Most previous analysis of Twitter user behavior has focused on individual information cascades and the social followers graph, in which the nodes for two users are connected if one follows the other. We instead study aggregate user behavior and the retweet graph with a focus on quantitative descriptions. We find that the lifetime tweet distribution is a type-II discrete Weibull stemming from a power law hazard function, that the tweet rate distribution, although asymptotically power law, exhibits a lognormal cutoff over finite sample intervals, and that the inter-tweet interval distribution is a power law with exponential cutoff. The retweet graph is small-world and scale-free, like the social graph, but less disassortative and has much stronger clustering. These differences are consistent with it better capturing the real-world social relationships of and trust between users than the social graph. Beyond just understanding and modeling human communication patterns and social networks, applications for alternative, decentralized microblogging systems-both predicting real-word performance and detecting spam-are discussed.

Improving and optimizing user-perceived smartphone performance requires understanding device, system, and application behavior for real-world workloads. However, measuring such performance is challenging due to the multi-threaded, asynchronous programming paradigms used in modern applications and the multiple layers of hardware and software used to respond to user input events. We address this challenge with Panappticon, a lightweight, system-wide, fine-grained event tracing system for Android that automatically identifies critical execution paths in user transactions. Panappticon monitors the application, system, and kernel software layers and can identify performance problems stemming from application design flaws, underpowered hardware, and harmful interactions between apparently unrelated applications. We carried out a 14-user, one-month study of an Android smartphone system instrumented with Panappticon, which revealed a number of specific problems and areas for improvement that may be of interest to system designers, application developers, and device manufactures.

Wireless networks are vulnerable to Sybil attacks, in which a malicious node poses as many identities in order to gain disproportionate influence. Many defenses based on spatial variability of wireless channels exist, but depend either on detailed, multi-tap channel estimation-something not exposed on commodity 802.11 devices-or valid RSSI observations from multiple trusted sources, e.g., corporate access points-something not directly available in ad hoc and delay-tolerant networks with potentially malicious neighbors. We extend these techniques to be practical for wireless ad hoc networks of commodity 802.11 devices. Specifically, we propose two efficient methods for separating the valid RSSI observations of behaving nodes from those falsified by malicious participants. Further, we note that prior signalprint methods are easily defeated by mobile attackers and develop an appropriate challenge-response defense. Finally, we present the Mason test, the first implementation of these techniques for ad hoc and delay-tolerant networks of commodity 802.11 devices. We illustrate its performance in several real-world scenarios.

Negative Bias Temperature Instability (NBTI) is a significant reliability concern for nanoscale CMOS circuits. Its effects on circuit timing can be especially pronounced for circuits with standby-mode equipped functional units, because these units can be subjected to static NBTI stress for extended periods of time. This article describes Internal Node Control (INC), in which the inputs to some individual gates are directly manipulated to prevent this static NBTI fatigue. We prove that the INC selection problem is NP -complete and present a linear-time heuristic that can quickly determine near-optimal placements. This near-optimality is confirmed by comparing results for small benchmarks against optimal solutions from a mixed integer linear programming formulation of our problem. We evaluate the heuristic on the ISCAS85 benchmarks and the Synopsys DesignWare Library. Our heuristic reduces static NBTI-induced delay over a ten year period by 30--60% and can reduce total path delay by an average 9.4% when NBTI degradation is severe. The INC placements and sleep signal routing require only a 1.6% increase in area.

Abstract-Increasing power densities due to process scaling, combined with high switching activity and poor cooling environments during testing, have the potential to result in high integrated circuit (IC) temperatures. This has the potential to damage ICs and cause good ICs to be discarded due to temperature-induced timing faults. We first study the power impact of scan chain testing for the ISCAS89 benchmarks. We find that the scan-chain test power consumption is 1.6¢ higher for at-speed testing than normal operating power consumption. We conclude that if the testing frequency is less than half of the normal frequency, then the testing power consumption may in fact be lower. However, due to differences in the cooling environments, the peak die temperatures may still be higher. Second, we present an optimal formulation for minimal-duration temperature-constrained test scheduling. Our results improve on the test schedule time of the best existing algorithm by 10.8% on average for a packaged IC thermal environment. We also present an efficient heuristic that generally produces the same results as the optimal algorithm, while requiring little CPU time, even for large problem instances.

Mobile ad-hoc networks of wireless devices (MANETs) hold the promise of providing network services without traditional infrastructures that could fall victim to manipulation and censorship. Unfortunately, current MANET systems suffer significant scalability problems, effectively precluding their use for general-purpose networking. We suggest tailoring MANETs to particular classes of application and leveraging application-specific properties to increase scalability. This paper describes the design of a scalable and secure MANET for text-based personal communication. Our design is based on geographic routing and uses human motion and communication patterns to facilitate location tracking and distribution, thereby increasing scalability above that of traditional geographic routing. We provide location privacy by transplanting mix-net techniques into MANETs.

Delay Tolerant Networks (DTNs) composed of commodity mobile devices have the potential to support communication applications resistant to blocking and censorship, as well as certain types of surveillance. We analyze the performance and energy consumption of such a network, and consider the impact of random and targeted denial-of-service and censorship attacks. To gather wireless connectivity traces for a DTN composed of human-carried commodity smartphones, we implemented and deployed a prototype DTN-based microblogging application, called 1am, in a college town. We analyzed the system during a time period with 111 users. Although the study provided detailed enough connectivity traces to enable analysis, message posting was too infrequent to draw strong conclusions based on user-initiated messages, alone. We therefore simulated more frequent message initiations and used measured connectivity traces to analyze message propagation. Using a flooding protocol, we found that with an adoption rate of 0.2% of a college town's student and faculty population, the median one-week delivery rate is 85% and the median delivery delay is 13 hours. We also found that the network delivery rate and delay are robust to denial-of-service and censorship attacks eliminating more than half of the participants. Using a measurement-based energy model, we also found that the DTN system would use less than 10.0% of a typical smartphone's battery energy per day in a network of 2,500 users.

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