Digital signatures are one of the fundamental security primitives in Vehicular Ad-Hoc Networks (VANETs) because they provide authenticity and non-repudiation in broadcast communication. However, the current broadcast authentication standard in VANETs is vulnerable to signature flooding: excessive signature verification requests that exhaust the computational resources of victims. In this paper, we propose two efficient broadcast authentication schemes, Fast Authentication (FastAuth) and Selective Authentication (SelAuth), as two countermeasures to signature flooding. FastAuth secures periodic single-hop beacon messages. By exploiting the sender's ability to predict its own future beacons, FastAuth enables 50 times faster verification than previous mechanisms using the Elliptic Curve Digital Signature Algorithm. SelAuth secures multi-hop applications in which a bogus signature may spread out quickly and impact a significant number of vehicles. SelAuth pro- vides fast isolation of malicious senders, even under a dynamic topology, while consuming only 15%--30% of the computational resources compared to other schemes. We provide both analytical and experimental evaluations based on real traffic traces and NS-2 simulations. With the near-term deployment plans of VANET on all vehicles, our approaches can make VANETs practical.

High throughput and lifetime are both crucial design objectives for a number of multihop wireless network applications. As these two objectives are often in conflict with each other, it naturally becomes important to identify the trade-offs between them. Several works in the literature have focused on improving one or the other, but investigating the trade-off between throughput and lifetime has received relatively less attention. We study this trade-off between the network throughput and lifetime for the case of fixed wireless networks, where link transmissions are coordinated to be conflict-free. We employ a realistic interference model based on the Signal-to-Interference-and-Noise Ratio (SINR), which is usually considered statistically sufficient to infer success or failure of wireless transmissions. Our analytical and numerical results provide several insights into the interplay between throughput, lifetime, and transmit power. Specifically, we find that with a fixed throughput requirement, lifetime is not monotonic with power-neither very low power nor very high power result in the best lifetime. We also find that, for a fixed transmit power, relaxing the throughput requirement may result in a more than proportional improvement in the lifetime for small enough relaxation factors. Taken together, our insights call for a careful balancing of objectives when designing a wireless network for high throughput and lifetime.

In wireless communications, the desired wireless signal is typically decoded by treating the sum of all the other ongoing signal transmissions as noise. In the networking literature, this phenomenon is typically abstracted using a wireless channel interference model. The level of detail in the interference model, evidently determines the accuracy of the results based upon the model. Several works in the networking literature have made use of simplistic interference models, e.g., fixed ranges for communication and interference, the capture threshold model (used in the ns2 network simulator), the protocol model, and so on. At the same time, fairly complex interference models such as those based on the SINR (signal-to-interference-and-noise ratio) have also been proposed and used. We investigate the impact of the choice of the interference model, on the conclusions that can be drawn regarding the performance of wireless networks, by comparing different wireless interference models. We find that both in the case of random access networks, as well as in the case of scheduled networks (where node transmissions are scheduled to be completely conflict-free), different interference models can produce significantly different results. Therefore, a lot of caution should be exercised before accepting or interpreting results based on simplified interference models. Further, we feel that an SINR-based model is the minimum level of detail that should be employed to model wireless channel interference in a networking context.

Scheduled wireless mesh networks (WMNs) represent an important paradigm in the development of high speed wireless access networks. As a consequence of (Aditya Karnik, 2006), it can be shown that the maximum throughput of a single gateway n node WMN is upper-bounded by 1/n, normalized with respect to the highest available data-rate. The situation is actually worse since with conventional omni-directional antennas, the achievable throughput is considerably lower at low powers. This paper makes a case for smart (directional) antennas. For the same power, smart antennas can provide higher range, and therefore shorter paths, as compared to omni-directional antennas. For the same range, smart antennas can operate at lower powers, reducing the interference, and thereby improving the spatial reuse. Although they cannot change the fundamental throughput limit of 1/n, smart antennas can achieve significantly higher throughput at lower powers, These insights are demonstrated through several numerical examples using the framework of (Aditya Karnik, 2006).

