Simon Heimlicher

Towards Realistic Mobility Models for Vehicular Ad-hoc Networks

In performance studies of vehicular ad hoc networks, the underlying mobility model plays a major role. In this paper, we investigate the influence of three mobility models on the performance of ad hoc network routing protocols (AODV and GPSR). As a benchmark, we use the popular random waypoint mobility model. Our second model is based on a vehicular traffic simulator that we proposed in earlier work. Finally, as third model, we propose a novel mobility model based on vectorized street maps and speed limit information extracted from a geographic information system. With the two considered routing protocols, the random waypoint mobility model tends to lead to substantially higher performance than with our own, presumably more realistic mobility models.

HEAT: Scalable Routing in Wireless Mesh Networks Using Temperature Fields

Existing unicast routing protocols are not suited well for wireless mesh networks as in such networks, most traffic flows between a large number of mobile nodes and a few access points with Internet connectivity. In this paper, we propose HEAT, an anycast routing protocol for this type of communication that is designed to scale to the network size and to be robust to node mobility. HEAT relies on a temperature field to route data packets towards the Internet gateways, as follows. Every node is assigned a temperature value, and packets are routed along increasing temperature values until they reach any of the Internet gateways, which are modeled as heat sources. Our major contribution is a distributed protocol to establish such temperature fields. The distinguishing feature of our protocol is that it does not require flooding of control messages. Rather, every node in the network determines its temperature considering only the temperature of its direct neighbors, which renders our protocol particularly scalable to the network size. We analyze our approach and compare its performance with OLSR through simulations with Glomosim. We use realistic mobility patterns extracted from geographical data of large Swiss cities. Our results clearly show the benefit of HEAT versus OLSR in terms of scalability to the number of nodes and robustness to node mobility. The packet delivery ratio with HEAT is more than two times higher than OLSR in large mobile scenarios and we conclude that HEAT is a suitable routing protocol for city-wide wireless mesh networks.

Routing Packets into Wireless Mesh Networks

Wireless mesh networks are a promising way to provide Internet access to fixed and mobile wireless devices. In mesh networks, traffic between mesh nodes and the Internet is routed over mesh gateways. On the forward path, i.e., from mesh nodes to Internet nodes, for all mesh nodes only route information for one destination, the gateways, needs to be maintained. However, on the backward path from the Internet to mesh nodes, an individual route for every mesh node is required. In this paper we investigate protocols for backward path routing in wireless mesh networks. Using simulation experiments with realistic mobility patterns of pedestrians and cars in cities, we compare three protocols, each of which represents a routing protocol family: (i) AODV with an extension for mesh networks, a reactive routing protocol, (ii) FBR, a proactive routing protocol, and (iii) GSR, a source routing protocol. Our results indicate that FBR has the highest packet delivery ratio but is not scalable to the network size. The extended AODV seems to be neither scalable nor does it achieve a high packet delivery ratio. A good compromise is provided by GSR, which is the most scalable to the network size and still achieves a high packet delivery ratio.

Routing in Large-Scale Wireless Mesh Networks Using Temperature Fields

Many wireless mesh networks are based on unicast routing protocols even though those protocols do not provide a particularly good fit for such scenarios. In this article, we report about an alternative routing paradigm, tailor-made for large multihop wireless mesh networks: field-based anycast routing. In particular, we present HEAT, a routing protocol based on this paradigm. In contrast to previous protocols, HEAT requires communication only between neighboring nodes. The underlying routing concept is a field similar to a temperature field in thermal physics. In extensive simulation experiments, we found that HEAT has excellent scalability properties due to a fully distributed implementation, and it provides much more robust routes than the unicast protocols, AODV and OLSR. As a consequence, in large-scale mobile scenarios, the packet delivery ratio with HEAT is more than two times higher, compared to AODV or OLSR. These promising results indicate that HEAT is suitable for large-scale wireless mesh networks that cover entire cities.

An empirical study of the impact of mobility on link failures in an 802.11 ad hoc network

A great deal of research has been done during the past few years in the area of wireless selforganizing networks. Generally, this research has been supported by either simulation or theoretical analysis, both relying on strong assumptions. However, a key point in coupling research and real-life applications is to understand how realworld conditions impact practical networking aspects. To gain more realistic insights, we deploy an indoor IEEE 802.11 mobile ad hoc network comprising 20 PDAs carried by volunteers for one week. In a subsequent analysis, we explore the impact of mobility and interference on the observed network behavior. A major finding of our analysis is that mobility is the most dominant cause of link failure for links with a long lifetime, whereas other causes (unrelated to mobility) are responsible for the breakage of links with short lifetimes. This inherent property could be used by network protocols in self-organizing networks to optimize link or route repair decisions depending on the age of a link at the time it fails.

