Realistic mobility simulation of urban mesh networks

It is a truism that today’s simulations of mobile wireless networks are not realistic. In realistic simulations of urban networks, the mobility of vehicles and pedestrians is greatly influenced by the environment (e.g., the location of buildings) as well as by interaction with other nodes. For example, on a congested street or sidewalk, nodes cannot travel at their desired speed. Furthermore, the location of streets, sidewalks, hallways, etc. restricts the position of nodes, and traffic lights impact the flow of nodes. And finally, people do not wander the simulated region at random, rather, their mobility depends on whether the person is at work, at lunch, etc. In this paper, realistic simulation of mobility for urban wireless networks is addressed. In contrast to most other mobility modeling efforts, most of the aspects of the presented mobility model and model parameters are derived from surveys from urban planning and traffic engineering research. The mobility model discussed here is part of the UDel Models, a suite of tools for realistic simulation of urban wireless networks. The UDel Models simulation tools are available online.     

Performance Improvements Provided by Route Diversity in Multihop Wireless Networks

In multihop wireless networks, the variability of channels results in some paths providing better performance than other paths. Although it is well known that some paths are better than others, a significant number of routing protocols do not focus on utilizing optimal paths. However, cooperative diversity, which is an area of recent interest, provides techniques for efficiently exploiting path and channel diversity. This paper examines the potential performance improvements offered by path diversity. Three settings are examined, namely, where the path loss and channel correlation are neglected, where path loss is considered, but channel correlation is neglected, and where path loss and channel correlation are both accounted for. It is shown that, by exploiting path diversity, dramatic improvements in the considered route metric may be achieved. Furthermore, in some settings, if the link statistics are held constant, then when path diversity is exploited, the route metric improves with path length. This implies that, if links statistics are fixed and if sufficient path diversity exists, then paths with more hops tend to support higher bit rates than paths with fewer hops. It is shown that such behavior occurs when a particular map has a nonzero fixed point.     

Structural stability of linear dynamically varying (LDV) controllers

LDV systems are linear systems with parameters varying according to a nonlinear dynamical system. This paper examines the robust stability of such systems in the face of perturbations of the nonlinear system. Three classes of perturbations are examined: differentiable functions, Lipschitz continuous functions and continuous functions. It is found that in the first two cases the system remains stable. Whereas, if the perturbations are among continuous functions, the closed-loop may not be asymptotically stable, but, instead, is asymptotically bounded with the diameter of the residual set bounded by a function that is continuous in the size of the perturbation. It is also shown that in the case of differential perturbations, the resulting optimal LDV controller is continuous in the size of the perturbation. An example is presented that illustrates the continuity of the variation of the controller in the case of a nonstructurally stable dynamical system.     

A new TCP for persistent packet reordering

Most standard implementations of TCP perform poorly when packets are reordered. In this paper, we propose a new version of TCP that maintains high throughput when reordering occurs and yet, when packet reordering does not occur, is friendly to other versions of TCP. The proposed TCP variant, or TCP-PR, does not rely on duplicate acknowledgments to detect a packet loss. Instead, timers are maintained to keep track of how long ago a packet was transmitted. In case the corresponding acknowledgment has not yet arrived and the elapsed time since the packet was sent is larger than a given threshold, the packet is assumed lost. Because TCP-PR does not rely on duplicate acknowledgments, packet reordering (including out-or-order acknowledgments) has no effect on TCP-PR's performance. Through extensive simulations, we show that TCP-PR performs consistently better than existing mechanisms that try to make TCP more robust to packet reordering. In the case that packets are not reordered, we verify that TCP-PR maintains the same throughput as typical implementations of TCP (specifically, TCP-SACK) and shares network resources fairly. Furthermore, TCP-PR only requires changes to the TCP sender side making it easier to deploy.     

Performance of 802.11 b/g in the interference limited regime

Signal to interference plus noise ratio, SINR, is one of the main factors that affects the quality of wireless communication. While the impact of white Gaussian noise on a wireless channel is well understood, impact of interference remains one of the less explored areas. With the deployment of dense mesh networks, the interference will be a dominant factor that affects the transmission errors. This paper explores the performance of 802.11 b/g when subject to interference. The findings are based on various controlled experiments in the laboratory setting. One finding of this work is that in contrast to communication over links where the noise is Gaussian, in 802.11 b/g, the probability of successfully transmitting a packet is dominated by the ability of the receiver to synchronize with the carrier. As a result, changing to a lower bit-rate with same synchronization scheme will not make the transmission more resilient to interference. Moreover, we found that the impact of interference is significant even if the interference is 10 dB below the strength of the noise. The significance of this result on bit-rate selection is briefly explored.     

Computational aspects of optimal schedules with power control and multiple bitrates selection in multihop wireless networks

A significant amount of research on optimal scheduling has focused on the case where links can transmit with only a single bit-rate and a single transmission power. This paper develops techniques to accommodate multiple bit-rates and multiple transmission powers as well as a continuum of transmission powers. Optimizing over a large number of bit-rates and transmission powers typically increases the computed throughput at the expense of increasing the computation time. This paper examines the trade-off between computed throughput and computation time, with emphasis on the case when the relationship between bit-rate and SINR is the same as it is for 802.11a/g. One finding of this paper is a progression of Pareto optimal schemes that increases the computed capacity. A second finding is that optimizing over two transmission powers and two or three bit-rates achieves nearly the same throughput as optimizing over a continuum of transmission powers and all supported bit-rates. However, optimizing over a continuum of transmission powers and all supported bit-rates is far more computationally complex than optimizing over two transmission powers and a few bit-rates.     

Investigation of effect of electrode polarity on electrochemistry and magnetohydrodynamics using tertiary current distribution in electroslag remelting process

Transport phenomena including the electromagnetic,concentration of ions,flow,and thermal fields in the electroslag remelting (ESR) process made of slag,electrod...     

A Numerical Investigation on the Electrochemical Behavior of CaO and Al2O3 in the ESR Slags

Metallurgical and Materials Transactions B - Field structures including electromagnetic, concentration of ions, and flow fields in an ESR-like process composed of a graphite crucible containing a...