On Premise Exchange Deployment
An on premise Exchange deployment, means exactly that. One or more Microsoft Exchange servers are deployed within your offices. This can be at a single office location or spread across multiple sites connected via a WAN for performance, availability and redundancy reasons.
There are a number of advantages and disadvantages to deploying Exchange on premise. Whatever your reasons for choosing Exchange, every Exchange environment must contain the following Exchange 2013 roles
Client Access Server (CAS)
The CAS server role is responsible for client side connectivity. Its what authenticates users with there mailbox’s and tells them where to go. Every Exchange deployment must have at least one server running this role. For added load balancing and redundancy it is recommended to deploy at least 2 CAS roles, in a load balanced deployment
The mailbox role is what is responsible for storing user data. Be it user mailboxes, archive mailboxes, or public folders. Another important feature of the mailbox role is the ability to route emails successfully around your organisation to ensure they get where they need to go. Every Exchange deployment must have at lease one server running this role. For added load balancing and redundancy we recommend that at least two (or more) exchange servers running this role are deployed in a Database Availability group for the highest possible mailbox availability for your users and business.