Characterizing 802.11n aerial communication

In Search And Rescue missions, Unmanned Aerial Vehicles (UAVs) equipped with cameras allow for efficient scanning of large areas. Yet, delivering high resolution images to rescuers also requires high-speed communication. In this paper, we investigate the potential and challenges of wireless communication between UAVs in such scenarios. Our fleet of UAVs supports both a high-bandwidth network for bulk data transfer (802.11n) as well as a long-range radio for control messages (XBee-PRO 802.15.4). Extensive experiments show that 802.11n performs poorly in highly mobile scenarios, as the throughput between UAVs drops far below the theoretical maximum as soon as they become airborne. We consider several potential causes and present an analysis of their impact in isolation.

End-to-end vs. hop-by-hop transport under intermittent connectivity

This paper revisits the fundamental trade-off between end-to-end and hop-by-hop transport control. The end-to-end principle has been one of the building blocks of the Internet; but in real-world wireless scenarios, end-to-end connectivity is often intermittent, limiting the performance of end-to-end transport protocols. We use a stochastic model that captures both the availability ratio of links and the duration of link disruptions to represent intermittent connectivity. We compare the performance of end-to-end and hop-by-hop transport over an intermittently-connected path. End-to-end, perhaps surprisingly, may perform better than hop-by-hop transport under long disruption periods. We propose the spaced hop-by-hop policy which is found to dominate (in terms of delivery ratio) the end-to-end policy over the whole parameter range and the basic hop-by-hop policy over most of the relevant range.

On Leveraging Partial Paths in Partially-Connected Networks

Mobile wireless network research focuses on scenarios at the extremes of the network connectivity continuum where the probability of all nodes being connected is either close to unity, assuming connected paths between all nodes (mobile ad hoc networks), or it is close to zero, assuming no multi-hop paths exist at all (delay-tolerant networks). In this paper, we argue that a sizable fraction of networks lies between these extremes and is characterized by the existence of partial paths, i.e., multi-hop path segments that allow forwarding data closer to the destination even when no end-to-end path is available. A fundamental issue in such networks is dealing with disruptions of end-to-end paths. Under a stochastic model, we compare the performance of the established end-to-end retransmission (ignoring partial paths), against a forwarding mechanism that leverages partial paths to forward data closer to the destination even during disruption periods. Perhaps surprisingly, the alternative mechanism is not necessarily superior. However, under a stochastic monotonicity condition between current vs. future path length, which we demonstrate to hold in typical network models, we manage to prove superiority of the alternative mechanism in stochastic dominance terms. We believe that this study could serve as a foundation to design more efficient data transfer protocols for partially-connected networks, which could potentially help reducing the gap between applications that can be supported over disconnected networks and those requiring full connectivity.

Now or later?: delaying data transfer in time-critical aerial communication

Search and rescue missions are entering a new era with the advent of small scale unmanned aerial vehicles (UAVs) with communication capabilities and embedded cameras. Yet, delivering high resolution images of the supervised surface to rescuers is time-critical. To help resolving this problem, we study how UAVs can take advantage of their controlled mobility to derive the optimum strategy for data transmission. Driven by real-world aerial experiments with both airplanes and quadrocopters equipped with 802.11n technology, we show that the UAV should not necessarily transmit as soon as a wireless link is established. Instead, it should wait until it reaches a suitable distance to the receiving UAV, only to transmit when the time to move to the new location and transmit is minimal. We then apply the principle of delayed gratification, where the UAV attempts to solve the tradeoff between postponing until it reaches this minimum and the impatience to deliver as much data as soon as possible, before any physical damage on-the-fly may occur. Our empirical-driven simulations demonstrate that the optimal distance of transmission greatly depends on the interplay of actual throughput, data size, UAV cruise speed, and failure rate, and that state-of-the-art UAVs can already benefit from our approach.

End-to-end vs. hop-by-hop transport

The transport layer has been considered an end-to-end issue since the early days of the Internet in the 1980s [1], when the TCP/IP protocol suite was designed to connect networks of dedicated routers over wired links. However, over the last quarter of a century, network technology as well as the understanding of the Internet has changed, and todays wireless networks differ from the Internet in many aspects. Since wireless links are unreliable, it is often impossible to sustain an end-to-end connection to transmit data in wireless network scenarios. Even if an end-to-end path exists in the network topology for some fraction of the communication, it is likely to break due to signal propagation impairments, interference, or node mobility. Under these circumstances, the operation of an end-to-end transport protocol such as TCP may be severly affected.