Departmental Papers (ESE)

Always Acyclic Distributed Path Computation

Saikat Ray — Tue, 17 Nov 2009 07:53:30 PST

Distributed routing algorithms may give rise to transient loops during path recomputation, which can pose significant stability problems in high-speed networks. We present a new algorithm, Distributed Path Computation with Intermediate Variables (DIV), which can be combined with any distributed routing algorithm to guarantee that the directed graph induced by the routing decisions remains acyclic at all times. The key contribution of DIV, besides its ability to operate with any routing algorithm, is an update mechanism using simple message exchanges between neighboring nodes that guarantees loop-freedom at all times. DIV provably outperforms existing loop-prevention algorithms in several key metrics such as frequency of synchronous updates and the ability to maintain paths during transitions. Simulation results quantifying these gains in the context of shortest path routing are presented. In addition, DIV's universal applicability is illustrated by studying its use with a routing that operates according to a non-shortest path objective. Specifically, the routing seeks robustness against failures by maximizing the number of next-hops available at each node for each destination.

Gas sensing properties of single conducting polymer nanowires and the effect of temperature

Yaping Dan — Tue, 03 Nov 2009 07:43:53 PST

We measured the electronic properties and gas sensing responses of template-grown poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS)-based nanowires. The nanowires had a 'striped' structure (gold-PEDOT/PSS-gold), and were typically 8 �m long (1 �m-6 �m-1 �m for the sections, respectively) and 220 nm in diameter. Single-nanowire devices were contacted with pre-fabricated gold electrodes using dielectrophoretic assembly. A polymer conductivity of 11.5 � 0.7 S cm−1 and a contact resistance of 27.6 � 4 kΩ were inferred from measurements on nanowires of varying length and diameter. The nanowire sensors detected a variety of odors, with rapid response and recovery (seconds). The response (ΔR/R) varied as a power law with analyte concentration. The power law exponent was found to increase with the molecular weight of the analyte and as a function of temperature. The detection limits are set by noise intrinsic to the device and are at the ppm level even for very volatile analytes.

Intrinsic Response of Graphene Vapor Sensors

Yaping Dan — Tue, 03 Nov 2009 07:34:52 PST

Graphene is a two-dimensional material with extremely favorable chemical sensor properties. Conventional nanolithography typically leaves a resist residue on the graphene surface, whose impact on the sensor characteristics has not yet been determined. Here we show that the contamination layer chemically dopes the graphene, enhances carrier scattering, and acts as an absorbent layer that concentrates analyte molecules at the graphene surface, thereby enhancing the sensor response. We demonstrate a cleaning process that verifiably removes the contamination on the device structure and allows the intrinsic chemical responses of the graphene monolayer to be measured. These intrinsic responses are surprisingly small, even upon exposure to strong analytes such as ammonia vapor.

Dielectrophoretically assembled polymer nanowires for gas sensing

Yaping Dan — Tue, 03 Nov 2009 07:32:13 PST

We measured the electronic properties and gas sensing response of nanowires containing segments of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) that were synthesized using anodic aluminum oxide (AAO) membranes. The nanowires have a "striped" structure of gold-PEDOT/PSS-gold and are typically 8 μm long (1 μm-6 μm-1 μm for each section, respectively) and 220 nm in diameter. Dielectrophoretic assembly was used to position single nanowires on pre-fabricated gold electrodes. A polymer conductivity of 11.5 � 0.7 S/cm and a contact resistance of 27.6 � 4 kΩ were inferred from resistance measurements of nanowires of varying length and diameter. When used as gas sensors, the wires showed a resistance change of 10.5%, 9%, and 4% at the saturation vapor pressure of acetone, methanol and ethanol, respectively. Sensor response and recovery were rapid (seconds) with excellent reproducibility in time and across devices. "Striped" template-grown nanowires are thus intriguing candidates for use in electronic nose vapor sensing systems.

Online Optimization of 802.11 Mesh Networks

Theodoros Salonidis — Wed, 28 Oct 2009 10:21:34 PDT

802.11 wireless mesh networks are ubiquitous, but suffer from severe performance degradations due to poor synergy between the 802.11 CSMA MAC protocol and higher layers. Several solutions have been proposed that either involve significant modifications to the 802.11 MAC or legacy higher layer protocols, or rely on 802.11 MAC models seeded with off-line measurements performed during network downtime.We introduce a technique for online optimization of 802.11 wireless mesh networks using rate control at the network layer. The technique is based on a lightweight model that characterizes the feasible rates region of an operational 802.11 wireless mesh network. Unlike existing 802.11 modeling approaches, the parameters of this model can be estimated online, incur minimal overhead and can be realized using standard probing mechanisms at the network layer. Using analysis and extensive measurements over a wireless mesh network testbed, we validate the assumptions on which the model is built, and explain the principles behind the choice and estimation of its parameters. The benefits of the model and its solution in terms of fairness, throughput and stability are demonstrated operationally for a range of multi-hop topologies and configurations.

