OSI Model in Computer Network

Last Updated on March 2, 2023 by Prepbytes

Since mid 90’s the data transfer became very popular and is one of the emerging needs of technology expansion as we have to not only transfer the data but also have to transfer the data faster and that’s where the OSI model comes into light. It helps in transferring the data between two devices by using its various layers also known as OSI layers and together all the layer work as one and perform the required task and OSI reference model is also used for design and implementation.

What is the OSI Model in Computer Networks?

The OSI (Open Systems Interconnection) Model is a reference model that defines how data is transmitted between two devices over a network. It provides a standardized approach to communication and helps to ensure the interoperability and compatibility of different systems and devices. The OSI Model breaks down the communication process into seven distinct layers, each with a specific function, to provide a clear understanding of how data is transmitted.

The clear and systematic approach of the OSI Model makes it an essential tool in the field of computer networking. It is widely used in the design and implementation of computer networks, as well as in network troubleshooting and management. The standardized approach of the OSI Model helps to ensure the interoperability and compatibility of different systems and devices, and provides a clear understanding of the communication process, making it easier to diagnose and resolve network issues.

In the OSI model in computer network, there are seven layers and combined they are known as OSI layers and they are

• Physical layer
• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

Physical Layer in OSI Model

The Physical Layer is the bottom-most or the first layer of the OSI Model and is responsible for the physical transmission of data between two devices. It is one of the OSI layers. This layer is concerned with the physical components of a network, including the cables, connectors, and other physical components that transmit data. The Physical Layer is responsible for converting digital signals from the Data Link Layer into electrical or optical signals for transmission over the network. It also defines the electrical, mechanical, and functional specifications for the connection between devices, including the type of cable, connector, and data transfer rate.

The Physical Layer is critical to the overall operation of a computer network, as it provides the foundation for all communication between devices. A failure at this layer can result in a loss of connectivity, or in data that is transmitted with errors, making it difficult or impossible to use the network. The Physical Layer is responsible for providing the basic connectivity between devices and is the foundation upon which the other layers of the OSI Model are built.

Functions of Physical Layer in OSI Model

The Physical Layer of the OSI Model is responsible for the following functions:

• Physical Connectivity: The Physical Layer is responsible for establishing and maintaining the physical connection between two devices, including the specification of the electrical, mechanical, and functional requirements for the connection.
• Data Transmission: The Physical Layer is responsible for transmitting data between two devices, including the conversion of digital signals from the Data Link Layer into electrical or optical signals for transmission over the network.
• Error Detection: The Physical Layer is responsible for detecting and reporting errors that occur during the transmission of data over the network, such as signal distortion or signal loss.
• Flow Control: The Physical Layer is responsible for controlling the flow of data over the network, including the rate at which data is transmitted, and the amount of data that is transmitted at any given time.
• Data Encoding: The Physical Layer is responsible for encoding data into a format that can be transmitted over the network, including the use of electrical or optical signals to represent the binary data.

Data Link Layer in OSI Model

The second layer in the OSI Model is the data link layer. This layer is responsible for the reliable transmission of data over a single link in a network. It takes the data from the higher layer, the Network Layer, and prepares it for transmission over the physical medium.

The Data Link Layer is responsible for dividing data into manageable units, known as frames, and adding a header and trailer to each frame. The header contains information such as the source and destination addresses, and the trailer includes a checksum or error-detection code.
the Data Link Layer is also responsible for providing a reliable data transfer service between the sender and receiver. This layer ensures that the data is transmitted correctly and in the correct order, and retransmits data if necessary.

Functions of Data Link Layer in OSI Model

The Data Link Layer of the OSI Model is responsible for the following functions:

• Framing: The Data Link Layer is responsible for dividing the data from the Network Layer into manageable units called frames, adding a header and trailer to each frame, and transmitting the frames over the physical medium.
• Error Detection and Correction: The Data Link Layer is responsible for detecting errors that occur during data transmission, and for correcting these errors. This layer adds a checksum or error-detection code to the frame header or trailer and uses this code to detect and correct errors.
• Flow Control: The Data Link Layer is responsible for controlling the flow of data between the sender and receiver, ensuring that data is transmitted at a rate that the receiver can handle. This layer uses mechanisms such as buffering and windowing to control the flow of data.
• Medium Access Control: The Data Link Layer is responsible for managing access to the physical medium, ensuring that multiple devices can share the medium and access it in a fair and efficient manner. This layer uses techniques such as carrier sensing, collision detection, and media access protocols to manage access to the physical medium.
• Reliable Data Transfer: The Data Link Layer is responsible for providing a reliable data transfer service between the sender and receiver. This layer ensures that data is transmitted correctly and in the correct order, and retransmits data if necessary.

