 most obvious service is a TDM trunk replacement service for PSTN access. To accomplish this, the SP has to include con- nectivity to the PSTN through a media gateway. Media gateways have existed for years with the role of transitioning TDM traffic into IP. Initially, this was for transit across an IP network, where the end traffic would ultimately be converted back to TDM via another media gateway. However, when used as a component of a SIP trunk solution, to provide interconnect to the PSTN, a media gateway serves the role of converting TDM traffic to IP that stays IP until it is converted into an analog signal at the customer premise. A media gateway has many functions, but the primary purpose of a media gateway to an SP offering SIP trunks for PSTN access is as an origination/termination device. Effectively, the solution is taking what was once a dedicated resource that each individ- ual enterprise owned, specically a TDM to IP gateway, and instead, using a shared resource owned by the SP. This is an important point to understand in the development of SIP trunks for PSTN access. Effectively, deploying SIP Trunking at an enterprise is the outsourcing and transfer in ownership, control, and location of the TDM gateway from the enterprise to the SP. This transfer in ownership and location of the media gateway away from the enterprise and into the SP resulted in a transfer of the use of the asset from being dedicated to a particular company to an asset shared among many customers. Because the asset is now Chapter 5: Components of SIP Trunks 67 shared, the capability to optimize for the particular requirements of a specific customer becomes much more difficult. Table 5-2 outlines some of the changes that have occurred with the location, network design, and configurations as customers have transitioned from TDM gateways owned by enterprises as a method to connect IP-PBX to the PSTN to the use of SIP Trunks for PSTN access by SPs. Table 5-2 Comparison of Media Gateway Architectures TDM Gateway Owned by SIP Trunk Ser