P, enterprise, or SMB customers were outlined. This chapter provided some details of the functions and inter- connect behavior of the various components. Some of the differences in the behavior when a component, such as a SBC, is deployed at a SP versus when the same device is used in an enterprise or SMB environment were discussed. Though all the components are not required for every deployment, when shown together, they illustrate the majority of the required interconnects of a solution. Figure 5-10 includes all the components described in this chapter for both enterprise and SP networks. 80 SIP Trunking LI and Supplementary Billing Server Services Call Agent Emergency Media Peer SIP Networks Enterprise Enterprise Firewall NAT ALG Voice IP Voice TDM Gateway Gateway Enterprise Enterprise Inter- Call Agent or application Session Manager Small IP PBX/TDM Customers Large Customers Customers Figure 5-10 Components of SIP Trunking Solutions Chapter 6 SIP Trunking Models This chapter covers the following topics: Understanding the traditional PSTN gateway connection model Choosing a SIP trunking model Centralized model Distributed model Hybrid model Considering trade-offs with centralized and distributed models Understanding the centralized model with direct media model This chapter explains the network topology alternatives for connecting a Session Initiation Protocol (SIP) trunk from a service provider (SP) into an enterprise network for Public Switched Telephone Network (PSTN) access. The sections of the chapter discuss the different methods of connecting, guide you through choosing which model is the best fit for your network, and describe the implications each model has on your network archi- tecture and implementation. The two fundamental models used for SIP trunking follow Centralized model: Calls from all sites in the network are routed to a central site through your Internet Protocol (IP) WAN and use a single, shared SIP trunk at a cam- pus or headquarters site. Distributed model: Calls from each local site to