An improved virtualization layer to support distribution of multimedia contents in pervasive social applications
Introduction
The steady advances in the realm of pervasive computing favored by the ever-growing popularity of smartphones, together with the new interaction patterns propelled by the social web, pave the way to a new era of information services tailored to the people׳s physical and social context (Drego et al., 2007, Zhou et al., 2012). Researchers in the area of pervasive social computing envisage many opportunities to enable meaningful interactions among groups of nearby people —either acquaintances or strangers— to help them make the most of their environment, ranging from the orchestration of activities in venues or events that attract people with potentially-related interests (e.g. museums, concert halls or campsites) to advances in the vision of smart cities, related to the planning of personal mobility or the celebration of location-based urban games (Schuster et al., 2013). As explained in Ben Mokhtar and Capra (2009), while pervasive computing aimed to facilitate the realization of users׳ daily tasks by changing the way they individually interact with the physical environment, pervasive social computing aims to facilitate the realization of those tasks that need to consider social relations between users. While the former placed a strong emphasis on the self component, the latter emphasises the social component. While web 2.0 websites enabled online interactions, pervasive social computing aims to change the way individuals interact with co-located people.
From the technological point of view, realizing the concept of pervasive social computing requires means to support the shared experiences by harnessing the communication and computing capabilities of the connected devices as an integrated whole. At the lowest level, due to physical proximity, the devices should be able to exchange data packets directly or within few hops in an ad-hoc network—rather than sending them out to remote Internet servers that would merely echo the same packets downlink—and harness the strengths of peer-to-peer (P2P), opportunistic networking (Sun, 2001, Conti and Giordano, 2013). The ad-hoc networks should serve as a channel to distribute a wealth of multmedia contents generated locally or downloaded from the Internet through any WiFi, DSRC, WiMAX, 3G or LTE connections available to the mobile devices. In doing so, it is necessary to deal with the challenges raised by the properties of the wireless medium, the varying topologies that emerge from the users׳ movements, and the fact that the devices may have limited memory, computational power and battery.
In this paper, we introduce an approach to support pervasive social applications by turning the mobile ad-hoc networks (MANETs) into more stable communication environments. Our proposal is based on an evolution of the virtualization layer presented in Dolev et al. (2004) and Brown et al. (2007) (called the Virtual Node Layer, VNLayer), which put forward procedures to create an infrastructure of stationary virtual nodes to ease the routing problem and the maintenance of persistent state information in the area covered by an ad-hoc network, notwithstanding the movements of the real nodes. Brown et al. (2007) discussed the advantages of the programming abstraction enabled by the VNLayer, whereas Wu et al. (2011) and Patil and Shah (2012) separately proved that a virtualized version of the AODV routing algorithm (Perkins et al., 2003) (called VNAODV) can outperform AODV itself in terms of route stability and packet delivery ratios. Later on, Wu proved the advantages of virtualization for another routing algorithm (RIP, Malkin, 2000) and ancillary protocols like DHCP (Wu, 2011). Here, we present a number of refinements to both the VNLayer and VNAODV, leading to new versions we have called VNLayer+ and VNAODV+. By means of simulation experiments and measurements on a real deployment, we prove that this solution achieves better performance than others in supporting a pervasive social application for History-related museums called REENACT (previously described in López-Nores et al., 2013a, Blanco-Fernández et al., 2014). Specifically, we have compared the performance achieved by the REENACT app in the distribution of multimedia contents with the five routing solutions shown in Fig. 1: plain AODV, VNAODV on top of the VNLayer, VNAODV+ on top of the VNLayer+, OLSR (Clausen et al., 2003) and ARA (Guenes et al., 2002). AODV, OLSR and ARA are the most representative examples of routing algorithms used for P2P distribution of contents in MANETs, along with DSDV (Perkins and Bhagwat, 1994) (an antecedent of OLSR) and Bee (Wedde and Farooq, 2005) (conceptually similar to ARA) (Gurumurthy, 2009, Tang et al., 2005, Sbai et al., 2010, Hwang and Hoh, 2009, Dhurandher et al., 2009, Castro et al., 2010, Barbeau, 2012, Wang et al., 2014).
