functions such as topology hiding and transcoding if it does not manage the media. The signaling Border Element can communicate with the media Border Element via a media resource control protocol such as H.248 or similar protocol. The direct media model has the following advantages: The media path is optimized. Signaling is centralized for optimal control and management. Chapter 6: SIP Trunking Models The direct media model has the following challenges: Remote sites using SRST for phone backup might not use the local media border ele- ment for SIP trunk access without having the signaling border element in contact. Presents the same DID number porting and other challenges characteristic of the centralized trunking model. Summary SIP trunks are becoming an increasingly viable option for small businesses and larger enterprises wanting to deploy new IP-based services to their users when communicating with customers and vendors external to the enterprise network. This chapter discussed the different architectural models for SIP trunking that can be deployed between the enterprise and the SP networks and the network changes this might precipitate on the design of your network. The considerations and trade-offs when choosing the most appropriate model for your business were explored in depth. The char- acteristics of the centralized and distributed SIP trunking models were discussed in detail, and a few practical variations of these were touched on. The questions you should ask of your IT team and of your SP to determine the most beneficial deployment should now be clear. Starting with the next chapter, the network design of connecting SIP trunks into the enterprise are explored in considerably more detail. For quick reference, the main points discussed in this chapter regarding the centralized, distributed, and hybrid SIP trunks models are summarized in the following sections. Centralized SIP trunking has the following implementation characteristics: A Border Element placed at a central location. A single SIP trunk IP