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The idea of layering helps to break a problem down into the simplest sort of flow diagram.
The idea of a Business Logic Layer is a technology driven concept.
Its aim is to separate the mass of complex software an application needs into three layers, the BLL laying below the User Interface Layer (at the top) but above the Data Access layer (at the bottom).
The idea of a single layer for the business logic seems insufficient except for helping to distinguish it from the other two, is would seem more sensible to call it the Business Logic Layers, giving you mind a little more room to think about the Business Logic as a series of layers itself.
I know this is obvious perhaps but it is worth mentioning.
Layering is a problem solving aide and as such should, where helpful, be applied to the Business Logic.
Various tool sets exist to help in this process; the idea has now manifested itself in what is currently called the Windows Workflow Foundation.
Layering forms a major part of the International Standards Organisations (ISO) Open Systems Interconnectivity (OSI).
Application
Presentation
Session
Transport
Network
Data link
Physical