OSI Model

Computer networks also require well-defined layers of functions and inter-layer protocols for having effective data communication between computers. In this regard the ISO developed the open systems interconnection connectivity reference model. Its purpose is to standardize the procedures for the exchange of information between various computer systems.

1) Physical Layer:

This is the lowest layer. It has direct interface with the physical media. The layer supports point – to – point or multipoint connection. It also supports both half and full duplex communication through serial or parallel data paths.

Four basic areas of media connectivity are defined here.

• Mechanical characteristics of communication device interface are defined in this layer.
• The Physical layer controls the electrical signal design used to convey data.
• The function logic of electric signals generated on certain wires in the physical media is specified in this layer.
• The Physical layer defines the procedures of protocols that control the sequence of events.

2) Data link Layer:

• The data link layer divides data traffic into transmission packets or frames.
• Data link functions provide the bits that separate and identify each field in a frame.
• This layer ensures that data are made transparent to all communication devices.
• This layer also provides flow control.
• This layer must ensure that data are delivered to the receiver in the same sequence in which they are transmitted.
• Error correction, detection and frame sequencing are important features of the data link layer.

3) Network Layer:

• The network layer handles the routing functions for data moving from one open system to another.
• This layer provides the addressing necessary to relay data through intermediate nodes. These nodes provide the connectivity between nonadjacent open systems.
• It ensures the data link layer frames are not lost.

4) Transport Layer:

• The transport layer is responsible for maintaining service for the user.
• It optimizes the resources that connect the two open systems.
• It establishes transport connections between session entities.
• This layer segments data into smaller or larger packets, which levels the flow of data between session entities.
• It provides communications flow control avoids congestion in the network.

5) Session layer:

• It provides support or session connection between open system.
• Multiple session connections may use sequentially one transport connection.
• Each session connection is handled by only one transport connection.
• Services provided include synchronizing the data interchange and controlling the two-way flow of data.

6) Presentation Layer:

• The presentation layer controls the syntax of data originated by two open systems.
• It controls the transformation of data as they pass between these systems to ensure that the exchange of data is meaningful to both systems.
• This layer negotiates the data syntax between two systems at the start of the connection and renegotiates a change in syntax if one is needed during the connection.
• This layer is responsible for formatting data.

7) Application Layer:

• The application layer is the highest OSI level. This layer contains application entities that control application process through interactions called application layer protocols. This layer controls operating system functions and the end-user interface with the system.