In the pure 3-tier / N-tier model, the application is divided into three formal tiers: the presentation tier (or GUI layer), the middle tier (or application server) and the data tier. Figure 2 shows a 3-Tier / N-Tier architecture. The key addition in this model is an application server which encapsulates the application's business logic . N-tier is used to describe this architecture since the application server may be implemented in one or more layers and it may be distributed in one or more places. For simplicity, the architecture will simply be referred to as "3-Tier", rather than N-Tier.
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