Improving TCP performance in ad hoc networks using signal strength based link management

Mobility in ad hoc networks causes frequent link failures, which in turn causes packet losses. TCP attributes these packet losses to congestion. This incorrect inference results in frequent TCP re-transmission time-outs and therefore a degradation in TCP performance even at light loads. We propose mechanisms that are based on signal strength measurements to alleviate such packet losses due to mobility. Our key ideas are (a) if the signal strength measurements indicate that a link failure is most likely due to a neighbor moving out of range, in reaction, facilitate the use of temporary higher transmission power to keep the link alive and, (b) if the signal strength measurements indicate that a link is likely to fail, initiate a route re-discovery proactively before the link actually fails. We make changes at the MAC and the routing layers to predict link failures and estimate if a link failure is due to mobility. We also propose a simple mechanism at the MAC layer that can help alleviate false link failures, which occur due to congestion when the IEEE 802.11 MAC protocol is used. We compare the above proactive and reactive schemes and also demonstrate the benefits of using them together and along with our MAC layer extension. We show that, in high mobility, the goodput of a TCP session can be improved by as much as 75% at light loads (when there is only one TCP session in the network) when our methods are incorporated. When the network is heavily loaded (i.e., there are multiple TCP sessions in the network), the proposed schemes can improve the aggregate goodput of the TCP sessions by about 14–30%, on average.     

Packet dropping statistics of a data-link protocol for wireless local communications

We consider a data transmission system over a wireless channel, where packets are queued at the transmitter. Automatic repeat request is employed as the error control scheme, by which packets corrupted due to channel impairments are immediately retransmitted. We study the statistics of the packet dropping process due to buffer overflow. In particular, we propose a Markov approximation to model such losses. The delay performance of the packets that are admitted to the queue is studied, and extensions of the analysis to general nonindependent arrival processes are also presented.     

Electrically short dipoles with a nonlinear load, a revisitedanalysis

We reexamine the characteristics of electrically short dipoles with nonlinear loads and, specifically, the early work of Motohisa Kanda (1980, 1983). Although this topic has been examined in great detail in the past, some inconsistencies between numerical and analytical results are apparent, and these have not been previously addressed. We show that these inconsistencies were due to only periodic sampling of the analytic solution, and an insufficient number of iterations in the numerical solutions, and we give corrected results. Additionally, some of the more significant analytical results, which were once thought to be impractical due to their complexity, are numerically implemented. We also show that a simple approximation accurately describes the behavior of these electrically short dipoles over a wide range of frequency and amplitude     

Parameter estimation with multiple sources and levels ofuncertainties

Least-squares designs are sensitive to errors in the data, which can be due to several factors including the approximation of complex models by simpler ones, the presence of unavoidable experimental errors when collecting data, or even due to unknown or unmodeled effects. We formulate a new design criterion that treats multiple sources of uncertainties in the data with possibly varied degrees of intensity. We show that the solution has a regularized form, with one regularization parameter for each source of uncertainty. The parameters turn out to be model dependent and can be determined optimally as the nonnegative roots of certain coupled equations. Applications in array signal processing and image processing are considered     

Electron scattering from acoustic phonons in quantum dots and other nanostructures

Scattering of electrons from acoustic phonons in semiconductor nanostructures occurs via an interaction, which we call the “ripple mechanism”, in addition to the usual deformation potential coupling. We provide a general derivation of this novel coupling mechanism and give detailed expressions for it which are valid for all nanostructure systems, including those with quasi-zero-, one- and two-dimensional geometries. Calculations are presented here of the electron scattering rates due to acoustic phonons for quantum dots in a variety of shapes. We find that scattering due to the ripple mechanism dominates that from the deformation potential for dot sizes less than 50 nm and that the ripple mechanism contribution can be much larger for smaller dot sizes.     

A study of bounds on limit cycles in digital filters

We give bounds on the self-sustained limit cycles in fixed-poin implementations of state-variable-form digital filters having general stable system matrices. These bounds, which are period independent and which concern limit cycles due to quantization errors in physical realizations of digital filters, are given in closed form for second-order sections. A numerical comparison shows that the bounds are smaller than corresponding ones reported in the literature. This is due to our use of real similarity transformations.     

Correlation estimation from compressed images

This paper addresses the problem of correlation estimation in sets of compressed images. We consider a framework where the images are represented under the form of linear measurements due to low complexity sensing or security requirements. We assume that the images are correlated through the displacement of visual objects due to motion or viewpoint change and the correlation is effectively represented by optical flow or motion field models. The correlation is estimated in the compressed domain by jointly processing the linear measurements. We first show that the correlated images can be efficiently related using a linear operator. Using this linear relationship we then describe the dependencies between images in the compressed domain. We further cast a regularized optimization problem where the correlation is estimated in order to satisfy both data consistency and motion smoothness objectives with a Graph Cut algorithm. We analyze in detail the correlation estimation performance and quantify the penalty due to image compression. Extensive experiments in stereo and video imaging applications show that our novel solution stays competitive with methods that implement complex image reconstruction steps prior to correlation estimation. We finally use the estimated correlation in a novel joint image reconstruction scheme that is based on an optimization problem with sparsity priors on the reconstructed images. Additional experiments show that our correlation estimation algorithm leads to an effective reconstruction of pairs of images in distributed image coding schemes that outperform independent reconstruction algorithms by 2–4 dB.     

