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OSI Model

OSI (Open System Interconnection) Model was developed by International Standards Organisation (ISO) to standardize the network architecture internationally.


It is a layered framework having seven layers.

The layers communicate with each other in a hiearchical manner where control is passed from one layer to another in the hierarchy beginning from the application layer at one computer, then to the bottom layer of that computer. From here the control passes to the bottom layer of the next computer and then back up in the hierarchy.

Layer 7 – Application (Highest)

It enables network access for the user by ensuring user authentication and quality of service. It specifies the environment for the network applications in which they can very well communicate with the network services like E-mail, shared database etc. Telnet, FTP(File Transfer Protocol) etc exist here.


Layer 6 - Presentation

It is concerned with the format(syntax & semantics) used for data transmission. It performs the basic functions of translating, Encrypting and Compressing the data thus freeing the user from data compatibility problems.

Layer 5 - Session

It is responsible for establishing, maintaining, synchronizing & terminating connections called as Sessions on a network. This is called dialog control. It manages the records of transmissions and controls the transmission errors.


Layer 4 - Transport

Its basic function is flow control and error control but that is performed end to end and not across a single link as is done in data link layer. It deals with the problems related with the transmission and reception of data packets & ensures the delivery of data in an errorfree format.


Layer 3 - Network

It transfers the data packets from source to destination via one or two networks by creating the logical paths called virtual circuits. This function of network layer is called Routing. It addresses & sequences data packets, manages traffic problems on the network and reports delivery errors also.


Layer 2 - Data Link

It performs node to node delivery of data and is responsible for dividing the stream of bits received from network layer into data frames & transfers them to the physical layer.It is divided into two sub-layers- MAC(Media Access Control )Layer – It controls network access and permission of data transmission.LLC(Logical Link Control layer)- It synchronizes the data frames, controls the flow of data, detects the errors in data and sometimes corrects them.

Layer 1 – Physical (Lowest)

It defines the mechanical, electrical & physical specifications of the interface & the transmission medium used for communication. It determines how a cable is attached with LAN card & is responsible for transmitting bit stream from one computer to another. Fast Ethernet, ATM etc are some of the protocols that exist here

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