Current capabilities of mobile handheld devices introduce them to new roles that were not possible less than a few years ago. In this paper, we look at personal mobile devices as an extension to the wireless infrastructure in environments that lack complete infrastructure coverage. We discuss various roles that these devices can play, starting with setting the devices as Internet relays that provide access to other devices not in range of the infrastructure access points. We then investigate server-like functionalities, in which the devices act as communication proxies or information caches in subnets disconnected from the rest of the network. We describe in detail the system architecture and an experimental implementation of these functionalities focusing on the practical issues of building the collective system.

Group recommendation, which makes recommendations to a group of users instead of individuals, has become increasingly important in both the workspace and people’s social activities, such as brainstorming sessions for coworkers and social TV for family members or friends. Group recommendation is a challenging problem due to the dynamics of group memberships and diversity of group members. Previous work focused mainly on the content interests of group members and ignored the social characteristics within a group, resulting in suboptimal group recommendation performance. In this work, we propose a group recommendation method that utilizes both social and content interests of group members. We study the key characteristics of groups and propose (1) a group consensus function that captures the social, expertise, and interest dissimilarity among multiple group members; and (2) a generic framework that automatically analyzes group characteristics and constructs the corresponding group consensus function. Detailed user studies of diverse groups demonstrate the effectiveness of the proposed techniques, and the importance of incorporating both social and content interests in group recommender systems.

Abstract Communication and information dissemination in times of disasters could have a critical role in aiding the emergency personnel and saving lives. Unfortunately, regular communication becomes more challenging at these times, as the infrastructure might be damaged or overloaded, and the requirements for communication (e.g., response time, capacity) becomes highly critical. Ad‐hoc networks forming between wireless devices have always been considered as a promising technology at disaster recovery situations. In this paper, we examine the need for efficient and robust information dissemination mechanisms on top of infrastructure‐less ad‐hoc networks. We argue that distributed rendezvous‐based approaches are more promising for emergency data dissemination than approaches based on network flooding or centralized storage. We present the design of a geographic rendezvous‐based architecture and evaluate its performance in detail against other variants and traditional approaches. The study shows its efficiency, high success rate in delivering data, and its robustness to failures and mobility; characteristics highly valuable at emergency situations. Copyright © 2008 John Wiley & Sons, Ltd.

In this work, the authors design and prototype PassItOn, a fully distributed opportunistic messaging system. Our goal is to build up a proof-of-concept platform on real mobile devices, and thus show the feasibility and potentials of utilizing human movements for dissemination applications. Meanwhile, we seek to shed lights on the design, implementation and deployment issues in building such systems, and thus stimulate new ideas and perspectives on addressing these issues. Moreover, we aim to offer a real testbed on which new mechanisms, protocols and use cases can be tested and evaluated.

The recent years have witnessed an explosive amount of user-generated contents, along with users' soaring demands on sharing these contents. An evidence of that is the blossom of hundreds of sharing sites on the web, among which the most famous are MySpace [1], Flickr [2], YouTube [3], Facebook [4] and LinkedIn [5]. Through these sites, users upload the contents they generate (blogs, pictures, videos, resumes, etc.), and make them available to their friends or the general public. A common characteristic of such Internet-based content sharing scenarios is that all contents are hosted by third-party servers on the Internet, and sharing happens when content consumers (regardless of where they are) access the hosting web sites.

We present an analytical, epidemiological, model for the overhead of information dissemination in mobile networks, which finds application in a range of settings, including resource discovery, information dissemination in sensor networks, location services and ad hoc network routing. We consider two principal information dissemination mechanisms, one for which nodes strictly relay information for other nodes, and the other for which they also take advantage of the information already being disseminated to insert their own data to share. The latter mechanism is denoted by path accumulation. We will show that the overhead with path accumulation scales as n3/2and without path accumulation as n5/4. With respect to the connection rate, both scale as radic(lambda). We also identify the cutoff point for path accumulation as xic= 1 - radic(2/radic(n)). Below that threshold, path accumulation should be preferred, while above, path accumulation does not provide substantial benefits. Our model proves extremely accurate against simulation data.

