fferent volumes of Voice over IP (VoIP) traffic across your enterprise IP network. The centralized model is also referred to as the aggregated model because it aggregates all calls from all sites and funnels them into a single SP entry point. Centralized SIP trunking is conceptually simple, and customers often gravitate to this early in their analysis of SIP trunking services. However, it looks better on paper than it does in a real network deployment. The trade-offs and points of analysis are discussed in more detail later in this chapter in the "Considering Trade-Offs with the Centralized and Distributed Models," section that compares the centralized and distributed models. Distributed Model The distributed model of SIP trunking predicates that each site has its own SIP trunk ter- mination of the appropriate capacity for calls to and from that site. Figure 6-3 depicts the network topology of an enterprise with one campus location and several remote sites with a dedicated SIP trunk terminating into each site, a model that constitutes distributed trunking. 86 SIP Trunking Softswitch Media Gateway SP VolP Session Border Campus Controller Border Element Site-SP RTP Integrated Service Router with Border Branch Element Offices Figure 6-3 Distributed SIP Trunk Model The distributed model is similar to a traditional TDM PSTN trunking model where each site has its own dedicated voice gateway. Because the placement of entry points into the network are the same as for TDM PSTN access, your dial plan and the routing of voice paths on your network typically do not change as you migrate from TDM access to SIP access for PSTN calls. However, your Cisco Unified Communications Manager (CUCM) or IP-PBX might need to be adjusted to do source-based routing (Calling Search Spaces if you use CUCM) in addition to routing calls based on the dialed destination number. Distributed SIP trunking is often attractive because it means virtually no changes to your network design and bandwidth provisioning as you migrate from TDM to IP trunkin