7 Layers of OSI Model



Initially Open System Interconnection was designed by International Organization of Standardization to make a framework on which to build suite of Open System Interconnection protocol. Vision was that OSI layer model protocol would be used to develop an international network that will not be dependent on the proprietary system.

As a reference model Open System Interconnection (OSI) model provides such an extensive list of functions and services that can occur at each layer. It also describes how each layer will interact with each other below or above.

OSI layer model is based on seven layers. Each layer performs its own functionality.


Application layer provides a mean for user to communicate over the different networks. It. exchanges data between programs running on the source and destination hosts. Hyper text transfer protocol (HTTP) works on application layer user can connect to World Wide Web through Hyper text transfer protocol. Simple Mail transfer Protocol (SMTP) is also a protocol of Application layer that deals with Mails. It is software based layer that perform three main functions.


Provides a way for the people to create message.


Are Responsible for establishing an interface in the network.


Provide rules and formats how data will be treated.


Presentation layer is responsible for coding and conversion It compress the message at sender end and decompress it at the receiver end. Also deal with Encryption it’s encrypts the data in a specific code at sender end and decrypt it at the receiver end.

Coding and compression format are GIFF, JPEG and TIFF


Session layer is responsible for maintaining the session that become idle or that are idle for long period between the source and destination point. It Initiate the dialogs and keep them active and restart the sessions that are disrupted or become idle for long period of time


Transport layer deals with segmentation of data and manage each piece of data it reassembled the segment into stream for application data. It deal with identifying the different applications. It controls the flow of data error checking transport layer deals with ports. In transport layer each protocol is known by a port number. Transport layer also track the individual communication between application on the destination and the source end. Transport layer is a connection oriented protocol; (TCP) Transmission Control Protocol, (UDP) User Datagram Protocol work on transport layer. TCP is a best effort delivery protocol. While UDP is a connection less protocol it sends the data to the receiver without initiating the session.


OSI Layer 3 provides services to exchange the data over the networks between identified end devices.  Layer 2 network layer address the data that where it will originate and what will be the destination of this data. Network layer encapsulate the data into packet from sender end and de-capsulate the data at the receive end. Network layer route the data between different networks. (IP) Internet protocol work on this layer that is the backbone of networking with out IP network cannot exist. IP has two versions. IPV4 and IPV6 ipv4 has 32 bits and it is dividing in to five classes. A, B, C, D, E. While ipv6 was designed for future use because ipv4 has not enough addressing space left so IPV6 was designed it used 128 bits large amount of addressing. Subnet mask is used to determine that ip address belongs to which class. Further VLSM (Variable length subnet mask) are used to break the network address in to different subnets so that large amount of addressing can be available. Routers are a layer 3 device.


Data link layer is responsible for the exchanging of data over a common local media. Data link layer encapsulate the data into frame. Data link layer is divided into two sub layers.

LOGICAL LINK CONTROL (LLC) that deals with software based layers it take the data from upper layer and send it to the MAC

Media Access Control (MAC) deals with hardware it takes the data from software based layer and sends it on physical layers Layer. MAC address is 48 bit long. Media sharing is used to determine that how media access controls is used. Whether there is a point to point network or multi access network. Switches are used on data link layers.

CYLIC REDUNDANCY CHECK (CRC) is used to check that frame arrives completely or it is lost. A random value is generated at sender end and at the receiver end if two values match each other it means frame is completely arrived otherwise it has been lost. Ethernet is used on data link layer.


Physical layer deals with transport of data across the network media. Different type of media is used. Physical layer receives the data from data link layer inform of bits and encapsulate the data into bits so that it can be transferred on the media and de-capsulate on the receiving end. Physical layer also deals with media connectors. Hubs work on physical layer. Speed of data depends upon the faster the media is faster it sends the data.


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