7 Layers of the OSI Model
Tags: network, networking
Published: 2023-05-15 11:31:15

The OSI Model or the Open Systems Interconnection Model is a conceptual framework which describes the functions of a networking system. It is used for the transfer of data over a network which moves through different layers. The seven layers are:

### 1. Physical Layer

- It is the bottom-most or the first layer of the OSI Model.

- It comprises the raw data which is further transmitted to the higher layers of the structure.

- Preparing the physical devices in the network and accepting the received data for transmission.
    
- The termination of connection between two nodes of a network also takes place at this stage.
    
- This layer converts the digital bits into electrical, radio, or optical signals.

- Examples of hardware in the physical layer are network adapters, ethernet, repeaters, networking hubs, etc.

### 2. Data Link Layer

- Access to get the data is achieved at this layer.
    
- It breaks the input data into frames which makes analysing the data easier.
    
- Ensures that the data received is free of any errors.
    
- It controls the flow of data in the stipulated time duration and along with a set speed of transmission.
    
- The data is sent to the next layer in the form of packets which are then reviewed for further processing.

- The data link layer is subdivided into two types of sublayers:

    - **Media Access Control (MAC) layer**- It is responsible for controlling how device in a network gain access to medium and permits to transmit data.

    - **Logical link control layer**- This layer is responsible for identity and encapsulating network-layer protocols and allows you to find the error.


### 3. Network Layer

- It acts as a network controller.

- Transferring of variable data from one node to another, connected in a network, takes place at this layer.

- Each node has a specific address and the network layer ensures that the data is sent to its destination address.
    
- The data is sent in the form of fragments which are then connected to each other once the processing is done.

### 4. Transport Layer

- The delivery of data packets is managed by the transport layer.

- It manages the flow of data, segmentation and desegmentation and error control.

- There are five classes of the transport protocol, starting from 0 and continuing till 4 (TP0 to TP4).

- Fragmentation and reassembly of data packets occur that this stage.

### 5. Session Layer

- The connection between the computers connected in a network is managed at this layer.

- Establishment, management and termination between the remote and local application takes place here.

- Authentication and authorisation happen at this layer.

- This layer can also terminate or end any session or transmission which is complete.

### 6. Presentation Layer

- The data is converted into the syntax or semantics which an application understands.

- Before passing on the data any further, the data is formatted at this stage.

- Functions including compression, encryption, compatible character code set, etc. are also done at this layer of the model.

- It serves as a data translator for the network.

### 7. Application Layer

- The interaction with the user or the user application takes place at this stage.

- When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit.

## Advantages of the OSI Model

- It helps to standardize router, switch, motherboard, and other hardware.

- Reduces complexity and standardizes interfaces.

- Facilitates modular engineering.

- Helps to ensure interoperable technology.

- Helps to accelerate the evolution.

- Protocols can be replaced by new protocols when technology changes.

- Provide support for connection-oriented services as well as connectionless service.

- It is a standard model in computer networking.

- Supports connectionless and connection-oriented services.

- Offers flexibility to adapt to various types of protocols.

## Disadvantages of the OSI Model

- Fitting of protocols is a tedious task.

- We can only use it as a reference model.

- Doesn’t define any specific protocol.

- In the OSI network layer model, some services are duplicated in many layers such as the transport and data link layers.

- Layers can’t work in parallel as each layer need to wait to obtain data from the previous layer.

## Summary

| Layer    | Name          | Function                                                                                                                                             | Protocols                                      |
|----------|---------------|------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------|
| Layer 7  | Application   | To allow access to network resources.                                                                                                                | SMTP, HTTP, FTP, POP3, SNMP                    |
| Layer 6  | Presentation  | To translate, encrypt and compress data.                                                                                                             | MPEG, ASCH, SSL, TLS                           |
| Layer 5  | Session       | To establish, manage, and terminate the session                                                                                                      | NetBIOS, SAP                                   |
| Layer 4  | Transport     | The transport layer builds on the network layer to provide data transport from a process on a source machine to a process on a destination machine.  | TCP, UDP                                       |
| Layer 3  | Network       | To provide internetworking. To move packets from source to destination                                                                               | IPV5, IPV6, ICMP, IPSEC, ARP, MPLS.            |
| Layer 2  | Data Link     | To organize bits into frames. To provide hop-to-hop delivery                                                                                         | RAPA, PPP, Frame Relay, ATM, Fiber Cable, etc. |
| Layer 1  | Physical      | To transmit bits over a medium. To provide mechanical and electrical specifications                                                                  | RS232, 100BaseTX, ISDN, 11.                    |

## Reference

- https://www.geeksforgeeks.org/layers-of-osi-model/

- https://byjus.com/govt-exams/osi-model-open-systems-interconnection/

- https://www.guru99.com/layers-of-osi-model.html