It contains one Primary Domain Controller and allows for about 40,000 users. The administrative overhead is relatively low and centralized security is maintained. In the master domain model, the domain that is trusted by all other domains on the network and acts as the central administrative unit for user and group accounts. Now a word on trust. There are basically two types of trusts.
Users can log on from computers in either domain to the domain that contains their account.
First off all exchange sites must support synchronous RPC, They must must run in the same NT Security environment. Permanent connections like lease lines, network connects etc. must exist. When considering connection speed it's important to keep in mind the relationship between bandwidth and the amount of messaging traffic you're expecting. In other words the backbone for 20,000 Exchange users isn't going to be 28,800K dial up modem connection. You should develop a mapping strategy to define the relationships between sites and domains.