Understanding the OSI model

What is OSI Model and How it is helpful in understanding Networking.

What is OSI model ?

OSI model stands for Open system inter connection model. OSI model defines & used to understand how data is transferred from one computer to another in the computer network.

Why OSI model come into action ?

Two computers with same operating system can be connected together with Lan cable & connectors sharing data with the help of Network interface card forms the Network.

in order to accomplish successful communication between two computers with two different operating systems or between computers networks or different architecture a seven layered OSI model was introduced by International organization for standardization ISO in year 1984.

The OSI Model

OSI Partitions into seven abstraction layers & the layers are

• Application layer,

• Presentation layer,

• Session layer,

• Transport layer,

• Network layer,

• Data link layer,

• Physical layer.

Now lets get into the layers from application layer to physical layer. Each layer is a package of protocols.

1) Application layer

Application layer is used by network applications, network applications that uses internet like Web browsers Google chrome, Safari, g-mail & skype.

web browsers is Network application running in pc, it doesn't resides in application layer but it is uses application layer protocols like(http\https) to do web surfing.

This layer takes the user input and input from the other applications to produce the data and to be produced over the network. and when the data is coming in from the network, it works in storing or displaying data to the end users..

There are Several application layer protocol http, https, Ftp, Telnet, Smtp, DHCP,DNS. Etc all this protocol collectively form application layer.

File Transfer uses Ftp Protocol,

web surfing uses HTTPS Protocol,

Emails uses SMTP protocol,

Virtual Terminal uses Tel-Net Protocol.

2) Presentation Layer

Presentation layer receives data from the application layer, this data will be in the form of characters and numbers, presentation layer converts characters and numbers into machine understandable binary format. this process is known as Translation.

Presentation layer performs three main functions

• Translation
• data compression,
• Encryption/decryption

3) Session layer

Managing session between the machines, for two devices on the network to communicate with each other session needs to be created to ensure authentication & security.

Session layer helps in

• Session management,

• Authentication,

• Authorization.

Web browser performs all functions of Application, Presentation & session layer.

4) Transport layer

transport layer controls the reliability of communication, segmentation, flow control & error control.

• Segmentation :- data received from session layer is divided into small data units called segments. each segments contains a source & destination port number & sequence number. port number helps to directs each segment to the correct application. sequence number helps to reassemble segment in the correct order to form correct message at the receiver.

• flow-control :- transport layer controls the amount of data being transmitted.

• error control :- transport layer helps in error control. if the data does not arrive at the destination, transport layer uses automatic repeat request in-order to retrieve the data.

  protocols used in transport layer are

• transmission control protocol(TCP)

• User datagram Protocol(UDP).

5) Network Layer

network layer works for transmission of received data segment from 1 Network to another different network.

functions of network layer are

• logical addressing
• routing
• path determination

Logical addressing : IP addressing done in network layer called logical addressing. every network have two different unique IP address, network layer assigns senders and receiver IP address to each segment to form IP packet. IP address are assigned to ensure each data packet to reach correct destination.

routing : is process of moving data packets from source to destination and it is based on logical address format ipv4 & ipv6.

Path determination : computer can be connected to internet server in many ways choosing the best possible path for data delivery can be termed as Path determination.

protocol used in path determination are OSPF, BGP, IS-IS.

6) Data Link layer

Once the network layer identifies where to send the data, the data link layer takes care of the data transfer between nearest network elements. It ensures that the data transfer is error-free over the physical layer.

functions of Data link layer are Physical addressing, framing and flow control.

• Physical addressing : Physical addressing refers to the addressing after a packet has arrived at the local network. To the Internet at large, machines on the same local network may have the same IP address. If that is the case, MAC addresses must be used to locate the right recipient of the message.

• Framing : it is a way to make sure that are transmitted are understandable to the receiver.

• Flow-control : This layer is responsible for making sure that all frames arrive at its correct destination when a communication channel is shared by multiple connections. It also ensures that frames arrive error-free by detecting and re-transmitting damaged and lost frames.

7) Physical Layer

Data from application layer has been segmented by transport layer placed into packets by network layer and framed by data link layer which is sequence of binary binary zero's and one's physical layer converts binary digits into electric signals transmit over media. it can be electrical signal, radio signal or air signal.

at the receiver receives signals convert it to bits and pass into data link layer as a frame, and it receives into application layer.

In this way OSI model helps in transferring data in different hosts.

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