OSI Model Explained: 7 Layers and Their Functions

The OSI (Open Systems Interconnection) model is a conceptual framework that represents the functions of a networking system in seven layers. A good understanding of this model can help networking professionals understand how networks operate and reduce challenges by dividing large data exchange processes into smaller segments. The smaller segments are easier to understand, manage, and troubleshoot. In this blog, we will explore the OSI model. We will discuss its advantages, disadvantages, and the functions of its seven layers in detail.  

Table of Contents (TOC)

• Key Highlights
• What is OSI Model?
• 7 Layers of OSI Model
  • Physical Layer
  • Data Link Layer
  • Network Layer
  • Transport Layer
  • Session Layer
  • Presentation Layer
  • Application Layer
• Advantages of OSI Model
• Disadvantages of OSI Model

Key Highlights

• The OSI (Open Systems Interconnection) model is a conceptual framework developed in 1984 for networking systems.

• It standardizes the way computer systems communicate over a network by dividing the process into seven layers.

• 7 Layers of the OSI Model:

  1. Physical Layer: Handles raw data transmission between devices via cables or wireless signals.
  2. Data Link Layer: Ensures error-free data transfer between nodes using frames and MAC addresses.
  3. Network Layer: Routes data packets to their destination using IP addresses and routing protocols.
  4. Transport Layer: Manages data flow, error checking, and delivery through TCP/UDP.
  5. Session Layer: Establishes, manages, and terminates communication sessions between devices.
  6. Presentation Layer: Translates, encrypts, and compresses data for application use.
  7. Application Layer: Provides a user interface and supports services like email and file transfer.

What is the OSI Model?

OSI is short for Open Systems Interconnection. Developed in 1984, the OSI model represents the seven layers that computer systems use to communicate over a network.

In simple terms, it specifies how a network functions and standardizes the way that systems send information to each other. It describes how information transmits from a network device to its destination through a physical medium as well as how it interacts with the application. The modern internet is based on the TCP/IP model instead of the OSI model. Yet, the OSI model is still widely used due to its various benefits.

7 Layers of the OSI Model Explained

The following are the seven layers of the OSI model:

Physical Layer

It is the lowest layer of the OSI Model that is responsible for the physical cable or wireless connection between network nodes. It transmits unstructured raw data bits from the physical layer of the sending device to the physical layer of the receiving device. The physical layer may contain network devices, such as cables, modems, hubs, switches, repeaters, and network adapters.

Functions of the Physical Layer

• Synchronization of bits
• Performs modulation – converts a signal from one form to another so that it can be physically transmitted over a communication channel
• It specifies the transmission rate (number of bits sent per second).
• This layer describes the physical network topologies or how different nodes or devices are arranged in a network (bus, tree, star, mesh, etc.).
• It specifies the transmission modes or how data flows between the two connected devices (simplex, half-duplex and full-duplex).

In a nutshell, this layer is “in charge of transmitting raw data, which is simply a series of 0s and 1s, using electrical, mechanical or procedural interfaces.”

Data Link Layer (DLL)

The data link layer ensures error-free node-to-node data transfer. It helps in data transfer between two devices on the same network. This layer takes packets from the network layer and converts them into smaller pieces called frames. It ensures flow control and error control in intra-network communication. This layer consists of two sub-layers:

• Media Access Control (MAC)
• Logical Link Control (LLC)

Functions of Data Link Layer

• It provides an error control mechanism to detect and retransmit damaged or lost frames.
• The data link layer performs framing. It receives the stream of bits from the network layer and divides them into manageable data units called frames.
• After creating frames, it adds physical addresses (MAC addresses) of the sender and/or receiver in the header of each frame.
• Access control: The MAC sub-layer of the DLL identifies which device has control over the channel at a time when multiple devices share a single communication channel.

In a nutshell, this layer is “in charge of establishing and terminating a connection between two physically connected network nodes.”

Network Layer

The network layer receives frames from the data link layer and sends them to their intended destinations according to the addresses in the frame. It uses logical addresses, such as IP (internet protocol) to find the best physical path for the data to reach its destination. Routers are an important component in this layer as they route information where it needs to go between networks.

Functions of the Network Layer

• Routing: The protocols in this layer identify the suitable routes from source to destination.
• Logical Addressing: It describes an addressing scheme to identify each device on the network uniquely.

In a nutshell, this layer is “in charge of breaking up segments into network packets, reassembling the packets on the receiving end, and determining the shortest and most secure path for transmitting data packets across a physical network.”

Transport Layer

It manages the delivery, error checking, flow control, and sequencing of data packets. It regulates the size and transfer of data between systems and hosts. For example -TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

Functions of Transport Layer

• Service-point addressing
• Ensures that each message reaches its destination completely.
• Maintains error and flow control from the source to the destination to ensure proper data transmission.
• Multiplexing and Demultiplexing
• Segmentation and Reassembly

In a nutshell, this layer is ” in charge of delivering an entire message from a source device application program to a destination device application program.

Session Layer

The session layer facilitates establishing connections, maintaining sessions, and authenticating. It creates communication channels between different devices, called sessions. It opens sessions, ensures they are functional during data transfer, and terminates communication sessions with the lower layers with the presentation and application layer.

Functions of Session Layer

• Establishes, maintains, and terminates sessions.
• Dialogue Controller: Enables two systems to start communication with each other in half-duplex or full-duplex.
• Synchronization: It also allows a process to add some checkpoints into a stream of data.

In a nutshell, this layer is “in charge of establishing, managing, synchronizing, and terminating sessions between end-user application processes.”

Presentation Layer

Also known as the translation layer, the presentation layer translates data so that it can be used by the application layer. This layer addresses the syntax and semantics of the information exchanged between the two systems. It performs translation, encryption, and compression of data.

Functions of Presentation Layer

• Translation: Translates data based on the application’s syntax or semantics
• Performs encryption and decryption of sensitive data before they are transmitted over common channels.
• Compression: Reduces the number of bits contained in the information to be sent.

In a nutshell, this layer is “in charge of ensuring that communications passing through it are in the format required by the receiving application.”

Application Layer

This layer is the closest to the end-user. It acts as a window for the application services to access the network and for showing the received information to the user.

Functions of Application Layer

• User interfaces (UI)
• Provides compatible support for different services such as e-mail, remote file transfer, etc.

In a nutshell, this layer is “in charge of ensuring that an application can communicate effectively with other applications running on different computer systems and networks.”

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Advantages of the OSI Model

The following are some of the key advantages of the OSI model:

• OSI model is a generic tool supported by a wide range of device manufacturers. It serves as a tool to develop any network model.
• Each layer is separate from the other layers. Changes in one layer do not impact the other layers unless there are changes in the layer interface.
• The OSI model highlights the different tasks in each layer. It helps all the devices that work with the OSI model to support each other.
• It is flexible.
• The model can work with both connection-oriented and connectionless services.

Disadvantages of the OSI Model

The following are some of the disadvantages of the OSI model:

• It is a theoretical model. It does not consider the availability of appropriate technology, which restricts its practical implementation.
• OSI model is complex in structure compared to a TCP/IP model.
• It is not as effective as the TCP/IP model.
• Some layers, including the session layer and presentation layer, have little functionality when deployed practically.

Conclusion

In this article, we learned about the seven layers of the networking model and their functions. The OSI model describes a 7-layer architecture in which each layer performs a specific function. All seven layers work collectively to transmit the data across the world. It is still a widely used model due to various reasons. It helps visualise how networks operate, troubleshoot networking problems, and much more. Knowledge of the OSI model can help understand modern computer network technology in a connection-oriented way.

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