 enterprise perspective. SIP trunks facilitate the communication of voice and video (and other real-time communications) over the Internet Protocol (IP) network within and between the realms of communications such as companies. SIP trunks are widely utilized to replace the tradi- tional Time Division Multiplexing (TDM) trunks to access the Public Switched Telephone Network (PSTN) from an Enterprise TDM or Internet Protocol Public Branch Exchange (IP-PBX). SIP trunks provide the platform for companies to communicate with other companies over IP networks. They also enable end-to-end IP communication to ensure rich media experience for end users. SIP trunking consists of several important components from the IP-PBX to the SP. This chapter describes the major components used to deploy SIP trunks within and outside of enterprises. This chapter also describes the differences in components for SIP trunks into large enterprises, medium-sized enterprises, and small-medium businesses. Although basic SIP technology knowledge is assumed for this book, a brief overview of basic SIP technology concepts such as SIP proxies, redirect servers, and back-to-back user agents are also given. SP Network Components SPs that offer a SIP trunking service for PSTN access have specific requirements to offer a service that they can make a profit on and a service that customers find compelling 58 SIP Trunking enough to purchase. Because the ultimate goal of a SP is to generate profit, its network design decisions are different then the decision factors of enterprise customers. The details of their network components are described in the subsequent sections. SP Network-Edge Session Border Controllers One of the most important components of a SIP trunking solution for SPs is the SPs' edge Session Border Controller (SBC). The edge SBC can be defined as the SBC that the SP uses to connect to end customers that it offers services to. One of the primary func- tions of the edge SBC is the capability to terminate the different IP addresses that each