We study the overhead of some mechanisms for information dissemination. We use reactive routing protocols in mobile ad hoc networks as our illustrative example. However, while reactive routing protocols are the main application for our results, they apply to different settings as well, which include publish/subscribe information dissemination mechanisms, information dissemination in sensor networks, and location services. We use a performance measure, defined in [8], which we call the efficiency xi of the information dissemination process, and use this efficiency to evaluate the overhead. The overhead is influenced by the use of a mechanism to increase the amount of information disseminated, at the cost of a higher overhead, called path accumulation. We compare the overhead of routing protocols with and without path accumulation. We observe that path accumulation is always more efficient at disseminating route information in the network, but the gain in efficiency decreases when it is normalized by the number of extra routes discovered, as more is known about the network. This means that the marginal return of path accumulation decreases over not using path accumulation. We propose a new route discovery mechanism which applies to most of the reactive protocols using a route request/route reply exchange. Our route discovery mechanism, denoted adaptive path accumulation adjusts the accumulated path in order to either: scale its bandwidth overhead down as more information is distributed in the network, or improve the network discovery by carefully selecting the routes to be propagated. We assess a range of adaptive path accumulation policies using numerical evaluation and show the effect of these policies on route discovery and packet overhead.

Recent experimental studies have shown that wireless links in real sensor networks can be extremely unreliable, deviating to a large extent from the idealized perfect-reception-within-range models used in common network simulation tools. Previously proposed geographic routing protocols commonly employ a maximum-distance greedy forwarding technique that works well in ideal conditions. However, such a forwarding technique performs poorly in realistic conditions as it tends to forward packets on lossy links. Based on a recently developed link loss model, we study the performance of a wide array of forwarding strategies, via analysis, extensive simulations and a set of experiments on motes. We find that the product of the packet reception rate and the distance improvement towards destination ( PRR × d ) is a highly suitable metric for geographic forwarding in realistic environments.

Despite the large amount of research in ad hoc networks, there are still several limitations on the scalability and performance of routing protocols, which require a more fundamental understanding of some of the basic mechanisms. In this paper, we focus on one of these mechanisms, which is route discovery, and analyze its extent and overhead in discovering routes to nodes in the network. Gwalani et al. (2003) has augmented route discovery in AODV with a mechanism called path accumulation, that allows additional routes to be propagated through the network. We provide detailed analysis and simulations for the potential benefits of path accumulation and apply it on one of the prominent ad hoc routing protocols, AODV. The results show that path accumulation can improve the performance of AODV and reduce its overhead significantly.

Summary form only given. In wireless ad hoc and sensor networks, building efficient and scalable protocols is a very challenging task due to the lack of infrastructure and the high dynamics. Geographic protocols, that take advantage of the location information of nodes, are very valuable in these environments. The state required to be maintained is minimum and the discovery overhead is low, in addition to the fast response to dynamics. In this paper, we present a state-of-the-art overview of geographic protocols providing basic functions such as geographic routing, geocasting, and geographic-based rendezvous mechanisms. We introduce also some of our work on assessing and improving the robustness of geographic protocols to nonideal realistic conditions corresponding to the real-world environments.

Summary form only given. In large-scale wireless networks such as mobile ad hoc and sensor networks, efficient and robust service discovery and data-access mechanisms are both essential and challenging. Rendezvous-based mechanisms provide a valuable solution for provisioning a wide range of services. We describe rendezvous regions (RRs) - a novel scalable rendezvous-based architecture for wireless networks. RR is a general architecture proposed for service location and bootstrapping in ad hoc networks, in addition to data-centric storage, configuration, and task assignment in sensor networks. In RR the network topology is divided into geographical regions, where each region is responsible for a set of keys representing the services or data of interest. Each key is mapped to a region based on a hash-table-like mapping scheme. A few elected nodes inside each region are responsible for maintaining the mapped information. The service or data provider stores the information in the corresponding region and the seekers retrieve it from there. We run extensive detailed simulations, and high-level simulations and analysis, to investigate the design space, and study the architecture in various environments including node mobility and failures. We evaluate it against other approaches to identify its merits and limitations. The results show high success rate and low overhead even with dynamics. RR scales to large number of nodes and is highly robust and efficient to node failures. It is also robust to node mobility and location inaccuracy with a significant advantage over point-based rendezvous mechanisms.

