## OSI REFERENCE MODEL:

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OSI MODEL:

 

APPLICATION LAYER (Interface between foreign application and the next down layer.)

PRESENTATION LAYER   (Data-formats, Encoding and Encryption)

SESSION LAYER   (Inter-host Communication)

TRANSPORT LAYER   (End-to-End connectivity, Error Detection and Correction)

NETWORK LAYER (Packet switching, Packet filtering, Path determination, Logical addressing, Routing)

DATA LINK LAYER (MAC, LLC, Hardware Addressing, Provides Control Information for Flow control)

PHYSICAL LAYER               (Signals over wires)

Ans> 

It was developed by ISO (International Organization of Standardization).

It never played a real part in today’s Internet, but is very useful for studying the TCP/IP model, on which our internet relies.

OSI is far better a model than TCP/IP, the only reason for TCP/IP to get selected because at that time the authority felt that the addressing in OSI model is more complex, but today if we look at IPv6 in TCP/IP, its very closely associated with the addressing in OSI model.

OSI model is known as a functional model.

It’s also known as Layered approach model, because it takes care of all the processes by grouping them into layers.

Here every protocol is divided according to its function into layers.

The top 3 layers deals with the communication between End applications, and the remaining 4 layers deals with Data transfer.



# Application Layer:

 It interfaces with the application which is not the part of the OSI model.

And provides network access to the application.


# Presentation Layer: 

Its known as Translator.

Its name “Presentation Layer” is apt, as it presents the data which is in readable format in Application  Layer of one OSI model to the Application Layer of another OSI model.

It handles the issues like Data formats, Encoding-Decoding, Encryption-Decryption.


# Session Layer:

 It Establishes, Maintains & Terminates the session between Presentation Layer entities.

 It keeps the data of 2 or more applications separate from each other

 Dialogue control between devices.



## All the functionality of above 3 layers are done before leaving the computer, this means that the Operating System is responsible for carrying out the jobs of the Application, Presentation and Session Layers'.





# Transportation Layer:

 It provides the reliable and unreliable services

 Reliable Service is provided using the TCP and Unreliable using the UDP.

**Reliable Transport:
 TCP uses the Acknowledgement, Sequencing, Flowcontrol to provide the Reliable service.

 Error Detection and Error Correction is done here.

 Flowcontrol:  It prevents the sender from overflowing the buffer of the receiver using   Windowing, Buffering & Collision Avoidance.
                    
**UnReliable Transport:

  Only Error Detection is done here but no Error Correction.

  Well Unreliable services are important during VOIP, Buffered Video.

 Example: If a packet got dropped during VOIP conversation, it will hardly matter anything to the speaker and the listener, but just think what if realible service was used, then all the conversation would be hampered as the sender side tries to send the dropped packet again.

**Assignment of Ports:

 Assignment of Source and Destination Port is done at this Layer.

                   Source Port : 1024 - 65536

                   Destination Port : 0- 1023 (Well Known Ports).


#Data Link Layer:

 Deals with Hardware Addressing.

This Layer actually lays the rules, that How the data is going to be Transferred, Who is the Source and Destination, When to Send and When to Stop.

** MAC (802.3) (Media Access Control):

Defines how the Packet is to be placed on media.

** LLC (802.2) (Logic Link Control): 

Tells which type of packet is present within the Frame.

Also informs that where the packet is destined to. (i.e. to which protocol in Network Layer).              

Basically Flow Control take place at Transport Layer, but its Control data is done under Data Link Layer.

SAP (SERVICE ACCESS POINT): Was a field in LLC which was replaced because of its less capacity of 2 bytes with SNAP (SUB NETWORK ACCESS PROTOCOL) which mainly deals in Data Transfer, Connection Management & Quality-Of-Service selection.

#Physical Layer:

  It converts all the above protocol, payload,etc in to electric signals and sends it to the destination.




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