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

Foreword. About the Editors. Preface. Acknowledgements. List of Contributors. 1 Introduction (Hannes Hartenstein and Kenneth P. Laberteaux). 1.1 Basic Principles and Challenges. 1.2 Past and Ongoing VANET Activities. 1.3 Chapter Outlines. 1.4 References. 2 Cooperative Vehicular Safety Applications (Derek Caveney). 2.1 Introduction. 2.2 Enabling Technologies. 2.3 Cooperative System Architecture. 2.4 Mapping for Safety Applications. 2.5 VANET-enabled Active Safety Applications. 2.6 References. 3 Information Dissemination in VANETs (Christian Lochert, Bjorn Scheuermann and Martin Mauve). 3.1 Introduction. 3.2 Obtaining Local Measurements. 3.3 Information Transport. 3.4 Summarizing Measurements. 3.5 Geographical Data Aggregation. 3.6 Conclusion. 3.7 References. 4 VANET Convenience and Efficiency Applications (Martin Mauve and Bjorn Scheruermann). 4.1 Introduction. 4.2 Limitations. 4.3 Applications. 4.4 Communication Paradigms. 4.5 Probabilistic, Area-based Aggregation. 4.6 Travel Time Aggregation. 4.7 Conclusion. 4.8 References. 5 Vehicular Mobility Modeling for VANETs (Jerome Harri). 5.1 Introduction. 5.2 Notation Description. 5.3 Random Models. 5.4 Flow Models. 5.5 Traffic Models. 5.6 Behavioral Models. 5.7 Trace or Survey-based Models. 5.8 Integration with Network Simulators. 5.9 A Design Framework for Realistic Vehicular Mobility Models. 5.10 Discussion and Outlook. 5.11 Conclusion. 5.12 References. 6 Physical Layer Considerations for Vehicular Communications (Ian Tan and Ahmad Bahai). 6.1 Standards Overview. 6.2 Previous Work. 6.3 Wireless Propagation Theory. 6.4 Channel Metrics. 6.5 Measurement Theory. 6.6 Emperical Channel Characterization at 5.9 GHz. 6.7 Future Directions. 6.8 Conclusion. 6.9 Appendix: Deterministic Multipath Channel Derivations. 6.10 Appendix: LTV Channel Response. 6.11 Appendix: Measurement Theory Details. 6.12 References. 7 MAC Layer and Scalability Aspects of Vehicular Communication Networks (Jens Mittag, Felix Schmidt-Eisenlohr, Moritz Killat, Marc Torrent-Moreno and Hannes Hartenstein). 7.1 Introduction: Challenges and Requirements. 7.2 A Survey on Proposed MAC Approaches for VANETs. 7.3 Communication Based on IEEE 802.11p. 7.4 Performance Evaluation and Modeling. 7.5 Aspects of Congestion Control. 7.6 Open Issues and Outlook. 7.7 References. 8 Efficient Application Level Message Coding and Composition (Craig L Robinson). 8.1 Introduction to the Application Environment. 8.2 Message Dispatcher. 8.3 Example Applications. 8.4 Data Sets. 8.5 Predictive Coding. 8.6 Architecture Analysis. 8.7 Conclusion. 8.8 References. 9 Data Security in Vehicular Communication Networks (AndreWeimerskirch, Jason J Haas, Yih-Chun Hu and Kenneth P Laberteaux). 9.1 Introduction. 9.2 Challenges of Data Security in Vehicular Networks. 9.3 Network, Applications, and Adversarial Model. 9.4 Security Infrastructure. 9.5 Cryptographic Protocols. 9.6 Privacy Protection Mechanisms. 9.7 Implementation Aspects. 9.8 Outlook and Conclusions. 9.9 References. 10 Standards and Regulations (John B Kenney). 10.1 Introduction. 10.2 Layered Architecture for VANETs. 10.3 DSRC Regulations. 10.4 DSRC Physical Layer Standard. 10.5 DSRC Data Link Layer Standard (MAC and LLC). 10.6 DSRC Middle Layers. 10.7 DSRC Message Sublayer. 10.8 Summary. 10.9 Abbreviations and Acronyms. 10.10 References. Index.

