 the PSTN use a local SIP trunk at that site. Several variations of these fundamental models are often a better fit for any particular network. However, it is important first to understand the characteristics of the fundamen- tal models and what these mean to your network before explaining the variations or deciding among them. In the sections following the initial discussion of the fundamental models, we explore the variations in more detail. 82 SIP Trunking Understanding the Traditional PSTN Gateway Connection Model Before diving into details of SIP trunking, it is instructive to review how traditional Time Division Multiplexing (TDM) PSTN trunking was done and why. Traditional TDM PSTN access from the enterprise network is accomplished with a voice gateway located at each site connected to the local PSTN central office. The PSTN trunk connectivity can be analog or digital, depending on the size of the site, the num- ber of channels of voice required, and the service available from the local provider. Calls to and from the PSTN are routed through the gateway and converted between TDM and IP by the voice gateway. Figure 6-1 shows a traditional PSTN trunk network connection topology. Campus Cisco Unified Communications Manager Voice Gateways 00000 00000 00000 10000 00000 00000 00000 00000 00000 00000 00000 00000 00000 Branch Offices Voice Gateway Figure 6-1 Traditional PSTN Trunk Connectivity Salient characteristics of traditional TDM connectivity from an enterprise network to the PSTN include Each site has a voice gateway and trunks to the PSTN. Local calls are almost always routed using the local site's PSTN voice gateway. Long distance calls might also be routed using this same gateway but are sometimes aggre- gated and routed via a central PSTN gateway at a campus site. Direct Inward Dial (DID) numbers at the site are tied to the physical location of the PSTN delivery into the site and use the local voice gateway. Toll-free service or incoming calls to the business's main number (that is, non-DID calls) are o