ISO-OSI Model Introduction

Introduction :

One of the best ways to understand networking is to study by which computers and other networking device transfer data. OSI model describes how data is transferred from an application on one computer to another. OSI model comprises seven different layers. Each layer handles specific function in transferring data over a network.


OSI Origin and Evolution

When network was first developed, computers could typically communicate with other computers from the same manufactures. For example: companies run either complete DECnet solution or an IBM solution but they couldn't use both. In order to overcome this problem, Open Systems Interconnection (OSI) reference model was developed by International Organization for Standardization (ISO).

OSI Model or OSI Reference model describes  the flow of data on a computer network layer has a composed of seven layers that describes the functions of data communication protocol. Easy layer of OSI model describes a  particular network function.

One layer in OSI model uses the services provided by the layer immediately below it. For example : the network layer has to deliver data from end to end. To do this, it uses the services provides by the data link layer to forward data to the next successive device along the end-to-end path.

The protocols in OSI model look like blocks stacked on upon another. Because of appearance, OSI model is also called as stack or protocol stack. In this, data is transmitted from one layer to the next, down the stack.
By dividing network function into logical smaller pieces, network related problems could be easily solved. OSI model reduces the impact of technological change on the entire network and allows extensibility. New application and network hardware can be easily added or replaced without requiring redesigning.

Seven layers in OSI Model

7.Application

6.Presentation

5.Session

4.Transport

3.Network

2.Data Link

1.Physical

Note : The easiest way to remember the seven layers is sentence: Please Do Not Throw Sausage Pizzas Away.


Advantages of OSI Model

• It divides network communication into smaller pieces, making it easier for people to understand and learn. People can easily understand protocol specification.
• It standardizes network components to allow multiple vendor development and support.
• Different types of network hardware and software can communicate with each other.
• Changes in one layer will not affect other layer.

Interaction between same layers on different computers

Each successive layer of the OSI model interacts with the layer one above it, or below it and its peer layer in order networked computer system. For example : Data Link Layer of Host A communicates with Network Layer, Physical Layer of the same systems and Data Link layer of Host B.

Layer D ( or Layer 4) has to interact with Layer D on another computer to successfully implement its function. For example: the transport layer can send data only when it receives the acknowledgment that the data is received, otherwise the sender will not know when it has to perform error recovery. Same way, the sending device encodes the distinction network layer address. It the router does not help by performing their network layer tasks, the data packets will not be delivered the current location.

In this figure, the application layer on HOST A communicates with application layer on HOST B. Same way, the transport, Session and Presentation Layer on HOST A communicate with respective layers in Host B. The lower three layers of the OSI model is concerned with the delivery of data and Router 1 is used for the purpose. Router 1 also communicates with Host B's Physical, Data Link and Network Layer.

Data Encapsulation

Each layer in OSI model is responsible for interacting with the peer layer in other networked computer systems. For example : when two computer communicate, one layer such as Data Link layer has to communicate with exact same layer ( in this case it is Data Link layer ) on the other computer. This is possible with the help of the own protocol. Each layer's protocol exchange information in the form of packets known as Protocol Data Unit ( PDU ).

Even though this type of communication takes place logically between peers, each layer depends on the layer below it for the actual delivery of data. Each layer transmits its PDU to the layer below. In this manner, the data for the lower layer is the PDU of the layer immediately above it.

This process of adding one PDU and encoding it with another is called as data encapsulation. This process is similar to the mail delivery system. Once the composition of the layer is ready, it is then placed in an envelope that gets the address to the destination. Then the mail is placed into the mail carrier's bag. The bag is then placed into the delivery truck. At the destination the bag is opened and the letter will be given to the appropriate mail carrier. The letter reaches the recipient who open it and reads the content. The layers is the OSI Model function in the same way.

Step 1 : The application message is converted into data for communication on the network.
Step 2 : The data obtained from the above step is added with a segment header by the transport system.
Step 3 : The segment is given the network header that contains source and destination logical addresses.
Step 4 : The packet or data-gram that is obtained from the above step is given frame header, specifies to the physical medium used.
Step 5 : The frame is then converted into series of bits and impressed as  electrical signal generated on the physical medium

The above five steps can be summarized as follows.

OSI Origin and Evolution