VANET : vehicular applications and inter-networking technologies

Direct radio-based vehicle-to-vehicle communication can help prevent accidents by providing accurate and up-to-date local status and hazard information to the driver. In this paper, we assume that two types of messages are used for traffic safety-related communication: 1) Periodic messages (ldquobeaconsrdquo) that are sent by all vehicles to inform their neighbors about their current status (i.e., position) and 2) event-driven messages that are sent whenever a hazard has been detected. In IEEE 802.11 distributed-coordination-function-based vehicular networks, interferences and packet collisions can lead to the failure of the reception of safety-critical information, in particular when the beaconing load leads to an almost-saturated channel, as it could easily happen in many critical vehicular traffic conditions. In this paper, we demonstrate the importance of transmit power control to avoid saturated channel conditions and ensure the best use of the channel for safety-related purposes. We propose a distributed transmit power control method based on a strict fairness criterion, i.e., distributed fair power adjustment for vehicular environments (D-FPAV), to control the load of periodic messages on the channel. The benefits are twofold: 1) The bandwidth is made available for higher priority data like dissemination of warnings, and 2) beacons from different vehicles are treated with ldquoequal rights,rdquo and therefore, the best possible reception under the available bandwidth constraints is ensured. We formally prove the fairness of the proposed approach. Then, we make use of the ns-2 simulator that was significantly enhanced by realistic highway mobility patterns, improved radio propagation, receiver models, and the IEEE 802.11p specifications to show the beneficial impact of D-FPAV for safety-related communications. We finally put forward a method, i.e., emergency message dissemination for vehicular environments (EMDV), for fast and effective multihop information dissemination of event-driven messages and show that EMDV benefits of the beaconing load control provided by D-FPAV with respect to both probability of reception and latency.

/pdf/vehicle-to-vehicle-communication-fair-transmit-power-control-cwgzu7utkg.pdf

Vehicle-to-Vehicle Communication: Fair Transmit Power Control for Safety-Critical Information

Existing position-based unicast routing algorithms which forward packets 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. Due to mobility, the information that a node receives about its neighbors becomes outdated, leading either to a significant decrease in the packet delivery rate or to a steep increase in load on the wireless channel as node mobility increases. In this paper, we propose a mechanism to perform position-based unicast forwarding without the help of beacons. In our contention-based forwarding scheme (CBF) the next hop is selected through a distributed contention process based on the actual positions of all current neighbors. For the contention process, CBF makes use of 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 position-based routing approach by means of simulation with ns-2. Our results show that CBF significantly reduces the load on the wireless channel required to achieve a specific delivery rate compared to the load a beacon-based greedy forwarding strategy generates.

/pdf/contention-based-forwarding-for-mobile-ad-hoc-networks-5ahq7csy23.pdf

Contention-based forwarding for mobile ad-hoc networks

NS-2, with its IEEE 802.11 support, is a widely utilized simulation tool for wireless communications researchers. However, the current NS-2 distribution code has some significant shortcomings both in the overall architecture and the modeling details of the IEEE 802.11 MAC and PHY modules. This paper presents a completely revised architecture and design for these two modules. The resulting PHY is a full featured generic module able to support any single channel frame-based communications (i.e. it is also able to support non-IEEE 802.11 based MAC). The key features include cumulative SINR computation, preamble and PLCP header processing and capture, and frame body capture. The MAC accurately models the basic IEEE 802.11 CSMA/CA mechanism, as required for credible simulation studies. The newly designed MAC models transmission and reception coordination, backoff management and channel state monitoring in a structured and modular manner. In turn, the contributions of this paper make extending the MAC for protocol researches much easier and provide for a significantly higher level of simulation accuracy.

Overhaul of ieee 802.11 modeling and simulation in ns-2

One key usage of VANET is to support vehicle safety applications. This use case is characterized by the prominence of broadcasts in scaled settings. In this context, we try to answer the following questions: i) what is the probability of reception of a broadcast message by another car depending on its distance to the sender, ii) how to give priority access and an improved reception rate for important warnings, e.g., sent out in an emergency situation, and iii) how are the above two results affected by signal strength fluctuations caused by radio channel fading? We quantify via simulation the probability of reception for the two-ray-ground propagation model as well as for the Nakagami distribution in saturated environments. By making use of some IEEE 802.11e EDCA mechanisms for priority access, we do not only quantify how channel access times can be reduced but also demonstrate how improved reception rates can be achieved. Our results show that the mechanisms for priority access are successful under the two-way-ground model. However, with a non-deterministic radio propagation model like Nakagami's distribution the benefit is still obvious but the general level of probability of reception is much smaller compared to two-ray-ground model. The results indicate that -- particularly for safety-critical and sensor network type of applications -- the proper design of repetition or multi-hop retransmission strategies represents an important aspect of future work for robustness and network stability of vehicular ad hoc networks.

/pdf/broadcast-reception-rates-and-effects-of-priority-access-in-48wnv4dy84.pdf

Hannes Hartenstein

Papers

VANET : vehicular applications and inter-networking technologies

Vehicle-to-Vehicle Communication: Fair Transmit Power Control for Safety-Critical Information

Contention-based forwarding for mobile ad-hoc networks

Overhaul of ieee 802.11 modeling and simulation in ns-2

Broadcast reception rates and effects of priority access in 802.11-based vehicular ad-hoc networks