Preface
The idea for this book was suggested to me by my colleague and friend Sergio Palazzo, who invited me to deliver a short course on “Mobility Models and Social Networks” at the 2010 Lipari Summer School on Mobile Computing and Communications. At the end of the course, when delivering his closing remarks, Sergio said that in his opinion the topic of the course was very interesting, and concluded with what sounded like “Why don't you write a book on this?” I have to admit that I did not take his suggestion too seriously, since I was sure that, given its importance within the field of mobile computing and networking, several books on the topic had already been published. However, I took his suggestion seriously enough to devote half a working day when I was back in the office to look up the published books on mobility models for short-range wireless networks. To my great surprise, the result of my Web-based research showed that, except for a few book chapters and survey papers, there was no book entirely focused on mobility modeling. After that morning, the idea of writing a book on this topic started to develop further, and I prepared a book proposal for submission and approval by John Wiley & Sons. Thus, a year and a half after the Lipari Summer School (it will be two years at the time of publication), Sergio's suggestion has become a reality.
As mentioned above, the general idea for the book and its organization is adapted from the short course I delivered at Lipari Summer School in 2010. In turn, the short course was an extension of an invited talk on the “Mathematics of Mobility” that I delivered at the Workshop on Mathematical Aspects of Large Networks in Barcelona, in 2009. Of course, the material contained in this book has been considerably extended and revised with respect to these presentations.
The aim of the book is to provide an exhaustive coverage of mobility models for next generation wireless networks. Some clarification about terminology is in order before proceeding further. By exhaustive, I mean that all topics related to mobility modeling are touched upon in this book, ranging from the theoretical characterization of stationary properties of mobility models to the design of mobility models aimed at resembling mobility features observed in the real world. “Exhaustive” does not mean that each mobility model presented in the wireless networking and computing literature is reported in this book. Instead, for each topic considered, a few, representative specific models are presented, while other mobility models are simply mentioned for reference at the ends of the relevant chapters. By next generation wireless networks, I mean short-range, mostly infrastructure-less wireless networks based on existing or forthcoming technology (e.g., WiFi, Bluetooth, ZigBee, etc.), which will complement traditional, infrastructured wireless networks (e.g., cellular) and make possible the realization of the ubiquitous computing paradigm. Application scenarios of next generation wireless networks include wireless local area network (WLAN)/mesh networks, vehicular networks, wireless sensor networks, and opportunistic networks. These innovative networks will allow us to gain ubiquitous Internet access, to improve safety and traffic conditions on the roads, to realize smart environmental monitoring, messaging with friends through multi-hop, phone-to-phone communication, etc. Since user mobility is a salient feature of next generation wireless networks, how to model user movement becomes a fundamental part of the wireless network performance evaluation process, which explains the considerable efforts devoted in recent years by the research community to understand, characterize, and model mobility. The goal of this book is to give the reader an organic view of this body of the literature, and to present mobility modeling as a scientific discipline, encompassing both theoretical and practical aspects related to this challenging task as part of the network performance evaluation process.
Audience
This book is intended for graduate students, researchers, engineers, and practitioners who are interested in acquiring a global view of the discipline of mobility modeling, which is at the basis of wireless network performance evaluation through analysis, simulation, or testbed-based data collection. In general, the book can serve as a reference resource for researchers, engineers, professionals, and developers working in the field of short-range wireless networking.
Although I have tried to make the book as self-contained as possible, including a brief introduction to short-range wireless networking and state of the art presentations of the application scenarios (WLAN/mesh networks, vehicular networks, etc.) considered, the reader is assumed to be familiar with basic concepts in wireless networking, as well as those in graph and probability theory.
Book Overview
The material contained in this book is organized as follows.
The first part of the book (Introduction) presents introductory material that is preparatory for what is described in the rest of the book.
Chapter 1 introduces next generation wireless networks, briefly describing the possible applications of these technologies in the near future, as well as the challenges that the designers of such networks will face.
Chapter 2 presents simple, yet widely accepted, models for next generation wireless networks. In particular, principles underlying radio signal propagation in the air are described in this chapter, as well as basic models for describing the occurrence of wireless links between network nodes, and energy consumption of a wireless node.
Chapter 3 motivates the need for mobility modeling in next generation wireless networks, clearly describing the importance of mobility modeling in the performance evaluation process. The chapter also describes the fundamental difference between mobility models for cellular vs. next generation wireless networks. Finally, the chapter introduces a taxonomy of existing mobility models for next generation wireless networks, and presents the CRAWDAD community resource for archiving wireless data.
The second part of the book (“General-Purpose” Mobility Models) presents the most representative mobility models that are not tailored to specific application scenarios.
Chapter 4 presents the well-known class of random walk models, which have been applied in several scientific disciplines. After describing some of the most representative variants of this model, the chapter discusses features of random walk mobility in the context of next generation wireless networks. Finally, this chapter presents some important theoretical properties of random walk models.
Chapter 5 is devoted to the most widely used mobility model for short-range wireless networks, namely, the random waypoint model. After describing the model and some of its variants, the chapter thoroughly discusses important properties of random waypoint mobility, such as node spatial distribution and average node speed.
Chapter 6 presents representative models of group mobility, in which the mobility pattern of a node is correlated with that of other nodes in the network (typically, with that of surrounding nodes). The chapter also presents other general-purpose mobility models, such as the smooth random mobility model and the Gauss–Markov mobility model, in which directional and speed changes of a node are smooth instead of abrupt, as in the mobility models presented in the previous chapters.
