Physically Unclonable Functions (PUFs) have proved to be an effective and low-cost measure against counterfeiting by providing device authentication and secure key storage services. Memory-based PUF implementations are an attractive option due to the ubiquitous nature of memory in electronic devices and the requirement of minimal (or no) additional circuitry. Dynamic Random Access Memory-- (DRAM) based PUFs are particularly advantageous due to their large address space and multiple controllable parameters during response generation. However, prior works on DRAM PUFs use a static response-generation mechanism making them vulnerable to security attacks. Further, they result in slow device authentication, are not applicable to commercial off-the-shelf devices, or require DRAM power cycling prior to authentication. In this article, we propose D-PUF, an intrinsically reconfigurable DRAM PUF based on the idea of DRAM refresh pausing. A key feature of the proposed DRAM PUF is reconfigurability , that is, by varying the DRAM refresh-pause interval, the challenge-response behavior of the PUF can be altered, making it robust to various attacks. The article is broadly divided into two parts. In the first part, we demonstrate the use of D-PUF in performing device authentication through a secure, low-overhead methodology. In the second part, we show the generation of true random numbers using D-PUF. The design is implemented and validated using an Altera Stratix IV GX FPGA-based Terasic TR4-230 development board and several off-the-shelf 1GB DDR3 DRAM modules. Our experimental results demonstrate a 4.3×-6.4× reduction in authentication time compared to prior work. Using controlled temperature and accelerated aging tests, we also demonstrate the robustness of our authentication mechanism to temperature variations and aging effects. Finally, the ability of the design to generate random numbers is verified using the NIST Statistical Test Suite.

The seven papers in this special issue focus on developments in the area of vehicular networks.

In this paper, we design and evaluate an energy efficient data retrieval architecture for continuous aggregate queries in wireless sensor networks. We show how the modification of precision in one sensor affects the sample-reporting frequency of other sensors, and how the precisions of a group of sensors may be collectively modified to achieve the target quality of information (QoI) with higher energy-efficiency. The proposed collective adaptive precision setting (CAPS) architecture is then extended to exploit the observed temporal correlation among successive sensor samples for even greater energy efficiency. Detailed simulations with synthetic and real data traces demonstrate how the combination of weak consistency semantics and temporal correlation can dramatically lower the energy consumption in practical sensor environments

An interesting feature of IEEE 802.11 wireless LAN cards is that they support multiple transmission modes. For example, the 802.11b cards support four transmission modes of I, 2, 5.5 and 11 Mbps, whereas, the 802.11a cards support eight transmission modes, up to a maximum of 54 Mbps. In this paper, we study layer four protocols over multi-rate multi-hop wireless networks and attempt to answer the question whether higher bandwidth links necessarily outperform lower bandwidth links in these networks. We examine this question by taking into account the transport layer protocols such as the TCP and UDP. While network capacity is a topic of active interest in the research community, a comparative study of the ad hoc network throughput at the different link bandwidths has not been made. We then propose a bandwidth-based ad hoc routing protocol and look at how to integrate QoS with routing.

We consider the problem of modeling TCP over multi-hop wireless networks using the IEEE 802.11 protocol. By identifying suitable regeneration instants, we are able to apply the standard technique of regenerative processes to compute the long term average throughput achieved by a single TCP session. Simulation results show that the proposed model predicts the TCP throughput to a very high level of accuracy. We then discuss how to extend this model to more general situations.

We use a Markov renewal reward approach to analyse the throughput achieved by a TCP connection spanning a multi-hop wireless network that uses the IEEE 802.11 MAC protocol. The significance of the paper is twofold: (a) the method used in the paper is simple and can be used for performance evaluation of wireless LANs in general, and, (b) the analytical results suggest that there can be a significant gain in TCP performance due to ACK thinning if the TCP receiver uses the delayed acknowledgement option of TCP. This is in accordance with other simulation based studies on similar lines.

This paper describes an algorithm to control non-adaptive sources in a simple and lightweight fashion that does not maintain per-flow state information. Using experimental results from simulations, we demonstrate that the algorithm outperforms current well-known algorithms for buffer management in providing better protection for TCP flows under varying degrees of attack from non-adaptive flows.

We use simulations to study the comparative routing overheads of three ad-hoc routing protocols, namely AODV, DSDV and DSR. In contrast to earlier studies, we focus exclusively on the energy consumption and not on other metrics such as the number of routing packets. In particular, we study the 'range effects' of the three protocols, i.e., how changes to the transmission power and transmission radius affect the overall energy consumed by routing-related packets. Due to the broadcast nature of the wireless medium, the energy spent in packet receptions is almost as important as the transmission power; using the number of transmissions as an indicator of the routing overhead can thus be fairly misleading. Our studies show that the energy overhead of the three protocols varies with the transmission power in distinct and non-obvious ways.

We study the dependence of the capacity of multi-hop wireless networks on the transmission range of nodes in the network with TCP regulated traffic. Specifically, we examine the sensitivity of the capacity to the speed of the nodes and the number of TCP connections in an ad hoc network. By incorporating the notion of a minimal acceptable QoS metric (loss) for an individual session, we argue that the QoS-aware capacity is a more accurate model of the TCP-centric capacity of an ad-hoc network. We study the dependence of capacity on the source application (Telnet or FTP) and on the choice of the ad-hoc routing protocol (ad-hoc on-demand distance vector - AODV, dynamic source routing - DSR or destination-sequenced distance vector - DSDV). We conclude that persistent and non-persistent traffic behave quite differently in an ad-hoc network.

