. How This Book Is Organized Although this book can be read cover-to-cover, it is designed with increasing levels of technical depth and practical network implementation as the chapters progress, geared toward readers of different interest levels. If you are an overview or trends reader, the chapters in Part I are for you: Chapter 1, "Overview of IP Telephony": Provides the introduction to IP Telephony and its essential components such as voice compression technologies, multiple voice signaling protocols, PBXs, Digital Signal Processing (DSPs) and TDM trunks, including an introduction to the latest development in IP Telephony and SIP Trunks. Chapter 2, "Trends in IP Telephony": IP telephony has traditionally been about con- necting new IP endpoints, such as IP PBXs from businesses or IP phones for con- sumers, to the traditional TDM-based PSTN. The latest trend is for the service providers (SP) to offer IP-based PSTN connectivity services or trunks. This enables traditional IP PBX customers, in both large and small businesses, to purchase IP trunking services as opposed to traditional TDM trunking services. This chapter pro- vides an overview of these industry migrations. Chapter 3, "Transitioning to SIP Trunks": Explains the stages that a company needs to plan or execute when transitioning its current systems from one that uses TDM or H.323 trunks to a system that uses SIP trunks, either for intra-application connec- tions or for PSTN access connectivity. Chapter 4, "Cost Analysis": Focuses on the cost structure of deploying the SIP trunks, one of the major criteria to select the SIP trunk service provider. If you are a network planner or designer, Part II's chapters are appropriate reading, with or without necessarily having read Part I, or perhaps reading them out of sequence: Chapter 5, "Components of SIP Trunks": Describes the major components used to deploy SIP trunks including call agents, session managers, session border con- trollers, billing servers, network infrastructure components, media gateways, an