We will review some of the major results in random graphs and some of the more challenging open problems. We will cover algorithmic and structural questions. We will touch on newer models, including those related to the WWW.

Random graphs

Intermittently connected mobile networks are sparse wireless networks where most of the time there does not exist a complete path from the source to the destination. These networks fall into the general category of Delay Tolerant Networks. There are many real networks that follow this paradigm, for example, wildlife tracking sensor networks, military networks, inter-planetary networks, etc. In this context, conventional routing schemes would fail.To deal with such networks researchers have suggested to use flooding-based routing schemes. While flooding-based schemes have a high probability of delivery, they waste a lot of energy and suffer from severe contention, which can significantly degrade their performance. Furthermore, proposed efforts to significantly reduce the overhead of flooding-based schemes have often be plagued by large delays. With this in mind, we introduce a new routing scheme, called Spray and Wait, that "sprays" a number of copies into the network, and then "waits" till one of these nodes meets the destination.Using theory and simulations we show that Spray and Wait outperforms all existing schemes with respect to both average message delivery delay and number of transmissions per message delivered; its overall performance is close to the optimal scheme. Furthermore, it is highly scalable retaining good performance under a large range of scenarios, unlike other schemes. Finally, it is simple to implement and to optimize in order to achieve given performance goals in practice.

/pdf/spray-and-wait-an-efficient-routing-scheme-for-50seq3wqos.pdf

Spray and wait: an efficient routing scheme for intermittently connected mobile networks

Wireless vehicular communication has the potential to enable a host of new applications, the most important of which are a class of safety applications that can prevent collisions and save thousands of lives. The automotive industry is working to develop the dedicated short-range communication (DSRC) technology, for use in vehicle-to-vehicle and vehicle-to-roadside communication. The effectiveness of this technology is highly dependent on cooperative standards for interoperability. This paper explains the content and status of the DSRC standards being developed for deployment in the United States. Included in the discussion are the IEEE 802.11p amendment for wireless access in vehicular environments (WAVE), the IEEE 1609.2, 1609.3, and 1609.4 standards for Security, Network Services and Multi-Channel Operation, the SAE J2735 Message Set Dictionary, and the emerging SAE J2945.1 Communication Minimum Performance Requirements standard. The paper shows how these standards fit together to provide a comprehensive solution for DSRC. Most of the key standards are either recently published or expected to be completed in the coming year. A reader will gain a thorough understanding of DSRC technology for vehicular communication, including insights into why specific technical solutions are being adopted, and key challenges remaining for successful DSRC deployment. The U.S. Department of Transportation is planning to decide in 2013 whether to require DSRC equipment in new vehicles.

Dedicated Short-Range Communications (DSRC) Standards in the United States

In this paper, we define and explore proofs of retrievability (PORs). A POR scheme enables an archive or back-up service (prover) to produce a concise proof that a user (verifier) can retrieve a target file F, that is, that the archive retains and reliably transmits file data sufficient for the user to recover F in its entirety.A POR may be viewed as a kind of cryptographic proof of knowledge (POK), but one specially designed to handle a large file (or bitstring) F. We explore POR protocols here in which the communication costs, number of memory accesses for the prover, and storage requirements of the user (verifier) are small parameters essentially independent of the length of F. In addition to proposing new, practical POR constructions, we explore implementation considerations and optimizations that bear on previously explored, related schemes.In a POR, unlike a POK, neither the prover nor the verifier need actually have knowledge of F. PORs give rise to a new and unusual security definition whose formulation is another contribution of our work.We view PORs as an important tool for semi-trusted online archives. Existing cryptographic techniques help users ensure the privacy and integrity of files they retrieve. It is also natural, however, for users to want to verify that archives do not delete or modify files prior to retrieval. The goal of a POR is to accomplish these checks without users having to download the files themselves. A POR can also provide quality-of-service guarantees, i.e., show that a file is retrievable within a certain time bound.

PORs: Proofs of Retrievability for Large Files

Vehicular networks are very likely to be deployed in the coming years and thus become the most relevant form of mobile ad hoc networks. In this paper, we address the security of these networks. We provide a detailed threat analysis and devise an appropriate security architecture. We also describe some major design decisions still to be made, which in some cases have more than mere technical implications. We provide a set of security protocols, we show that they protect privacy and we analyze their robustness and efficiency.

