In the centralized model, Session Manager controls Communications Managers or telephony servers at all locations. The centralized configuration is a good choice if you have multiple sites and particularly, if you have sites with fewer users than the capacity of a single application server. See the following illustration for an example. You might have 8,000 users at your Atlanta site and 2,000 users at the Boston site. If you already have the telephony infrastructure in place in Boston, you can get Atlanta to handle the Boston traffic also and maximize the capacity of your servers.
Similarly, if you have a number of small offices with a couple of hundred or a thousand users at each location, each requiring only a fraction of a server, you might require significantly fewer servers than you would in a distributed configuration. By optimum utilization of the application servers, a centralized configuration lowers your total cost of ownership (TCO). The trade-off is you have more voice traffic on WAN and no local survivability.