Design of an effective loss-distinguishable MAC protocol for 802.11 WLAN

In the IEEE 802.11 WLAN standard, the backoff algorithm assumes that all losses are due to collisions, while the automatic rate fallback algorithm assumes that all losses are due to link errors. The coexistence of these two types of losses in real networks reduces the efficiency of currently used algorithms. We propose a loss-distinguishable MAC layer protocol for 802.11 WLAN. No PHY layer modification is needed. Analysis shows that the new protocol is effective and has negligible overhead.     

Design and analysis of a propagation delay tolerant ALOHA protocol for underwater networks

Acoustic underwater wireless sensor networks (UWSN) have recently gained attention as a topic of research. Such networks are characterized by increased uncertainty in medium access due not only to when data is sent, but also due to significantly different propagation latencies from spatially diverse transmitters—together, we call these space-time uncertainty. We find that the throughput of slotted ALOHA degrades to pure ALOHA in such an environment with varying delay. We therefore propose handling this spatial uncertainty by adding guard times to slotted ALOHA, forming Propagation Delay Tolerant (PDT-)ALOHA. We show that PDT-ALOHA increases throughput by 17–100% compared to simple slotted ALOHA in underwater settings. We analyze the protocol’s performance both mathematically and via extensive simulations. We find that the throughput capacity decreases as the maximum propagation delay increases, and identify protocol parameter values that realize optimal throughput. Our results suggest that shorter hops improve throughput in UWSNs.     

Improving protocol robustness in ad hoc networks through cooperative packet caching and shortest multipath routing

A mobile ad hoc network is an autonomous system of infrastructure-less, multihop, wireless mobile nodes. Reactive routing protocols perform well in this environment due to their ability to cope quickly against topological changes. This paper proposes a new routing protocol named CHAMP (caching and multiple path) routing protocol. CHAMP uses cooperative packet caching and shortest multipath routing to reduce packet loss due to frequent route failures. We show through extensive simulation results that these two techniques yield significant improvement in terms of packet delivery, end-to-end delay and routing overhead. We also show that existing protocol optimizations employed to reduce packet loss due to frequent route failures, namely local repair in AODV and packet salvaging in DSR, are not effective at high mobility rates and high network traffic.     

Impact of mobility on TCP/IP: an integrated performance study

This paper presents a simulation analysis of the impact of mobility on TCP/IP augmented with features to support host mobility in wide area networks. Our results show that the existing version of TCP can yield low throughput in highly mobile environments due to the fact that TCP cannot discriminate packets dropped due to hand-offs with those dropped due to congestion in one or more network resources. As a result, TCP invokes a congestion recovery process when packets are lost during internetwork hand-offs of the mobile host. We investigate a proposal in which the transport layer explicitly receives information from the network layer of any ongoing mobility. We show that by effectively capitalizing this information, TCP can appropriately extend the slow-start phase in the recovery process and achieve higher throughput. Based on the simulation analysis we also show the robustness of this scheme in the presence of both host mobility and network congestion     

Negative bias temperature instability: What do we understand?

We present a brief overview of negative bias temperature instability (NBTI) commonly observed for in p-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) when stressed with negative gate voltages at elevated temperatures and discuss the results of such stress on device and circuit performance and review interface traps and oxide charges, their origin, present understanding, and changes due to NBTI. Next we discuss some of the models that have been proposed for both NBTI degradation and recovery and p- versus n-MOSFETs. We also address the time and energy dependence effects of NBTI and crystal orientation. Finally we mention some aspect of circuit degradation. The general conclusion is that although we understand much about NBTI, several aspects are poorly understood. This may be due to a lack of a basic understanding or due to varying experimental data that are likely the result of sample preparation and measurement conditions.     

On the Throughput Capacity of Cooperative Multi-hop Wireless Ad hoc Networks with Multi-flow

Cooperative relaying techniques are known to provide spatial diversity for wireless fading channels. In contrast to non-cooperative transmission (direct transmission), they increase link reliability, provide higher capacity, reduce transmit power, and extend transmission range. Mostly the gains of cooperative relaying are shown for single flow scenarios in the absence of inter-flow interference. In this paper we study the effect of inter-flow interference on the capacity of cooperative networks in multi-hop multi-flow settings. We used the conflict graph to model the interference and find the additional constraints introduced due to cooperative transmission by using the concept of cliques on the conflict graph, which can be used to capture the interference relation among links. We formulated the multi-commodity flow problem for network capacity using linear programming, and employed a clique based analysis of the conflict graph to compute interference constraints. It is observed that the throughput drops significantly when cooperative transmissions are used in the network. We also found that the hop counts increase when cooperative links are used that is due to avoiding interfering links, which results in losing the benefits of shortest-path routing.     