Cooperative profit sharing in coalition based resource allocation in wireless networks

Alireza Aram — Tue, 27 Oct 2009 11:33:40 PDT

We consider a network in which several service providers offer wireless access service to their respective subscribed customers through potentially multi-hop routes. If providers cooperate, i.e., pool their resources, such as spectrum and base stations, and agree to serve each others' customers, their aggregate payoffs, and individual shares, can potentially substantially increase through efficient utilization of resources and statistical multiplexing. The potential of such cooperation can however be realized only if each provider intelligently determines who it would cooperate with, when it would cooperate, and how it would share its resources during such cooperation. Also, when the providers share their aggregate revenues, developing a rational basis for such sharing is imperative for the stability of the coalitions. We model such cooperation using transferable payoff coalitional game theory. We first consider the scenario that locations of the base stations and the channels that each provider can use have already been decided apriori. We show that the optimum cooperation strategy, which involves the allocations of the channels and the base stations to mobile customers, can be obtained as solutions of convex optimizations. We next show that the grand coalition is stable in this case, i.e. if all providers cooperate, there is always an operating point that maximizes the providers' aggregate payoff, while offering each such a share that removes any incentive to split from the coalition. Next, we show that when the providers can choose the locations of their base stations and decide which channels to acquire, the above results hold in important special cases. Finally, we examine cooperation when providers do not share their payoffs, but still share their resources so as to enhance individual payoffs. We show that the grand coalition continues to be stable.

Shared Versus Separate Networks - The Impact of Reprovisioning

Soumya Sen — Tue, 20 Oct 2009 07:48:53 PDT

As networks improve and new services emerge, questions arise that affect service deployments and network choices. The Internet is arguably a successful example of a network shared by many services. However, combining heterogeneous services on the same network need not always be the right answer, and technologies such as virtualization make deploying new services on separate networks increasingly more viable. So, which is the right option? The question is not unique to networks, and there is a large body of work in the manufacturing systems literature that explores the trade-off between flexible and dedicated plants. This paper highlights an important feature missing from these earlier works, namely, the ability to ``reprovision'' resources in response to changes in demand. It demonstrates that this feature alone can affect the choice of network solutions, and argues for models that incorporate it.

Quantifying Acute Myocardial Injury Using Ratiometric Fluorometry

Mahsa Ranji — Tue, 13 Oct 2009 08:35:45 PDT

Early reperfusion is the best therapy for myocardial infarction (MI). Effectiveness, however, varies significantly between patients and has implications for long-term prognosis and treatment. A technique to assess the extent of myocardial salvage after reperfusion therapy would allow for high-risk patients to be identified in the early post-MI period. Mitochondrial dysfunction is associated with cell death following myocardial reperfusion and can be quantified by fluorometry. Therefore, we hypothesized that variations in the fluorescence of mitochondrial nicotinamide adenine dinucleotide (NADH) and flavoprotein (FP) can be used acutely to predict the degree of myocardial injury. Thirteen rabbits had coronary occlusion for 30 min followed by 3 h of reperfusion. To produce a spectrum of infarct sizes, six animals were infused cyclosporine A prior to ischemia. Using a specially designed fluorometric probe, NADH and FP fluorescence were measured in the ischemic area. Changes in NADH and FP fluorescence, as early as 15 min after reperfusion, correlated with postmortem assessment infarct size (r=0.695, p<0.01). This correlation strengthened with time (r=0.827, p<0.01 after 180 min). Clinical application of catheter-based myocardial fluorometry may provide a minimally invasive technique for assessing the early response to reperfusion therapy.

Layers and layer interfaces in wireless networks

Alejandro Ribeiro — Tue, 13 Oct 2009 08:07:02 PDT

This paper proposes an optimal architecture for wireless networks based on layers and layer interfaces. In the presence of fading the architecture is shown to be optimal. The result follows from a subgradient descent algorithm on the dual function of a generic wireless networking optimization problem. The fact that these non-convex optimization problems have nonetheless zero duality gap is exploited.

Microscale inverse acoustic band gap structure in aluminum nitride

Nai-Kuei Kuo — Thu, 01 Oct 2009 13:06:05 PDT

This work presents the design and demonstration of a microscale inverse acoustic band gap (IABG) structure in aluminum nitride (AlN) with a frequency stop band for bulk acoustic waves in the very high frequency range. Conversely to conventional microscale acoustic band gaps, the IABG is formed by a two-dimensional periodic array of unit cells consisting of a high acoustic velocity material cylinder surrounded by a low acoustic velocity medium. The periodic arrangement of the IABG array induces scattering of incident acoustic waves and generates a stop band, whose center frequency is primarily determined by the lattice constant of the unit cell and whose bandwidth depends on the cylinder radius, the film thickness, and the size of the tethers that support the cylinder. A wide band gap (>13% of the center frequency) is formed by the IABG even when thin AlN films are used. The experimental response of an IABG structure having a unit cell of 8.6 �m and an AlN film thickness of 2 �m confirms the existence of a frequency band gap between 185 MHz and 240 MHz.