g. It is essentially a one-to-one upgrade, and you can convert each site, or a group of sites, with little to no effect on the rest of the network or existing dial plan routing. The impact of a SIP trunk outage is also minimal to the network as only calls to and from that site are affected, unlike in the centralized model where a large number of sites in the network are impacted by a single SIP trunk failure. Hybrid Model Centralized trunking implies a single SIP trunk for your entire network, regardless of country and continental boundaries and delays of routing remote site voice streams through a potentially geographically distant central site. Distributed trunking implies every site has a SIP trunk, no matter how small. The five-to-ten-person office that makes one call at most in the busy hour hardly warrants a dedicated SIP trunk. The extremes of both the centralized and distributed trunking models are usually impractical for the average medium business or enterprise network. Chapter 6: SIP Trunking Models 87 The hybrid model depicted in Figure 6-4 is a middle ground that accrues most of the ben- efits of both models. Regional-SP RTP Branch-Campus RTP Softswitch Media Gateway SP VolP Session Border Campus Controller Border Element Integrated Branch Services Router Offices Figure 6-4 Hybrid SIP Trunk Model A number of sites in your network have a SIP trunk termination, but not all do. The desired number of multiple SIP trunk entry points might be small, in which case you would likely connect your major sites, or the largest site within each geographic region. Smaller sites, in particular geographies or regions, aggregate into a larger site with a SIP trunk in their area. Aggregating the calls from a site implies sufficient IP bandwidth must be available. If a number of remote sites have restricted IP bandwidth, you should consid- er connecting a SIP trunk into those sites to lower the number of calls and bandwidth crossing your WAN to the head-office site. This determination is similar to the way tradi- tional