Multi tier architecture

  In software engineering, multi-tier architecture (often referred to as n-tier architecture) is a client–server architecture in which the presentation, the application processing, and the data management are logically separate processes. 

  For example, an application that uses middleware to service data requests between a user and a database employs multi-tier architecture. The most widespread use of multi-tier architecture is the three-tier architecture.

Advantages:-

1.Scalability:

      Consider an example-

      A small business can begin running all tiers on a single machine. As traffic and business increases, each tier can be expanded and moved to its own machine and then clustered. This is just one example of how N-Tier Architecture improves scalability and supports cost-efficient application building.

2.Readability and reusability

3.Robust:

     If a business changes database vendors, they just have to replace the data tier . The business logic tier and the presentation tier remain unchanged. Likewise, if the presentation layer changes, this will not affect the integration or data layer.

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An n-Tier application usually has three tiers, and they are called the presentation tier, the business tier and the data tier. Let's have a look at what each tier is responsible for. 
1.Presentation Layer
     Presentation Layer is the layer responsible for displaying user interface and "driving" that interface using business tier classes and objects. In ASP.NET it includes ASPX pages, user controls, server controls and sometimes security related classes and objects. 
2.Business Tier
     Business Tier is the layer responsible for accessing the data tier to retrieve, modify and delete data to and from the data tier and send the results to the presentation tier. This layer is also responsible for processing the data retrieved and sent to the presentation layer. 
     Often this layer is divided into two sub layers: the Business Logic Layer (BLL), and the Data Access Layers (DAL). Business Logic Layers are above Data Access Layers, meaning BLL uses DAL classes and objects. DAL is responsible for accessing data and forwarding it to BLL. 
3.Data Tier
     Data tier is the database or the source of the data itself. Often in .NET it's an SQL Server or Access database, however it's not limited to just those. It could also be Oracle, mySQL or even XML. In this article we will focus on SQL Server, as it has been proven to be the fastest database within a .NET Application. 


Whats the difference between logical and physical layers
-->A logical layer means that layers are separate in terms of assembly or sets of classes, but are still hosted on the same server. 
-->Physical layer means that those assemblies or sets of classes are hosted on different servers with some additional code to handle the communication between the layers. 
E.g. remoting and web services.