 address to SP. All remote site calls hairpin through the campus site where SIP trunk terminates. The centralized model has the following advantages: Operational and management savings Equipment savings due to less equipment needed in fewer sites Central call routing and dial-plan management Simplified provisioning of PSTN access Consolidation of oversubscription Potential per-minute call rate savings because of a larger volume of calls from a sin- gle point 98 SIP Trunking The centralized model has the following challenges: Bandwidth, QoS, and CAC concerns as RTP for remote site to PSTN calls traverse WAN twice. Nonoptimal media routing. Number porting of DID numbers to the aggregated SIP trunks. High availability in the campus (single point of failure). Survivability (backup branch call routing). Correct routing of calls to emergency services; emergency calls might need to con- tinue to use TDM trunks. Legal, regulatory, and geographical considerations. Failover must be provided at each site with SRST and TDM PSTN ports. Distributed SIP trunking has the following implementation characteristics: A Border Element placed at each site. A SIP trunk IP address per site to SP. Calls flow directly from the remote site to the SP. The distributed model has the following advantages: The call media path is optimized. DID numbers remain local to the area where they were issued and do not have to be ported. The remote site Border Element can also act as the local MTP and the SRST router; the same router can also maintain TDM PSTN trunks if required. Emergency call location information is tied to the local site (Border Element) and is hence more accurate. A one-to-one mapping between TDM PSTN interfaces and SIP trunks simplifies the migration. CAC is handled the same way as for TDM PSTN trunking. High availability is inherent as the SIP trunk remains active during SRST failover, and each site has its own trunk making the sites independent of each other for PSTN call routing. Bandwidth requirements between branch sites and the