Logical versus Physical Tiers
The three services layers are often called tiers, or in this case, three logical tiers. They might be separated by well-defined interfaces, thereby allowing for less troublesome changes in the future, but they are considered logical because they may be implemented on only two physical tiers. Figure 8-2 depicts the notion of logical versus physical tiers.
Figure 8-2. Logical versus physical tiers
This very common implementation scheme represents a logical three-tiered solution that is actually implemented on two physical tiers: the client and server. Called fat-client or client-centric, this solution initially served ...
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