The picture below will gives an idea about the design changes from exchange 2007 middle tier to exchange 2010 middle tier
However this is nothing to do with the Outlook Anywhere Access (RPC over HTTP). Outlook Anywhere Access still uses the RPC proxy component on the client access server.
The design looks good, now let us see what the benefits or the goals of it. The MAPI and directory lookup are now moved into middle tier, no more direct access to the directory server and the exchange mailbox server. This helps in improved performance on the mailbox server, as we all know that one of the issues that we faced in previous versions of exchange was related to the exchange mailbox server performance. This also helped in single channel in which all data access occurs. The single channel access helps in terms of when a failover or a switchover happens to the databases in a DAG (Database Availability Groups). In simple words, client experience is better comparing any other exchange versions. The client experience will be like a slight disconnect during the failover/switchover of mailbox databases (about 30 sec) and will restore it automatically.
The middle tier design of single path access in exchange 2010 allows to handle more concurrent connections per mailbox servers. This helps in designing mailbox servers with more number of mailboxes in it. Exchange 2010 mailbox server could handle 250,000 connections when compared to the 64,000 limit in Exchange 2007.
Yes, now you will have another question, what will happen if the CAS server goes down? This is when the importance of a CAS array (Client Access Server Array) comes in. As the name implies this is an array of CAS servers. In this instead of connecting to the FQDN of a CAS server, the clients will use the CAS array URL (as mail.exchangedictionary.com). This will give a redundancy if one server fails in a CAS Array. The array FQDN should be updated in all the mailbox databases. The CAS array can also be configured with windows NLB (or any other third party NLB), so that the load balancing is also present. As a result clients will no longer connect to the mailbox servers or directory servers directly. So to improve the redundancy at CAS server level we can configure a CAS array and associate it with the database for better client experience.
Hope you enjoyed reading this.-Praveen