The paper is organized as follows. First of all, Section 2 includes an overview of routing protocols for MANETs. Then, we present the main procedures of the VNLayer (Section 3) and details of how AODV was adapted to work on top of it (Section 4). The refinements we have included in the VNLayer+ and in VNAODV+ are presented in Sections 5 and 6, respectively. Section 7 presents the REENACT application, the scenarios of our measurements and simulations and the comparison results. Finally, conclusions are given in Section 8 along with the motivation of our ongoing work.
Section snippets
Overview of MANET routing protocols
During the last fifteen years, the wireless networking community has come up with a large number of routing protocols for ad-hoc networks of mobile devices, going from early proposals that would fit a wide range of scenarios to more recent algorithms focused on specific constraints (in terms of mobility parameters, energy limitations, knowledge of the physical location of the nodes, etc). Accordingly, whereas the first surveys would only consider centralized/distributed and
Background on the VNLayer
The VNLayer defines procedures for mobile devices to collaboratively emulate an infrastructure of stationary virtual nodes (VNs) that can be addressed as static routing devices and maintain persistent state information. To this aim, it divides the geographical area of an ad-hoc network into square regions, whose size is chosen so that every physical node (PN) in a region can send and receive data, at least, from every other physical node in that region and in the neighboring ones. The virtual
From AODV to VNAODV
As explained in Section 2, AODV is a reactive routing protocol, which means that communication routes are created only when there are data packets to deliver. Its operation can be summed up as follows:
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Route discovery: When a node needs to forward a data packet but it does not know a route to the intended destination, it buffers the packet and broadcasts a Route Request message (RREQ). The neighbors in turn broadcast the RREQ to their neighbors till it reaches the destination node or a node that
VNLayer+: enhancements to the VNLayer
To start working with the VNLayer and VNAODV, we replicated their implementations as per the extensive details included in Wu (2011), and then checked the validity of the software by replicating some of the simulation experiments reported in Wu et al. (2011) and Patil and Shah (2012). The results were practically identical to those already published, so we moved on to using VNAODV in different scenarios. In doing so, we identified the following sources of inefficiency in the VNLayer constructs:
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VNAODV+: enhancements to VNAODV
During the implementation of VNAODV as per the details given in Wu (2011), we identified several potential inefficiencies that were later confirmed by our simulations experiments. These shortcomings at the network layer pile up on the limitations of the VNLayer discussed in Section 5. In this regard, it was already noticed in Wu (2011) that any failures at the virtualization layer to preserve state information entail a risk of forwarding packets into dead-ends or forming routing loops, which is
Experiments with the REENACT application
REENACT is a pedagogical approach aimed at engaging groups of museum visitors into immersive collective experiences to improve their understanding of historical battles and wars. This proposal brings together different elements used during the last decade to improve the pedagogy of History through technology, including smartphones and tablets (Loidl, 2006, Akkerman et al., 2009), videogames for learning (Charsky and Ressler, 2011, Froschauer et al., 2012), location-based and virtual reality
Conclusions and future work
Judging from the results of the preceding section, we can confirm that the evolutions brought by the VNLayer+ and VNAODV+ with regard to the VNLayer and VNAODV do serve to support the distribution of multimedia contents within groups of mobile devices spanning indoor and outdoor environments, in a representative application of pervasive social computing. On one hand, the VNLayer+ turns the ad-hoc networks into more responsive and reliable communication environments than possible with the
Acknowledgments
This work has been supported by the European Regional Development Fund (ERDF) and the Galician Regional Government under agreement for funding the Atlantic Research Center for Information and Communication Technologies (AtlantTIC), by the European Union Seventh Framework Programme (FP7/2007–2013) under grant Agreement no. 287966 (EXPERIMEDIA project), and by the Ministerio de Educación y Ciencia (Gobierno de España) research project TIN2013-42774-R (partly financed with FEDER funds).
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