Hi folks! So with the new version of Exchange 2013, there are huge changes to server roles!
What might you ask? For one, there are only two server roles now! CAS (Client Access Server) and Mailbox roles! Microsoft did away with the complexity and reduced the roles to just two.
Also the connectivity between the roles makes use of https now. The CAS servers literally act as dumb proxies that forward requests to the mailbox roles and session affinity is no longer required at the load balancer. The mailbox role is responsible essentially for everything else.
This makes better use of your servers and makes it much easier to design an Exchange architecture. Of course in smaller environments you can combine the CAS and mailbox roles on the same server.