OSI REFERENCE MODEL

INTRODUCTION

OSI Reference model stands for open system interconnected reference model.

It was introduced in 1974.

It explains how the data is transmitted from one system to another system with proper format.

It contains 7 layers.

Each layer perform different function.

It is not a protocol but protocol supports OSI reference model.

OSI Reference Model

Physical layer

Physical layer is concerned with sending the data in the form of bits.

It controls the define voltage and bit rate needed for transmission.

It deals with electrical and mechanicalspecification of the cables and connectors.

Repeaters are example for physical layer’

Factors of physical layer

Signal encoding.

Medium

Bit synchronisation.

Transmission type.

Topology.

Multiplexing.

Interface.

Signal type.

Data link layer

This layer takes the data or message from the network layer and provides for their actual transmission.

Synchronisation of data which received from network layer.

It takes the packets from the upper layer and converts it in to frames.

It takes bits from lower layer and converts them in to packets.

Error detection.

Framing is also done by this layer.

Ensures link level reliability.

Network layer

It receives the packets from the transport layer and convert them in to frames.

It is responsible for sending the data to destination node by choosing the route.

It converts the address in to logical address(IP address).

It also controlls the conjesion problems.

Establishing the connection .

Sending the data.

Releasing the connections.

Transport layer

After receiving the data from the session layer,it splits in to packets.

It adds sequence number to the packets.

It is reponsible for error control and flow control.

It is also reponsible for sending the data correctly.

At the destination node the sequence number is removed .

It reassembles the data and send the data to the above layer.

Session layer

The received data from presentation layer are converted in the form of sessions.

The sessions are divided in to sub-sessions.

It avoids retransmission by adding check point features.

It reassembles the sub-sessions and hand overs to presentation layer.

It is also responsible for synchronization.

Presentation layer

The function of presentation layer is.

It encodes the given data.

The transmission of data from source to destination depends upon the destination node.

Data is compressed before sending the data to the destination node.

The received data is decompressed at the presentation layer.

Application layer

It contains many protocols.

Like www.,http etc.

It connects the address to the server.

Once it connects to the protocols it handover to the presentation layer.