In comparison to two tier three tier architecture is more scalable and more secure

In three tier architecture the project is divided into three sections UI, BO and DAL. Due to this logical separation when we make changes in one section it does not ripple out throw out the project. Due to this projectmaintenance cost and complications are minimized.

UI: User Interface is a top layer in the application and it’s responsible for Look and feel and its run on client machine or user’s computer.

BO: Business Object resides in the middle below the User Interface and it’s responsible for validation
.
DAL: Data Access Layer is last layer or bottom layer of an application. All the database connectivity is done in this layer.

Below diagram show the concept of Three Tier Architecture in a better manner.
 

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