IP trunking. In most parts of the world, TDM PSTN trunking is a regulated service in which the costs for installation and recurring monthly costs are well understood or regulated or both. SIP trunking on the other hand is an unregulated service, and these costs can vary widely between providers and geographic regions. Although per-minute call costs might perhaps be cheaper on SIP trunks, you must look at all the costs associated with validating, installing, and running the trunks in production compared to TDM trunking. SIP trunking is not as mature as TDM trunking, and feature operation and features offered on different SIP trunk services vary. This requires you to run through a fairly extensive test and verification procedure and trials before installing a SIP trunk for your production voice calls. Tip Evaluate all costs related to SIP trunking, not just per-minute call rates. Factor in a validation test cycle before running SIP trunks in production. Chapter 6: SIP Trunking Models High Availability A distributed SIP trunk design provides inherent high availability and redundancy because every site has its own access and can use one of numerous neighboring sites' access if its own is unavailable. Additionally, each site's SIP trunk carries only a small vol- ume of traffic; if you lose one of these SIP trunks, the impact on your business is rela- tively contained. A centralized SIP trunk design introduces a number of availability considerations. Because many remote sites' traffic is aggregated onto a single physical connection, there can be a considerable business impact if the SIP trunk becomes unavailable. A central SIP trunk is also often sized for hundreds or thousands of simultaneous connections, making the loss of this number of calls at once a severe impact to ongoing business in your network. If a TDM trunk went down, it was usually a T1's or E1's worth of lost calls while neigh- boring T1/E1 connections continued to operate. If a 2000-session SIP trunk goes down though, it takes the equivalent of 84 T1s o