The paper describes an algorithm to control flows from nonadaptive sources in the Internet in a simple and lightweight fashion. The algorithm, called XCHOKe, is an extension to CHOKe and RED. The paper presents experimental results from simulations that demonstrate that the algorithm outperforms current well-known algorithms for buffer management in providing better protection for TCP flows under varying degrees of attack from non-adaptive flows.

We study the performance metrics associated with TCP-regulated traffic in multi-hop wireless networks that use a common physical channel (e.g., IEEE 802.11). In contrast to earlier analyses, we focus simultaneously on two key operating metrics - the energy efficiency and the session throughput. Using analysis and simulations, we show how these metrics are strongly influenced by the radio transmission range of individual nodes. Due to tradeoffs between the individual packet transmission energy and the likelihood of retransmissions, the total energy consumption is a convex function of the number of hops (and hence, of the transmission range). On the other hand, the TCP session throughput decreases supra-linearly with a decrease in the transmission range. In certain scenarios, the overall network capacity can then be a concave function of the transmission range. Based on our analysis of the performance of an individual TCP session, we finally study how parameters such as the node density and the radio transmission range affect the overall network capacity under different operating conditions. Our analysis shows that capacity metrics at the TCP layer behave quite differently than corresponding idealized link-layer metrics.

We propose k-SACK, a TCP variant that has considerably improved throughput characteristics over lossy links. A k-SACK source does not consider every packet loss as an indication of congestion. It uses the selective acknowledgement option to estimate a parameter lookahead-loss, which is used in congestion detection. Our results demonstrate that k-SACK maintains a steady performance in a non-lossy environment while showing considerable performance improvement over paths spanning lossy links. Throughput gains of more than 90% are observed over links with packet loss rates of the order of 5%. We show that when k-SACK is used over wireline Internet links, it does not harm the existing TCP variants significantly. k-SACK is simple to implement and does not involve any additional overheads.

In networks with mobile nodes, power consumption is an important constraint due to the limited battery power available at the mobile nodes. Reduction of power consumption in mobile devices can be achieved by keeping these devices in a low power (standby) mode during periods of inactivity. This may, however, result in increased end-to-end delays leading to violation of the quality of service (QoS) parameters. We propose medium access control (MAC) scheduling policies for centrally controlled TDD wireless systems. The proposed policies reduce the power consumption of mobile devices by putting them into low power mode intelligently, using probabilistic estimates of inactivity based on the previous traffic arrival pattern. The policies ensure that the QoS parameters such as end-to-end packet delays are not violated.

Emerging technologies such as Bluetooth are expected to become a ubiquitous solution for providing short range, low power, low cost, pico-cellular wireless connectivity. Bluetooth is a master driven time division duplex (TDD) system that supports an asynchronous channel for data traffic as well as synchronous channels for voice traffic. Data applications running over Bluetooth such as http, ftp and real audio will need transport layer protocols such as TCP and UDP to send packets over the wireless links. In this paper we study several schemes designed to improve the performance of asynchronous data traffic over a Bluetooth piconet that supports multiple active slaves. We propose and compare a number of segmentation and reassembly policies and MAC scheduling algorithms with a view towards enhancing the performance of transport layer sessions. We investigate the effect of different FEC and ARQ schemes at the baseband level, using a two-state Markov channel model for the Bluetooth RF link. We also study how the presence of circuit-switched voice impacts the performance of data traffic.

Owing to the absence of any static support structure, ad-hoc networks are prone to link failures. The 'shortest path seeking' routing protocols may not lead to stable routes. The consequent route failures that ensue, lead to the degradation of system throughput. This paper suggests a routing protocol wherein the route selection is done on the basis of an intelligent residual lifetime assessment of the candidate routes. Schemes for performance enhancement with TCP and non-TCP traffic in ad-hoc networks are proposed. The protocol is backed by simulations in ns that show excellent adaptation to increasing network mobility. We have also introduced new route cache management and power aware data transmission schemes.

Properly designed handover procedures in cellular wireless systems are essential for maintaining continuity of a call in progress and for minimizing the probability of forced call termination and signaling and switching load on the network. We model a channel exchange scheme that involves exchange of channels between two mobiles that are moving in opposite directions across the handover area of adjacent cells. The channel exchange method is interesting since it yields low values of handover failure probability as compared to the case with no channel exchange. We compare the performance of the cellular system with and without channel exchange through simulation and analysis. For realistic values of new call arrival rate, it is found that the handover failure probability obtained from analysis is in close agreement with that obtained through simulation.

The authors model a parallel processing system comprising several homogeneous computers interconnected by a communication network. Jobs arriving to this system have a linear fork-join structure. Each fork of the job gives rise to a random number of tasks that can be processed independently on any of the computers. Since exact analysis of fork-join models is known to be intractable, the authors resort to obtaining analytical bounds to the mean job response time of the fork-join job. For jobs with a single fork-join and, probabilistic allocation of tasks of the job to the N processors, they obtain upper and lower bounds to the mean job response time. Upper bounds are obtained using the concept of associated random variables and are found to be a good approximation to the mean job response time. A simple lower bound is obtained by neglecting queueing delays. They also find two lower bounds that include queueing delays. For multiple fork-join jobs, they study an approximation based on associated random variables. Finally, two versions of the join-the-shortest-queue (JSQ) allocation policy (i.e., JSQ by batch and JSQ by task) are studied and compared, via simulations and diffusion limits.>