 including both scalability and high availability: Scalability: Routing all calls from the entire enterprise over a single or a small number of centralized SIP trunk access points means that you are looking at a SIP trunk ca- pacity of several hundred to several thousand connections for all enterprises except the really small ones. This implies border handling session capacity equipment that often far outstrips any single TDM gateway that exists in the typical enterprise. Most enterprise gate- ways are in the 1 to 16 T1/E1 range that equates up to between 384 to 480 ses- sions. Even a T3 gateway, a relative rarity in the average enterprise, presents only 672 sessions. Some of the redundancy schemes covered in the remainder of this section simultane- ously address scalability mechanisms including higher-capacity equipment and load balancing over clusters of individual boxes. High Availability: The more sessions that are concentrated into a single physical pipe, the larger the business impact to your organization of this single point of fail- ure. For this reason few enterprises truly deploy a single SIP trunk entry point into their networks; there are almost always multiple points. Chapter 7: Design and Implementation Considerations 131 Redundancy also becomes a much more pressing consideration because of the potentially large session capacity of SIP trunks. TDM gateway redundancy amount- ed to alternative routing over a different gateway when there was a failure. But when a single failure can now easily impact more than a 1000 calls, and potentially the routing of all PSTN-destined calls, the need for mitigation of such a failure escalates. You can deploy several strategies to protect against the business impact of a SIP trunk failure: Local and geographical SIP trunk redundancy Border element redundancy Load balancing and clustering PSTN TDM gateway failover The handling for emergency calls that you decide on (see the "Emergency Calls" section earlier in this chapter) might affect considerations for the redundan