Chapter 7 introduces the important class of random trip models and presents results concerning the existence and characterization of a stationary regime for any mobility model belonging to this class. The class of random trip models includes, among others, some of the mobility models presented in the previous chapters, such as the random waypoint and random walk models.
The third part of the book (Mobility Models for WLAN and Mesh Networks) presents the most representative mobility models for WLAN and mesh networks.
Chapter 8 describes state of the art user scenarios and the prospects for WLAN and mesh networks.
Chapter 9 presents and discusses the extensive research work that has been performed in analyzing real-world WLAN traces and in deriving prominent features of WLAN mobility. The material presented in this chapter is fundamental for providing guidelines on the design of mobility models aimed at resembling movement patterns of WLAN users, which are reported in Chapter 10.
Chapter 10 presents representative WLAN mobility models, belonging to two different categories. The first model, called LH, aims at reproducing user/access point registration patterns observed in real-world WLAN traces. In this model, the interest is in reproducing WLAN usage patterns rather than the physical movement of users. The second model presented in the chapter, called KKK, is instead aimed at modeling the physical mobility of a WLAN user.
The fourth part of the book (Mobility Models for Vehicular Networks) presents the most representative mobility models for vehicular networks.
Chapter 11 describes state of the art user scenarios and the prospects for vehicular networks.
Chapter 12 describes the differences between the macroscopic and microscopic approaches to vehicular mobility modeling, and explains why microscopic mobility models are more relevant for vehicular network performance evaluation.
Chapter 13 presents and discusses in detail representative microscopic mobility models for vehicular movement, and in particular the simulation of urban mobility (SUMO) model. The chapter ends with a discussion of the challenges related to integrating vehicular mobility and wireless network simulation, and presents a tool designed for this purpose.
The fifth part of the book (Mobility Models for Wireless Sensor Networks) presents the most representative mobility models for wireless sensor networks.
Chapter 14 describes state of the art user scenarios and the prospects for wireless sensor networks.
Chapter 15 categorizes mobility models for wireless sensor networks into models for passive and active mobility of sensor nodes. The chapter then presents representative mobility models for passive movement of wireless sensor nodes caused by external forces like ocean flows, animal movements, etc.
Chapter 16 introduces models for active movement of wireless sensor nodes, that is, movement of sensor nodes and/or data collection entities endowed with autonomous motion capabilities, able to control and optimize their movement pattern.
The sixth part of the book (Mobility Models for Opportunistic Networks) presents the most representative mobility models for opportunistic networks.
Chapter 17 describes state of the art user scenarios and the prospects for opportunistic networks.
Chapter 18 describes the basic mechanisms of message routing in opportunistic networks, as well as basic concepts in opportunistic network mobility models.
Chapter 19 introduces basic notions of social network analysis, and presents fundamental properties that have been observed when analyzing human individual mobility data.
Chapter 20 describes mobility models aimed at reproducing human individual mobility patterns, which are known to be heavily influenced by social relationships between individuals.
The seventh part of the book (Case Studies) presents two case studies showcasing the importance of mobility modeling in the wireless network performance evaluation process.
Chapter 21 starts with a discussion of the implications of stationary properties of the random waypoint mobile networks on wireless network simulation accuracy. Then, the chapter introduces techniques aimed at improving simulation accuracy—the so-called “perfect simulation” methodology.
Chapter 22 discusses the effects of different assumptions regarding node mobility on conclusions that can be drawn about the asymptotic performance of opportunist networks.
Finally, this book contains two appendices, collecting basic definitions and notions of the theories underlying the field of mobility modeling. In particular, Appendix A.1 collects basic notions of probability and stochastic process theory, while Appendix B.1 introduces the asymptotic notation and then collects basic definitions from graph theory and miscellaneous notions that have been used in the book.
How To Use This Book
Informally speaking, the first part of the book provides basic concepts and definitions related to next generation wireless networks and mobility modeling that will be used in the rest of the book. While a reader who is familiar with the field of wireless networking can probably skip Chapter 1, she/he should probably not miss Chapters 2 and 3, which introduce the network models and basic mobility concepts used in the remainder of the book.
The second part of the book can be considered as its fundamental part, which the reader should not skip. In fact, in this part important classes of mobility models such as random walk and random waypoint models are introduced and studied, and important notions such as stationarity of a mobility model are presented. The mobility models and notions introduced in this part will be repeatedly called upon in the rest of the book.
The next four parts of the book can be treated as nearly independent, stand-alone parts, each one focusing on a specific application scenario: WLAN/mesh networks (Part Three), vehicular networks (Part Four), wireless sensor networks (Part Five), and opportunistic networks (Part Six). To a large extent, these parts are independent of each other with minimal cross-referencing, and the reader interested in a specific application scenario could focus on the corresponding part only.
Finally, the last part of the book presents two case studies. The first one refers to the well-known random waypoint mobility model, and requires a careful reading of Chapters 5 and 7 at least. The second case study focuses on opportunistic networks, and a careful reading of the corresponding part of this book is a prerequisite for fully understanding the material presented there.
The two appendices at the end are intended to provide a unique reference point for the concepts and notions from probability theory, stochastic process theory, graph theory, etc., used in the book: if the reader is not sure about a certain notion mentioned in the text, she/he can refer to the appropriate appendix to find a formal definition. In the same vein, I have included a list of the abbreviations used in the book.