Access ISDN/Frame Relay Server Software Workstation Server Switch Token Token Ring Terminal Ciscoworks Modem Server Server Workstation Switch Printer Laptop Front End Cluster Multilayer Mainframe Processor Controller Switch DSU/CSU Gateway Router Bridge DSU/CSU Catalyst Switch Network Cloud Line: Ethernet Line: Serial Line: Switched Serial Command Syntax Conventions The conventions used to present command syntax in this book are the same conventions used in the IOS Command Reference. The Command Reference describes these conven- tions as follows: Boldface indicates commands and keywords that are entered literally as shown. In actual configuration examples and output (not general command syntax), boldface indicates commands that are manually input by the user (such as a show command). Italic indicates arguments for which you supply actual values. Vertical bars (I) separate alternative, mutually exclusive elements. Square brackets ([]) indicate an optional element. Braces ({}} indicate a required choice. Braces within brackets ([{ }]) indicate a required choice within an optional element. Introduction Today is an unusually interesting time in the field of unified communications as we stand at the dawn of a major industry transition in how communications flow between busi- nesses. The network implementation to enable this communications flow is drastically changing in terms of technology, products, connectivity, service provider offerings, and cost. One of these changes is the transition to Public Switched Telephone Network (PSTN) trunking technology. The industry is active with discussions of Session Initiation Protocol (SIP) trunking, but the appreciation of its real implications is just beginning. This book is written to help you understand and navigate these uncertain and fast-changing waters. Deciding when to change your network to take advantage of SIP trunking, how much of your network to change, and at what pace are all daunting decisions, and there might not be a single answer for all. The decisions you mak