Network Layer in OSI Model

The Network Layer is referred as the third layer of the OSI Model. This layer is responsible for the delivery of data between devices over a network. The Network Layer takes the data from the upper layer, the Transport Layer, and routes it to its intended destination.
Congestion control is the process of managing the flow of data through the network to prevent network congestion, which can result in delays and lost data.
The network Layer is responsible for providing a logical communication path between devices, allowing them to exchange data over the network. This layer also provides a level of network abstraction, allowing the higher layers of the OSI Model to operate independently of the underlying network technology.

Functions of Network Layer in OSI Model

The Network Layer of the OSI (Open Systems Interconnection) Model performs the following five functions:

• Network Addressing: The Network Layer assigns unique network addresses to devices on the network, allowing them to be identified and located.
• Routing: The Network Layer is responsible for determining the best path for data to travel from its source to its destination, taking into account factors such as network congestion, network availability, and network topology.
• Congestion Control: The Network Layer manages the flow of data through the network to prevent network congestion, which can result in delays and lost data.
• Logical Communication Path: The Network Layer provides a logical communication path between devices, allowing them to exchange data over the network.
• Network Abstraction: The Network Layer provides a level of network abstraction, allowing the higher layers of the OSI Model to operate independently of the underlying network technology.

Transport Layer in OSI Model

The Transport Layer is termed as the fourth layer of the OSI Model. It sits between the Network Layer and the Session Layer and is responsible for end-to-end data transmission between applications.
The main functions of the Transport Layer are to provide reliable, efficient data transmission, and to ensure that data is delivered to its intended destination in the correct order and without errors. To achieve these goals, the Transport Layer provides services such as flow control, error checking, and segmentation.
It provides services such as flow control, error checking, and segmentation to ensure that data is transmitted efficiently, accurately, and reliably over the network. These services are critical to the overall operation of a computer network, as they ensure that data is delivered to its intended destination in the correct form.

Functions of Transport Layer in OSI Model

The Transport Layer of the OSI Model performs the following five functions:

• Segmentation and Reassembly: The Transport Layer breaks down data into smaller segments and reassembles them at the receiving end. This reduces the chances of network congestion and makes data transmission more efficient.
• End-to-end Delivery: The Transport Layer ensures that data is delivered to its intended destination, handling issues such as errors and congestion along the way.
• Flow Control: The Transport Layer manages the flow of data between the sender and receiver to prevent the overloading of the network.
• Error Checking: The Transport Layer performs error checking to ensure that data is transmitted accurately and without errors.
• Multiplexing: The Transport Layer allows multiple applications to share the same network connection, allowing multiple streams of data to be transmitted simultaneously over the same link.

Session Layer in OSI Model

The Session Layer can be understood as the fifth layer of the OSI Model. It is responsible for establishing, maintaining, and ending communication sessions between applications. A session is a logical connection between two applications that allows them to exchange data in an organized and controlled manner.

The Session Layer provides services that allow applications to exchange data in a controlled and organized manner. By establishing, maintaining, and ending communication sessions, the Session Layer helps ensure that data is transmitted accurately and efficiently between applications.

Functions of Session Layer in OSI Model

The Session Layer of the OSI Model performs the following functions:

• Session Establishment: The Session Layer is responsible for establishing a communication session between applications. This involves negotiating the parameters of the session and establishing a logical connection between the applications.
• Session Management: The Session Layer is responsible for maintaining the communication session between applications. This involves ensuring that data is transmitted accurately and without errors and resolving any issues that may arise during the session.
• Session Termination: The Session Layer is responsible for ending the communication session between applications. This involves releasing any resources that were allocated for the session and closing the logical connection between the applications.
• Token Management: The Session Layer is responsible for managing tokens that control access to shared resources. Tokens are used to ensure that only one application has access to a shared resource at a time.
• Synchronization: The Session Layer is responsible for synchronizing the communication between applications. This ensures that data is transmitted in a predictable and organized manner and that both applications have the necessary information to continue the session.

Presentation Layer in OSI Model

The Presentation Layer can be referred as the sixth layer of the OSI Model. It is responsible for converting the data generated by the Application Layer into a standard format that can be transmitted over the network and ensuring that the data received is in a format that can be understood by the receiving application.

The Presentation Layer provides services that ensure that data is transmitted in a standardized and secure manner over the network. By converting the data generated by the Application Layer into a standard format and providing encryption, compression, and other services, the Presentation Layer helps ensure that data is transmitted efficiently and securely between applications.