In the absence of location errors, geographic routing - using a combination of greedy forwarding and face routing - has been shown to work correctly and efficiently. The effects of location errors on geographic routing have not been studied before. In this work we provide a detailed analysis of the effects of location errors on the correctness and performance of geographic routing in static sensor networks. First, we perform a micro-level behavioral analysis to identify the possible protocol error scenarios and their conditions and bounds. Then, we present results from an extensive simulation study of GPSR and GHT to quantify the performance degradation due to location errors. Our results show that even small location errors (of 10% of the radio range or less) can in fact lead to incorrect (non-recoverable) geographic routing with noticeable performance degradation. We then introduce a simple modification for face routing that eliminates probable errors and leads to near perfect performance.

Recent experimental studies have shown that wireless links in real sensor networks can be extremely unreliable, deviating to a large extent from the idealized perfect-reception-within-range models used in common network simulation tools. Previously proposed geographic routing protocols commonly employ a maximum-distance greedy forwarding technique that works well in ideal conditions. However, such a forwarding technique performs poorly in realistic conditions as it tends to forward packets on lossy links. We identify and illustrate this weak-link problem and the related distance-hop trade-off, whereby energy efficient geographic forwarding must strike a balance between shorter, high-quality links, and longer lossy links. The study is done for scenarios with and without automatic repeat request (ARQ).

Geocasting is the delivery of packets to nodes within a certain geographic area. For many applications in wireless networks, geocasting is an important and frequent communication service. The challenging problem in geocasting is distributing the packets to all the nodes within the geocast region with high probability but with low overhead. According to our study we notice a clear tradeoff between the proportion of nodes in the geocast region that receive the packet and the overhead incurred by the geocast packet especially at low densities and irregular distributions. We present two novel protocols for geocasting that achieve high delivery rate and low overhead by utilizing the local location information of nodes to combine geographic routing mechanisms with region flooding. We show that the first protocol (GFG) has close-to-minimum overhead in dense networks and that the second protocol (GFPG) provides guaranteed delivery without global flooding or global network information even at low densities and with the existence of region gaps. A practical version of the second protocol (GFPG) has the desirable property of perfect delivery at all densities and close-to-minimum overhead at high densities. We evaluate our mechanisms and compare them using simulation to the currently proposed geocasting mechanisms. The results show the significant improvement in delivery rate (up to 63% higher delivery percentage in low density networks) and reduction in overhead (up to 80% reduction) achieved by our mechanisms.

Congestion control is a major requirement for multicast to be deployed in the current Internet. Due to the complexity and conflicting tradeoffs, the design and testing of multicast congestion control protocols is difficult. In this paper, we present a novel framework for systematic testing of multicast congestion control protocols. In our framework, we first design an appropriate model for the studied protocols based on the protocols specifications and correctness conditions, and then we develop an automated search engine to generate all possible error scenarios and filter these errors to come up with a selected set of scenarios that we evaluate in more detailed simulations. Our methodology helps in identifying the potential problems of the studied protocols and points to possible improvements. We hope that this will provide a valuable tool to expedite the development and standardization of such protocols.

No abstract available.

Fairness to current Internet traffic, particularly TCP, is an important requirement for new protocols in order to be safely deployed in the Internet. This specifically applies to multicast protocols that should be deployed with great care. In this paper we provide a set of experiments that can be used as a benchmark to evaluate the fairness of multicast congestion control mechanisms when running with competing TCP flows. We carefully select the experiments in such a way to target specific congestion control mechanisms and to reveal the differences between TCP and the proposed multicasting protocol. This enables us to have a better understanding oft he proposed protocol behavior and to evaluate its fairness and when violations can happen. To clarify our experiments we carry them on a single-rate case study protocol, pgmcc, using NS-2 simulations. Our analysis shows the strengths and potential problems of the protocol and point to possible improvements. Several congestion control mechanisms are targeted by the experiments such as timeouts, response to ACKs and losses, independent and congestion losses effect. In addition, we evaluate multicast mechanisms such as the effect of multiple receivers, group representative selection, and feedback suppression when there is network support.