Characteristics of the OSI Layers

The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers.

The Upper Layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another type in the OSI model.

The Lower Layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for e.g.) and is respoinsible for actually placing information on the medium.

Two sets of Layers make up the OSI Layers
Application	Application
	Presentation
	Session
Data Transport	Transport
	Network
	Data Link
	Physical

OSI Reference Model

Modern computer networks are designed in a highly structured way. To reduce their design complexity, most networks are organized as a series of layers, each one built upon its predecessor.
Open System Interconnection (OSI) Reference Model was developed by the International Standard Organization (ISO) to provide a framework for understanding how information is sent from one computer to another. The model is so called because it deals with connecting open systems - that is, the system is open for communication with other systems. The OSI model describe seven layers, with each layer intended to provide a well defined service in order to ensure data has been successfully transmitted between devices.

These layers are:

• Application - Provides different services to the applications.
• Presentation - Converts the information
• Session - Handles problems which are not communication issues.
• Transport - Provides end to end communication control.
• Network - Routes the information in the network.
• Data Link - Provides error control between adjacent nodes.
• Physical - Connects the entity to the transmission media.