Impact of device physics on DG and SOI MOSFET linearity

The influence of different physical mechanisms on MOSFET linearity is analyzed using 2D TCAD device simulations. In particular, the RF linearity performance of 50 nm gate length SOI and DG-MOSFETs are investigated and compared with traditional bulk MOSFETs. We employ the hydrodynamic (HD) transport model to account for non-equilibrium carrier dynamics and the density gradient approximation for quantum mechanical effects. Impact ionization of channel carriers and self-heating effect (SHE) are also accounted for in the thin-body devices. Our results disclose the relationship between various aspects of device physics and linearity. We show that linearity performance is particularly sensitive to non-local effects and are lowered due to SHE. Quantum mechanical effects appear to have a small positive impact on linearity. Drift-diffusion approximation is found to be unreliable for linearity analysis of DG MOSFETs due to large overestimation from this model. We also observe that linearity has an anomalous monotonous dependence on the ambient temperature.     

Characterization of soft breakdown in thin oxide NMOSFETs based onthe analysis of the substrate current

We have investigated the properties of soft breakdown (SBO) in thin oxide (4.5 nm) nMOSFETs with measurements of the gate and substrate leakage currents using the carrier separation technique. We have observed that, at lower gate voltages, the level of the substrate current exhibits a plateau. We propose that the observed plateau is due to the Shockley-Hall-Read (SHR) generation of hole-electron pairs in the space charge region and at the Si-SiO₂ interface. At higher voltages, the substrate current steeply increases with voltage, due to a tunneling mechanism, trap-assisted or due to a localized effective thinning of the oxide, from the substrate valence band to the gate conduction band, which becomes possible for gate voltages higher than the threshold voltage. The proposed interpretation Is consistent with the results of measurements performed at different operating conditions, in the presence of light and in the case of substrate reverse bias. The presented results are also useful for characterizing the performance of MOSFETs after SBD     

Theory of mode-locked semiconductor lasers

We present a theoretical study of semiconductor mode-locked lasers at a phenomenological level. We use the slow absorber model of New and Haus, but extend the analysis by taking into account the shift in the gain maximum due to the changing number of carriers. In our analysis of the resulting equation, we show that due to stability requirements the shortest attainable pulse duration is limited. The use of self-phase modulation due to a slow medium can at most lead to a shortening of the pulse by a factor of 2. We show that the shifting of the gain maximum due to the changing number of carriers in the laser is a crucial aspect in the operation of mode-locked semiconductor lasers. We further find that the end mirrors must be reflecting more than about half of the light intensity to prevent a self-pulsation type instability similar to the relaxation oscillation     

LED pulse distortion in single-mode fibres near the wavelength of zero dispersion

We show that the broadening of LED pulses in single-mode fibres due to higher-order dispersive effects, which are significant near the wavelength of zero dispersion, is considerably less than that predicted on the basis of RMS theory.     

Overlay networks of in situ instruments for probabilistic guarantees on message delays in wide-area networks

Messages transported over wide-area networks are subject to various delays at the hosts and intermediate nodes. In addition to bandwidth limits, the delays have an apparent "random" component due to the complicated dynamics of the network traffic. We consider that the messages sent over the network are subjected to three types of delays: 1) propagation delays along the links; 2) delays due to bandwidth of the links; 3) "other delays" at the hosts and intermediate nodes which are randomly distributed according to unknown distributions. We propose an overlay network of in situ instruments on such a network to collect delay measurements, compute paths and route messages. We propose regression methods to compute a path whose message delay is close to the optimal expected delay with a high probability, based entirely on measurements. The delay distributions are arbitrary and this guarantee is the best kind possible for this network. We then present a simple multiple path method for achieving low end-to-end delays. This overlay network is implemented over the Internet using user-level daemons that realize paths among themselves without explicit support from the underlying network routers. Internet measurements show that this method achieves significantly higher aggregated bandwidths compared with the default paths.     

Call performance for a PCS network

It is well known that, due to the mobility of a portable and limited channel availability, calls of portables may not be completed due to being blocked or terminated during the call initiation or the handover process. The characteristics of the call-completion and call-holding times for both a complete call and an incomplete call are of critical importance for establishing the actual billing process in the PCS network. We derive the call-completion probability (hence, call-dropping probability) and the effective call-holding time distributions for complete/incomplete calls with a general cell-residence time and a general call-holding time are analyzed, and general computable formulas are obtained. We show that when call-holding times are Erlang distributed, easy-to-compute formulas for the probability of a call completion and the expected effective call-holding times for both a complete call and an incomplete call can be derived     

Characterization of wavelength-selective fiber-optic devices usinga modified phase-shift method

We present a novel measurement setup that can be used for the complete characterization of fiber Bragg gratings and wavelength selective fiber-optic devices. Our setup is based on the phase-shift method (PSM), which we have modified to include the measurement of polarization-induced effects such as polarization-dependant loss (PDL) and polarization-mode dispersion (PMD). We measure the spectral response of devices used in transmission and in reflection, the wavelength dependency of the group delays due to chromatic and polarization-mode dispersion, and the wavelength dependency of the polarization-dependent loss. Experimental results are presented and sources of error are discussed. Comparisons with the Jones matrix eigenanalysis method for the measurement of PDL and differential group delay due to PMD have been carried out