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OSI Layers: Definition and Functions of the 7 Osi Layers

Last updated: Jun 20, 2024

What Is OSI Layer?

OSI, or Open System Interconnection, is an International Organization for Standardization model for the communication of computer systems with standard protocols.

OSI layers are specific layers of the system with its own functionality. The 7 layers of OSI work together to send data from one person to another around the world. 

The OSI model provides a theoretical basis for understanding network communications but is not directly implemented. 

Instead, specific protocols and technologies are often developed based on the principles outlined in the OSI model to facilitate efficient data transmission and network operations.

So, why are the OSI layers important? The OSI model is useful for troubleshooting network problems, although it is no longer followed by the Internet. 

The OSI model can dissect and solve the problem of why a device cannot connect to the Internet. 

If the problem can be narrowed down to a specific layer of the model, you can save effort and time by not doing unnecessary work. 

 

7 OSI Layers and Their Explanations

As mentioned before, OSI has 7 layers that work by their functions. Here are the OSI layers in order from 1 to 7. 

 

1. Physical Layer

The first OSI layer is the physical layer, which is the lowest of the other layers. This layer is responsible for the physical connection of devices and contains information in the form of bits. 

These individual bits are transmitted from one node to another. Then, when this layer receives the data, converts the signal from 0 to 1 and sends it to the data link layer (the layer that stitches the frames together).

The Physical Layer of both devices must also agree on signalling conventions so that a 1 can be distinguished from a 0 on both devices. The other functions of the physical layer are

  • Defines how data flows between two connected devices. 
  • Defines how different nodes or devices on a network are organized. 
  • Defines the transmission rate. 
  • Synchronizes bits by providing a clock that controls the sender and receiver. 

 

Examples of physical layer devices include cables, modems, hubs, and so on. The physical layer is also commonly referred to as the lower layer or hardware layer.

 

2. Data Link Layer (DLL)

The next OSI layer is the data link layer, which is responsible for the data delivery from one node to another on the same network. Examples of DLL devices include switches and bridges.

DLLs take packets from the network layer and break them into small pieces called frames. This layer ensures that data can be transmitted between nodes without problems. 

When data arrives on a network, the DLL is responsible for transmitting it to the host using a MAC address. 

The DLL has other sub-layers; Logical Link Control (LLC) and Media Access Control (MAC). The other functions of the DLL are as follows:

  • The MAC sublayer ensures which device has control over a channel when a single communication channel is shared by multiple devices. 
  • Controls the flow by coordinating the amount of data that can be sent.
  • Ensures that the data rate remains constant at both ends to prevent data corruption. 
  • Detects and retransmits corrupt or lost frames. 
  • Adds the MAC address of the recipient and/or sender to the header of each frame. 
  • Provides a means of transmitting bits to the receiver. 

 

3. Network Layer

Next, the network layer is responsible for transferring data from one host to another host on a different network. 

This also responsible for packet traffic, such as choosing the fastest route for packets to reach their destination. 

Some examples of network layer devices are routers and switches. This layer also has other functions, such as

  • Defines an addressing scheme to identify different devices on the internetwork. Devices circulating on the network have different IP addresses, which are then placed in the header by the network layer.
  • Determines the appropriate route (routing).
  • Breaks down Transport Layer segments into smaller pieces called packets, and then reassembles them when the packets reach the receiving device. 

 

4. Transport Layer

The fourth of the 7 OSI layers is the Transport Layer. This layer provides services to the application layer and receives services from the network layer. 

Examples of its devices include Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and others. 

The transport layer is operated by the operating system and communicates with the application layer. 

The other functions of the transport layer are as follows

  • Provides end-to-end data delivery. 
  • Determines the success of the data transmission and retransmits it if there are errors (flow and error control). 
  • At the sending end, the Transport Layer receives data from the Session Layer and performs segmentation (data segregation).
  • At the receiving end, the Transport Layer receives the data and reassembles it. This layer also adds the source and destination port numbers to the header.
  • Specify the service point address so that the data can be sent to the correct destination. 

 

5. Session Layer

Next, the Session Layer is responsible for establishing and maintaining connectivity, authentication, and security. In general, this layer function to open and close the connection between two devices.

The time between opening and closing a connection is called a session. This layer ensures that there is enough time to open the connection so that data can be transferred smoothly, and then closes it so that no resources are wasted. 

In addition, the functions of these are as follows

  • Create, use, and terminate connections. 
  • Add checkpoints, which are data synchronization points. This point can detect errors so that data can be resynchronized, ensure that upstream messages are not prematurely truncated, and ensure that data is not lost. 
  • Facilitates communication between two systems.

 

6. Presentation Layer

Presentation layer extract the data and convert it to the required format. In short, the presentation layer prepares the data for use by the application layer. 

This layer is responsible for translating, encrypting, and compressing data. Data encryption is the process of translating data into a code format. The encrypted data is called ciphertext and the decrypted data is called plaintext. 

Meanwhile, the function of data compression is to reduce the number of bits that need to be transmitted over the network.  

 

7. Application Layer

The last of the OSI layers located at the top of the layer is the application layer. This layer creates the data that is sent over the network. 

This layer also allows applications to access the network and provide information to users. This is the only layer that interacts with the user. An example of a device is a web browser. 

This layer has protocols such as HTTP and SMTP (Simple Mail Transfer Protocol, a protocol for email communication).

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