The Director is an Office Communications Server 2007 Standard Edition server or Enterprise pool that does not host users. However, as a member of an Active Directory domain, the Director has access to Active Directory to authenticate remote users and routing traffic to the appropriate server or Enterprise pool. Active Directory access is required to perform authentication, which the edge servers do not have because they are deployed in the perimeter network outside Active Directory.
We recommend that you deploy Directors to support external user access, but they are not required. You could deploy a similar topology without using a Director. If no Director is deployed, the authentication is performed on the Front End Server in the pool.