Question

How many layers are in OSI reference model?

Answer 1

OSI reference model: OSI reference model is an ISO standard which defines a networking framework for implementing the protocols in seven layers. These seven layers can be grouped into three categories:

Network layer: Layer 1, Layer 2 and layer 3 are the network layers.

Transport layer: Layer 4 is a transport layer.

Application layer. Layer 5, Layer 6 and Layer 7 are the application layers.

There are 7 layers in the OSI reference model.

1. Physical Layer

1. It is the lowest layer of the OSI reference model.
2. It is used for the transmission of an unstructured raw bit stream over a physical medium.
3. Physical layer transmits the data either in the form of electrical/optical or mechanical form.
4. The physical layer is mainly used for the physical connection between the devices, and such physical connection can be made by using twisted-pair cable, fibre-optic or wireless transmission media.

2. DataLink Layer

1. It is used for transferring the data from one node to another node.
2. It receives the data from the network layer and converts the data into data frames and then attach the physical address to these frames which are sent to the physical layer.
3. It enables the error-free transfer of data from one node to another node.
4. Functions of Data-link layer:
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6. Frame synchronization: Data-link layer converts the data into frames, and it ensures that the destination must recognize the starting and ending of each frame.
7. Flow control: Data-link layer controls the data flow within the network.
8. Error control: It detects and corrects the error occurred during the transmission from source to destination.
9. Addressing: Data-link layer attach the physical address with the data frames so that the individual machines can be easily identified.
10. Link management: Data-link layer manages the initiation, maintenance and, termination of the link between the source and destination for the effective exchange of data.

3. Network Layer

1. Network layer converts the logical address into the physical address.
2. It provides the routing concept means it determines the best route for the packet to travel from source to the destination.
3. Functions of network layer:
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5. Routing: The network layer determines the best route from source to destination. This function is known as routing.
6. Logical addressing: The network layer defines the addressing scheme to identify each device uniquely.
7. Packetizing: The network layer receives the data from the upper layer and converts the data into packets. This process is known as packetizing.
8. Internetworking: The network layer provides the logical connection between the different types of networks for forming a bigger network.
9. Fragmentation: It is a process of dividing the packets into the fragments.

4. Transport Layer

1. It delivers the message through the network and provides error checking so that no error occurs during the transfer of data.
2. It provides two kinds of services:
3. Connection-oriented transmission: In this transmission, the receiver sends the acknowledgement to the sender after the packet has been received.
4. Connectionless transmission: In this transmission, the receiver does not send the acknowledgement to the sender.

5. Session Layer

1. The main responsibility of the session layer is beginning, maintaining and ending the communication between the devices.
2. Session layer also reports the error coming from the upper layers.
3. Session layer establishes and maintains the session between the two users.

6. Presentation Layer

1. The presentation layer is also known as a Translation layer as it translates the data from one format to another format.
2. At the sender side, this layer translates the data format used by the application layer to the common format and at the receiver side, this layer translates the common format into a format used by the application layer.
3. Functions of presentation layer:
4. Character code translation
5. Data conversion
6. Data compression
7. Data encryption

7. Application Layer

1. Application layer enables the user to access the network.
2. It is the topmost layer of the OSI reference model.
3. Application layer protocols are file transfer protocol, simple mail transfer protocol, domain name system, etc.
4. The most widely used application protocol is HTTP(Hypertext transfer protocol ). A user sends the request for the web page using HTTP.