/pdf/securing-vehicular-ad-hoc-networks-4sgegi8hgk.pdf

Securing vehicular ad hoc networks

When performing wireless network simulations, the lack of precise channel modeling in simulator frameworks becomes a serious problem. Often deterministic models are used for packet propagation, which describe real conditions insufficiently. To close this gap we extended the OMNeT++ Mobility Framework to support probabilistic propagation models. We provide an implementation for the Log-Normal-Shadowing, Nakagami, Rayleigh and Rice wave propagation models and set up a framework that allows easy integration of additional models in future.

Due to the characteristics of probabilistic radio models a fixed maximum packet propagation range encounters the problem of inaccurate simulation results as relevant events may be suppressed. On the other hand, unlimited packet propagation, which guarantees for correct simulation runs, causes unnecessary simulation overhead. In this work we present an approach to limit the event delivery to the area where the probability that the event is relevant to the simulation exceeds an adjustable threshold. In order to validate our extensions we successfully performed a detailed crosscheck with the network simulator NS-2 and run a performance evaluation and comparison.

/pdf/introducing-probabilistic-radio-propagation-models-in-omnet-581jyxcbks.pdf

Introducing probabilistic radio propagation models in OMNeT++ mobility framework and cross validation check with NS-2

In ad hoc networks, autonomous nodes collaborate to route information through the network. Commonly, nodes are end-systems and routers at the same time. In cases in which nodes have a notion of their geographic position, position-based routing protocols can be used; these protocols' properties of statelessness and fast adaptability to changes in the topology match very well with the characteristics of ad hoc networks.Position-based routing does not require the maintenance of routes; instead, forwarding decisions are made locally, based only on the node's own position, the positions of its neighbors, and the position of the destination. The routing algorithms are complemented by location services through which a node can obtain the position of a packet's destination.We will introduce the main components of position-based routing, discuss position-based routing algorithms as well as location services, and present an application scenario where position-based routing can be used for intervehicle communication.

Position-based routing in ad hoc wireless networks

Existing position-based unicast routing algorithms, where packets are forwarded in the geographic direction of the destination, require that the forwarding node knows the positions of all neighbors in its transmission range. This information on direct neighbors is gained by observing beacon messages each node sends out periodically. The transmission of beacons and the storage of neighbor information consumes resources. Due to mobility, collected neighbor information can quickly get outdated which in turn can lead to packet drops. In this paper, we propose a mechanism to perform position-based forwarding without the help of beacons or the maintenance of neighbor tables. In our contention-based forwarding scheme(CBF) the next hop is selected through a distributed contention process using biased timers. To avoid packet duplication, the first node that is selected suppresses the selection of further nodes. We propose three suppression strategies which vary with respect to forwarding efficiency and suppression characteristics. We analyze the behavior of CBF with all three suppression strategies and compare it to an existing greedy routing approach by means of simulation with ns-2. Our results demonstrate that CBF is a promising strategy for position-based routing.

Beaconless Position-Based Routing for Mobile Ad-Hoc Networks

Vehicular Dedicated Short Range Communication (DSRC) promises to reduce accidents by enabling assistance systems such as cross-traffic assistance. This application needs movement information from vehicles that are potentially in Non-Line-Of-Sight (NLOS) due to buildings at intersection corners. The amount of NLOS is determined by the placements of buildings. From another point of view, buildings influence the DSRC wireless channel by creating radio signal reflections and diffraction. Here, existing NLOS path-loss models for cellular communication with below roof-top base stations hint that the spatial placement of buildings has influence on the NLOS reception quality. This paper analyzes building positioning in the City of Munich with respect to DSRC. In a first step, it is investigated how much the line-of-sight is blocked by buildings. In a second step, the location of buildings is abstracted and the resulting data clustered to find a set of representative buildingposition scenarios. The presented scenario knowledge can be used in future research to configure meaningful simulations that investigate the radio channel at intersections under load and to select representative intersections for radio propagation field tests.

Hannes Hartenstein

Papers

Introducing probabilistic radio propagation models in OMNeT++ mobility framework and cross validation check with NS-2

Position-based routing in ad hoc wireless networks

Beaconless Position-Based Routing for Mobile Ad-Hoc Networks

Vehicular safety communication at intersections: Buildings, Non-Line-Of-Sight and representative scenarios

Introducing Probabilistic Radio Propagation Models in OMNeT++ Mobility Framework and Cross Validation Check with NS-2