In a wireline network, nodes form links with only those nodes they are wired to, and the links do not interfere with one another. In contrast, in a wireless network, signal transmissions are intrinsically broadcast, and suffer from mutual interference. In several physical layer technologies, a wireless signal is decoded by treating the sum of all the other on-going signal transmissions as noise. Hence, from a networking standpoint, there is a need to model wireless channel interference. An accurate interference model is especially important in a multi-hop network context, since there could be several simultaneous wireless transmissions. Several works in the literature have made use of simplified interference models. Some works assume a fixed range for communication and interference, while others are based on concepts like capture threshold where the desired signal strength is compared with interference from a single node at a time, rather than cumulatively. In particular, the latter model is used in ns2 which is the most common simulation tool. Under isotropic pathloss, the capture threshold model is also equivalent to the protocol model proposed by Gupta and Kumar, which is now the subject of a lot of analytical activity notably through conflict graph based problem formulations. We investigate the accuracy and appropriateness of the capture threshold based interference model, by comparing it with one based on the SINR (signal-to-interference-and-noise ratio) with additive interference calculation. We find that both in the case of random access networks, as well as in the case of scheduled networks (where node transmissions are scheduled to be completely conflict-free), a simplified interference model such as the capture threshold model, can produce significantly different results compared to an additive interference based model. Therefore, a lot of caution should be exercised before accepting or interpreting results based on simplified interference models.

Abstract Multi‐hop wireless networks arise in the context of ad hoc networks, sensor networks, and mesh networks, and their performance depends critically on the underlying medium access control (MAC) protocol. Inspite of the large body of work devoted to MAC protocols and associated problems, the relative importance of these problems is still not well understood. This is because most of the previous work focuses on designing a protocol to solve a particular problem, or on identifying scenarios where a protocol will not work well. In addition, most of the work is also based on simplistic assumptions about the physical wireless medium, like fixed ranges for communication and interference, or concepts like capture threshold where the desired signal strength is compared with interference from a single node at a time, rather than cumulatively. Our paper seeks to address these issues. We believe it is extremely critical that (i) we develop an understanding of the relative significance of the problems affecting MAC protocols, and that (ii) we use a realistic model for the physical channel for design and performance evaluation. Towards this end, we evaluate the performance of three currently proposed MAC protocols, IEEE 802.11 [1], RI‐BTMA [2], and DUCHA [3] under a realistic channel model with additive interference. Since these protocols solve or suffer from different sets of problems, our evaluation provides a differential diagnosis of the severity of these problems. Based on our observations, we propose a simple and robust two channel MAC protocol (entitled 2CM) that is based on IEEE 802.11 augmented with a busy‐tone channel. The 2CM protocol (i) mitigates the hidden node problem considerably, (ii) does not waste bandwidth in terms of logical control channels, and (iii) provides a reliable link layer acknowledgment. Through extensive simulations, we show that 2CM offers a consistently high throughput performance while not sacrificing link layer reliability in a variety of scenarios, thereby vindicating our approach. Copyright © 2006 John Wiley & Sons, Ltd.

The problem of connectivity in multi-hop wireless networks has been extensively studied in the literature and general results for node density requirements have been obtained (P. Gupta and P.R. Kumar, Proc. 37th IEEE Conf. of Decision and Control, 1998). These results have been obtained based on the implicit assumption of a generic routing protocol, capable of exhaustively searching all possible routes, between all pairs of nodes. Hence these results would be too optimistic in several practical cases, where the routing protocols are not generic but optimized for specific applications. In this paper, we provide a framework for defining the appropriate notion of connectivity that reflects the underlying network architecture and protocols. Based on this framework, we define and analyze connectivity requirements for two network architectures proposed in the literature, namely, GAF (geographic adaptive fidelity) with Manhattan routing (Y. Xu et al., Proc. Int. Conf. on Mobile Comp. and Netw., 2001), proposed for ad-hoc networks and AIMRP (address-light integrated MAC and routing protocol) which employs tier-based routing in sensor networks (S.S. Kulkarni et al., submitted for publication in IEEE Trans. on Networking, Dec. 2003). By comparing the critical node density requirements for connectivity, obtained through our framework, with the results in Gupta and Kumar, we show that the earlier results are too optimistic and hence it is important to consider the underlying routing protocol to dimension the density of nodes appropriately.

In this paper, we consider the problem of supporting video transmission over Bluetooth public access (Blue PAC) networks. BluePAC networks are an example of what are broadly called wireless personal area networks (WPAN). We compare BluePAC networks with WPANs based on the IEEE 802.11 technology. We compare the two based on the following performance measures: link delay distribution and packet loss rate. We also see how the performance of a video connection in a BluePAC network can be enhanced by reserving bandwidth by means of a deficit round robin (DRR) polling algorithm in Bluetooth. To perform the above the above study, we have used network models provided by the network simulator (NS-2), extended to model Bluetooth by Bluehoc from IBM. The results show that Bluetooth performs significantly better than IEEE 802.11 MAC in terms of packet loss rates, while 802.11 is better in terms of the packet delay distribution.