Shyam Parekh

A quick simulation method for excessive backlogs in networks of queues

Excessive backlogs in stable open Jackson networks are studied. Although these events occur rarely, they can be critical, since they can impair the functioning of the network. The use of simulation to estimate their probability is attempted. Since a direct simulation of a rare event takes a very long time, a method is discussed for changing the network to speed up the simulation, using a heuristic method. It is shown by examples that the method can be several orders of magnitude faster than direct simulations. >

Economics of Femtocells

Femtocells or home base stations are a proposed solution to the problem of degraded indoor service from the macrocell base station in future 4G data networks. In this paper, we study user incentives for the adoption of femtocells and their resulting impact on network operator revenues. We model a monopolist network operator who offers the option of macrocell access or macro+femtocell access to a population of users who possess linear valuations for the data throughput. We compare the revenues from two possible spectrum schemes for femtocell deployment; the split spectrum scheme, where femtocells and macrocells operate on different frequencies and do not interfere, and, the common spectrum scheme, where they operate on the same frequencies (partially or fully) and interfere. Our results suggest that the optimal pricing scheme always charges a higher price for the femtocell service, i.e., the operator does not offer any subsidies for adoption. Yet, at the optimal prices, almost full adoption of femtocells is achieved even for many common spectrum schemes that degrade macrocell capacity. Femtocell deployments provide huge revenue gains when macrocell capacities are low. However, in this range, even common spectrum schemes that heavily degrade the macrocell capacity perform comparably to the split spectrum scheme. Some common spectrum schemes with moderate macrocell degradation yield revenues comparable or higher than the split spectrum scheme at all levels of macrocell congestion.

A Quality-Driven Cross-Layer Solution for MPEG Video Streaming Over WiMAX Networks

Extensive efforts have been focused on deploying broadband wireless networks. Providing mobile users with high speed network connectivity will let them run various multimedia applications on their wireless devices. Satisfying users with different quality-of-service requirements while optimizing resource allocation is a challenging problem. In this paper, we discuss the challenges and possible solutions for transmitting MPEG video streams over WiMAX networks. We will briefly describe the MPEG traffic model suggested by the WiMAX Forum. A cross-layer solution for enhancing the performance of WiMAX networks with respect to MPEG video streaming applications is explained. Our solution uses the characteristics of MPEG traffic to give priority to the more important frames and protect them against dropping. Besides, it is simple and compatible with the IEEE 802.16 standards and thus easily deployable. It is shown that the proposed solutions will improve the video quality over WiMAX networks.

Quality of Service Architectures for Wireless Networks: Performance Metrics and Management

Quality of Service Architectures for Wireless Networks: Performance Metrics and Management presents a comprehensive collection of quality of service mechanisms for various access technologies and introduces existing and traditional approaches to traffic management for wireless technologies. With expert international contributions, this critical mass of knowledge covers both academic and industry-based state-of-the-art techniques and ongoing research.

Some performance trade-offs associated with ATM fixed-length vs. variable-length cell formats

The authors compare asynchronous transfer mode (ATM) fixed-length-cell (FLC) versus variable-length-cell (VLC) formats with regard to memory requirements, delays, and efficiency considerations. In particular, they develop models for the comparison of these formats at an access multiplexer. Results show that at moderate outgoing link utilizations, the memory requirements for an FLC format are less than those for a VLC format. However, at higher outgoing link utilizations, the opposite is true, due to the inherent efficiency advantage of a VLC format. At moderate outgoing link utilizations, delays for short messages are found to be comparable. However, at higher outgoing link utilizations, a VLC format has an edge for short messages because of the efficiency advantage. It is concluded that the performance differences found here do not overwhelmingly favor either an FLC or a VLC format, and thus it is important to factor in other issues as well.<<ETX>>

Traffic-Aware Video Streaming in Broadband Wireless Networks

With increasing implementation of broadband wireless networks and extensive deployment of multimedia services such as Video on Demand (VoD) or IPTV, the demand for video streaming applications will increase and more people will use their wireless devices to reach numerous video contents available in the Internet. Streaming real-time video in wireless networks is a challenging problem due to the stringent service requirements of video traffic and impairments of wireless channels. Providing the required Quality of Service (QoS) through efficient resource allocation is a complicated problem that service providers are confronting. In this paper, we propose a traffic-aware, cross-layer solution for enhancing the perceived video quality at the end user in wireless networks. Our solution incorporates the characteristics of the MPEG traffic to give more priority to the more important frames and to protect them against dropping when available resources of the network are not sufficient for providing the desired QoS to the traffic flow. It is shown that the proposed solution will improve the perceived video quality over the broadband wireless networks.

Communication Networks: A Concise Introduction

This book results from many years of teaching an upper division course on communication networks in the EECS department at University of California, Berkeley. It is motivated by the perceived need for an easily accessible textbook that puts emphasis on the core concepts behind current and next generation networks. After an overview of how todays Internet works and a discussion of the main principles behind its architecture, we discuss the key ideas behind Ethernet, WiFi networks, routing, internetworking and TCP. To make the book as self contained as possible, brief discussions of probability and Markov chain concepts are included in the appendices. This is followed by a brief discussion of mathematical models that provide insight into the operations of network protocols. Next, the main ideas behind the new generation of wireless networks based on WiMAX and LTE, and the notion of QoS are presented. A concise discussion of the physical layer technologies underlying various networks is also included. Finally, a sampling of topics is presented that may have significant influence on the future evolution of networks including overlay networks like content delivery and peer-to-peer networks, sensor networks, distributed algorithms, Byzantine agreement and source compression. Table of Contents: The Internet / Principles / Ethernet / WiFi / Routing / Internetworking / Transport / Models / WiMAX & LTE / QOS / Physical Layer / Additional Topics

Instability and geometric transience of the aloha protocol

In this paper, we give a simple probabilistic proof to show that the discrete time Markov chain underlying the slotted uncontrolled Aloha protocol is geometrically transient. Let P be the (¿ × ¿) transition matrix of this Markov chain. Let Pn denote the northwest (n × n) corner truncation of P and ßn. its largest eigenvalue. We establish that, as a consequence of geometric transience, ß = lim ßn as n ¿ ¿, exists and that 0 < ß < 1. Note that the largest eigenvalue of P equals 1. We propose 1/(1-ß) as a performance measure which we show to be the limit of certain expected exit times.

DEDI: A framework for analyzing rank evolution of random network coding in a wireless network

We develop a framework called DEDI based on differential equations (DE) and differential inclusions (DI) to describe the rank evolution of random network coding (RNC). The DEDI serves as a powerful numerical and analytical tool to study RNC and we demonstrate this via numerical examples as well as an alternate proof of a well known result on RNC - a multicast at rate R exists if and only if a unicast at rate R exists separately for each destination.

Dynamic topology control in ad hoc networks with directional links

Networks emerging as increasingly significant to US DoD visions comprise dynamic, heterogeneous terrestrial, airborne, and/or space-based nodes, some of whose links are directional, variable performance, and inherently unstable. Our work investigates an approach for managing the topology of such ad hoc networks with free-space optical (FSO) links that optimizes network performance metrics, e.g., the total network throughput. Unlike previous approaches, our dynamic approach to topology control incorporates node motions and link quality to drive connectivity decisions and multiple topology updates accordingly in order to maintain connectivity and maximize throughput. In addition to incorporating the constraints and limitations of the network capabilities and resources, the approach features a model formulation of link impairment for a generic FSO system. We describe this model and show how it can be used to guide topology control decisions for optimal network performance. We then present a formulation of a multi-period, multi-layer (network and physical) topology control decision problem and compare example solutions to alternative approaches