OSI Model: An In-Depth Review

June 8, 2024 • ☕️☕️ 8 min read • 🏷 computer, software, network

Translated by author into: English

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In the field of networking and information technologies, a common framework is needed so that different systems can work in harmony with each other. In this context, the OSI (Open Systems Interconnection) Model is a basic conceptual model used to understand how various networking and communication protocols work and how they interact with each other. The OSI Model was developed by the International Organization for Standardization (ISO) and published in 1984.

The OSI Model consists of seven layers, and each layer represents a specific network function. These layers describe how hardware and software components interact with each other and provide a reference point for better managing network communications.

The modern Internet is not based on OSI but on the simpler TCP/IP model. But the OSI 7-layer model is still widely used because it helps visualize and communicate how networks work and helps isolate and troubleshoot network problems. Today, the OSI model is used primarily as a reference model to teach computer professionals the fundamentals of computer networks.

1. Physical Layer

The Physical Layer is the lowest layer of the OSI Model and defines the characteristics of the physical communication medium. This layer handles the physical aspects of data transmission and deals with physical components such as cables, connectors, electrical signals, optical signals. The main tasks of the Physical Layer are:

- Selection of Physical Medium: Determining the medium to be used for data transmission (e.g. copper cables, fiber optic cables, radio waves).

- Signal Transmission: Converting digital data into analog signals and transmitting these signals over the physical medium.

- Ports: Management and identification of physical ports of devices.

- Cable Connections: Physical configuration and management of cables and connectors.

Example protocols: IEEE 1394, DSL, ISDN

2. Data Link Layer

The Data Link Layer ensures reliable data transmission and transmits data units called frames using the services offered by the physical layer. The main tasks of this layer are:

- Framing Packaging of data into frames.

- Error Control: Detection and correction of errors that may occur during transmission (for example, CRC - Correction Codes).

- Flow Control: Ensuring that data is transmitted in a stable manner between receiver and transmitter.

- MAC Address Management: Directing data packets to the correct destination with physical addressing.

The Data Link Layer can be further divided into two sublayers:

-MAC (Media Access Control) Sublayer: Controls access to the data transmission medium and carries out the physical addressing function of frames.

-LLC (Logical Link Control) Sublayer: Manages error control and flow control functions.

Example protocols: SLIP, PPP

3. Network Layer

The Network Layer deals with routing and transmitting data packets between networks. This layer ensures that the most appropriate path is selected between the source and target devices and the data packets are transmitted over this path. The main tasks of the Network Layer are:

- Routing: Transmitting data packets between the source and destination through the most appropriate route.

- Logical Addressing: The use and management of logical addresses such as IP addresses.

- Packaging: Organizing data into packages.

- Traffic Control: Management of network traffic and optimization of data transmission paths.

Example protocols: IP, ARP, RARP, BOOTP, ICMP

4. Transport Layer

Transport Layer ensures end-to-end data transmission and performs functions such as reliability, error control, and flow control to ensure accurate transmission of data. The main tasks of this layer are:

- Connection Control: Establishing, managing and terminating connections between end devices.

- Error Control: Ensuring the integrity of the data and correcting transmission errors.

- Flow Control: Stable transmission of data and prevention of buffer overflows.

- Segmentation and Reassembly: Dividing large blocks of data into smaller segments and reassembling them at the receiver.

Example protocols: TCP, UDP

5. Session Layer

The Session Layer manages communication sessions between two devices. This layer deals with establishing, managing and terminating sessions. The main tasks of the Session Layer are:

- Session Setup and Management: Initiating and managing communication sessions between two devices.

- Synchronization: Determination of synchronization points so that data is transmitted in the correct order.

- Session Control: Ensuring the continuity of sessions and restarting them when necessary.

Example protocols: NFS, RPC, X WİNDOWS SYSTEM, APPLE TALK SESSİON PROTOCOL

6. Presentation Layer

The Presentation Layer performs the necessary operations to make the data understandable at the application layer. This layer is responsible for operations such as data formatting, encryption and compression. The main tasks of the Presentation Layer are:

- Data Formatting: Conversion of different data formats (e.g. ASCII, EBCDIC, JPEG).

- Encryption: Encryption processes to ensure the security of data.

- Compression: Compression operations to reduce data size.

Example protocols: GIF, JPEG, TIFF, EBCDIC, ASCII

7. Application Layer

Application Layer is the layer where users and applications provide direct access to network services. This layer exchanges data between applications through various network services and protocols. The main tasks of the Application Layer are:

- Network Services: Providing network services such as e-mail, file transfer, remote access.

- Application Protocols: Data transmission via protocols such as HTTP, FTP, SMTP.

- User Interfaces: Management of interfaces through which users access network services.

Example protocols: SSH, Telnet, FTP, DNS, HTTP

Importance of OSI Model

OSI (Open Systems Interconnection) Model plays many important roles in the field of information technologies. Here are some key advantages and details of this model:

Standardization

The OSI Model ensures interoperability of systems and protocols developed by different manufacturers. This allows network devices and software to be universally compatible. For example, a router developed by one manufacturer can communicate seamlessly with a switch developed by another manufacturer. This standardization encourages market expansion and the development of more innovative solutions.

Education

The OSI Model makes networking topics easier to understand and teach. The seven-layer structure of the model allows complex network processes to be broken down into simpler and more understandable parts. This provides students and beginners with great convenience in understanding network technologies. For example, each layer, from the physical layer to the application layer, focuses on specific functions and tasks, thus simplifying the structuring of training materials and lesson plans.

Troubleshooting

It offers a structured approach to identifying and resolving network problems. The OSI Model allows solving network problems by isolating them over certain layers. This helps network administrators and technicians identify the source of the problem quickly and effectively. For example, when a connection problem occurs, a systematic approach is taken to determine whether the problem originates from the physical layer or from higher layers.

Development

It provides a reference framework for the development of new network protocols and technologies. The OSI Model provides a structure to ensure that new technologies and protocols are compatible with existing systems. This makes it easier for developers and engineers to comply with certain standards when creating innovative solutions. For example, when a new communications protocol is developed, it is determined how this protocol will fit into specific layers of the OSI Model and how it will interact with other protocols.

Independence Between Layers

The OSI Model ensures that each layer performs a specific function, allowing these layers to operate independently of each other. This independence ensures that changes made in one layer do not affect other layers. For example, an improvement made at the data link layer does not affect operations at the application layer, providing flexibility and maintainability.

Universal Acceptance

As a globally accepted standard, the OSI Model serves as the foundation of global networking. This universal acceptance ensures harmonious and seamless communication between different countries and organizations. For example, an international company can establish a reliable and compatible network infrastructure between its branches in different countries.

The OSI Model plays a vital role in the world of networking and information technology. Consisting of seven layers, this model explains each stage of data transmission in detail and helps us understand how network components work together. Thanks to the OSI Model, network problems can be solved faster and more effectively, new technologies can be developed and network communications can be made more reliable. That’s why the OSI Model is an indispensable tool for network experts and information technology professionals.

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