Functions of Presentation Layer in OSI Model

The Presentation Layer of the OSI Model performs the following functions:

• Data Formatting: The Presentation Layer is responsible for formatting the data generated by the Application Layer into a standard format that can be transmitted over the network. This includes converting the data into a standardized representation, such as ASCII or EBCDIC.
• Encryption and Decryption: The Presentation Layer is responsible for encrypting data prior to transmission and decrypting data upon receipt. This helps ensure the confidentiality and integrity of the data being transmitted over the network.
• Compression and Decompression: The Presentation Layer is responsible for compressing data prior to transmission and decompressing data upon receipt. This helps reduce the amount of data transmitted over the network, increasing network efficiency.
• Data Translation: The Presentation Layer is responsible for translating data from one format to another, as necessary. This may be necessary when data is being transmitted between systems that use different data representations.
• Data Integrity: The Presentation Layer is responsible for ensuring the integrity of the data being transmitted over the network. This includes checking for errors in the data and correcting any errors that are found.

Application Layer in OSI Model

The Application Layer is the highest layer or the seventh Layer of the OSI Model and is responsible for providing the interface between the applications and the network. This layer is responsible for enabling the applications to access the network and use the services provided by the other layers of the OSI Model.
The Application Layer provides the applications with the services they need to interact with the network and enables the applications to communicate with each other and access the resources they need. This layer is responsible for defining the protocols and services that are needed to support the applications, and for providing the security and authentication services that are needed to protect the network.

Functions of Application Layer in OSI Model

The Application Layer of the OSI Model is responsible for the following functions:

• Application Interfacing: The Application Layer is responsible for providing the interface between the applications and the network, enabling the applications to access the network and use the services provided by the other layers of the OSI Model.
• Network Service Provision: The Application Layer is responsible for providing the applications with the services they need to perform their functions, such as the ability to send and receive data and the ability to access network resources.
• Protocol Definition: The Application Layer is responsible for defining the protocols that the applications use to communicate with each other over the network, including protocols for email, file transfer, and other common network services.
• Security and Authentication: The Application Layer is responsible for providing the security and authentication services that are needed to protect the network and the applications that use it, including the services that are needed to authenticate users and devices, and to ensure that the data transmitted over the network is secure and protected from unauthorized access.
• Resource Sharing: The Application Layer is responsible for enabling the applications to share network resources, such as printers and file servers, and for providing the services that are needed to manage and coordinate the use of these resources.

Characteristics of the OSI Model

The OSI Model has several characteristics that make it a widely used reference model in computer networking. Some of these characteristics include:

• Modularity: The OSI Model divides the communication process into seven distinct layers, each with a specific function. This modular approach allows for an easier understanding of the communication process and helps in troubleshooting and problem-solving.
• Standardization: The OSI Model provides a standardized approach to communication, which allows for compatibility and interoperability between different systems and devices.
• Hierarchical Structure: The seven layers of the OSI Model are organized in a hierarchical manner, with each layer building upon the previous one. This structure helps to simplify the communication process and provides a clear understanding of the relationships between different components.
• Abstraction: Each layer in the OSI Model provides a level of abstraction, which isolates the implementation details of lower layers from higher layers. This abstraction makes it easier for network designers and developers to focus on the implementation of specific layers, without being concerned about the implementation details of other layers.
• Separation of Concerns: The OSI Model separates the different functions involved in communication into separate layers. This separation of concerns helps to simplify the communication process and makes it easier to understand and manage.
• Reference Model: The OSI Model provides a reference model that can be used to guide the design, implementation, and maintenance of computer networks. It provides a common understanding of the communication process, which helps to ensure that different systems and devices are compatible and can communicate with each other.
  Conclusion
  The OSI Reference Model is a theoretical framework for understanding the functioning of a computer network. It provides a standardized way of describing the various layers of a computer network and the functions they perform.
  The OSI Model is comprised of seven layers, each of which is responsible for a specific set of functions, The OSI Model has been extremely influential in the development of computer networking, and it continues to be widely used as a reference for the design and implementation of modern computer networks. By providing a clear and comprehensive understanding of the functions performed at each layer of a computer network, the OSI Model has helped to ensure the development of robust and interoperable computer networks.

Frequently Asked Questions

1. Why was the OSI Model created?
The OSI Model was created to provide a standardized way of describing the communication process between different systems and networks. It was developed by the International Organization for Standardization (ISO) to facilitate interoperability between different networks and systems.

2. What is the difference between the OSI Model and the TCP/IP Model?
The OSI Model and the TCP/IP Model are two theoretical frameworks for describing the communication process between different systems and networks. The OSI Model is a seven-layer model that provides a standardized way of describing the communication process, while the TCP/IP Model is a four-layer model that is used as the basis for the Internet Protocol (IP).

3. What are the physical layer standards in networking?
There are several physical layer standards in networking, including Ethernet, Wi-Fi, Bluetooth, and Fiber Optic. These standards define the specifications for transmitting data over a physical medium, including the type of cable or transmission medium, data transfer speed, and communication protocols.

4. How does the physical layer ensure reliable data transmission?
The physical layer uses various methods to ensure reliable data transmission, including error detection and correction, flow control, and data encryption. Additionally, the physical layer may use redundant connections to increase reliability and availability.