, vehicular and mesh networks) ||...

Documents