aff, the voice organization might want to control the SIP trunk service whereas the data organization might not want this connection to risk its already existing and secured data infrastructure. This situation might be an argument for bringing in a separate physical connection that carries only the SIP trunk traffic and terminates on dedicated routers and border controllers supervised entirely by the voice team. Centralized Model As part of the TDM to IP transformation of PSTN trunking, many customers have a strong desire to consolidate all trunking into a central point in the network to gain opera- tional cost benefits. SIP trunking is seen as a way to achieve this goal because it has few geographic and size (how many channels can be delivered on the provider connection) limitations. The centralized model of SIP trunking predicates that a single SIP trunk, of the appropri- ate capacity for all calls to and from all sites in the enterprise, is delivered on a single physical connection. By virtue of serving the entire enterprise, this SIP trunk typically terminates in a large campus or data center site and carries hundreds or thousands of simultaneous channels. Figure 6-2 depicts the network topology of an enterprise with one campus location and several remote sites with a single, or aggregated, SIP trunk. Chapter 6: SIP Trunking Models 85 Softswitch Media Gateway SP VolP Session Border Campus Controller Border Element HQ-SP RTP Branch-HQ RTP Integrated Branch Service Router Offices Figure 6-2 Centralized/Aggregated SIP Trunk Model Calls to or from the site that terminates the SIP trunk are routed much as they were for TDM PSTN trunking. Remote offices, however, no longer have any direct PSTN connec- tivity, and all calls to and from the PSTN from remote offices are now routed (often referred to as hairpinning) through the central site to reach the SIP trunk entry point. This is usually a different call routing path from how TDM PSTN calls from the remote sites would have been routed and, therefore, constitutes di