OSI Reference Model

Why to learn OSI reference Model?

The answer is too simple that It tells usthat how communication takes placebetween computers on internet but how??

Nowadays, When we talk aboutcommunication of computer we got toknow a term communication protocols.What are they??

Set of rules which defines the rulesfollowed by all involved in the process ofcommunication but what are theyactually??

What if someone is using some otherrules/Protocols?

If it is necessary to communicate withcomputers following totally differentprotocols then we have to connect a devicecalled Gateway between such systems.Gateways are computers loaded with specialsoftware, which do the job of translationwork from one protocol to other so thatcommunication between heterogeneoussystems may take place.

The Seven-layer Architecture of the OSI model

PHYSICAL LAYER

TRANSMISSION MEDIUM

From Data link Layer To Data link Layer

• Physical layer is the bottom layer of OSI model

• It is responsible for the actual physical connection between the devices. Such physical connection may be made by using twisted pair cable, fiber-optic, coaxial cable or wireless communication.

• Physical layer is concerned with transmittingraw bits over a communication channel.

Function of Physical layer

• Transforming bits into signals

• Bit Synchronization

• Provides physical characteristics of interfaces and medium

• Bit rate control

• Line configuration

• Transmission mode

• Physical topologies

• Multiplexing

• Circuit switching

DATA LINK LAYER

Functions of Data link layer

• Framing

• Physical Addressing

• Flow control

• Error control

• Access Control

• Feedback

Network Layer

Functions of Network layer

• Logical Addressing

• Routing

Transport Layer

• Transport layer provides two types of services• Connection oriented

• Connectionless

Session Layer

Function of Transport layer

• Segmentation of message into packet and reassembly of packets into message

• Connection management

• Service point addressing

• Flow control

• Error control

Function of Session layer

• Establishment, maintaining and ending a session

• Dialog control

• Dialog separation or Synchronization

Presentation Layer

• Different computers may use different codingschemes for data transmission. For examplesome computers may send the mostsignificant bit of a byte first and leastsignificant bit last and in some othercomputers it may be other way round, thesequence may be different at byte level also.The job of the presentation layer is to do thejob of code translation and present the datain the format used at the destinationcomputer.

Functions of presentation layer

• Data Presentation or Translation

• Data Encryption

• Data compression

Application Layer

• This is the layer with which users have directinterface. At the transmitting computer themessage originates at this layer and at thedestination computer the message is finallyreceived by this layer and presented to theuser in desired format. The data packets atthe application layer are known as messages

Functions of Application Layer

• Network Virtual Terminal

• File transfer, access and management (FTAM)

• Mail services

• Directory services

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