There are specific reference models that define the services required to move data from one point to another and which layers will be responsible for those services. In this article, we will examine how the first of these models, the **OSI (Open System Interconnection) Reference Model**, works.

To ensure that devices within computer networks can communicate without compatibility issues, a global standard set of rules has been established. These standard protocols form the basis of the **OSI (Open System Interconnection) Reference Model**, developed by the **ISO (International Organization for Standardization)**. Its general structure defines the communication process of a node within a multi-layered framework.

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OSI Reference Model Architecture and 7 Layers

The OSI model defines 7 layers on an end system, each with different functionalities. Data passes through all layers, and distinct tasks are assigned to each one. This ensures that the work done and the methods used are separated, providing healthier communication. The primary task of these layers is to provide data flow to the layer above them; the data is processed sequentially through the layers until it is ready for transmission. Similarly, when it reaches the destination, it passes through these layers again to be delivered, ensuring successful communication. All other protocol stacks and network concepts are based on and explained using the OSI model.

The OSI reference model is 7-layered and has a hierarchical architecture. The layers below serve the layer above them and expect the layer below them to provide a service in order to fulfill their duties. The **Application Layer**, at the top of the hierarchy, serves programs and user applications.

The 7 Layers of the OSI Reference Model.

7. Application Layer

The Application layer encompasses the functions necessary for network access and communicates directly with the application programs that the user interacts with. It is the layer where incoming and outgoing data is visible. This is where users interface with the computer and perform their operations. Many protocols, such as File Transfer, E-mail, file access, and network management, are defined at this layer.

6. Presentation Layer

The Presentation layer involves the formatting and translation of the incoming data to be understood by the recipient, which prevents communication incompatibility between different devices. It handles functions such as data compression/decompression, encryption/decryption, and character conversion.

5. Session Layer

The Session layer covers the establishment, management, and termination of sessions between end nodes. In addition to these operations, it ensures the logical continuity of communication by allowing transmission to resume from where it left off if the session breaks down.

4. Transport Layer

The Transport Layer ensures that data transfer is obtained error-free at the end-user. It also has the task of monitoring transmission sequentially. The information blocks created by this layer are called **segments**. It segments or reassembles the data whole according to the data's status. It checks whether the data has traveled from its source address to the destination address, and if a problem occurs, it retransmits the data.

3. Network Layer

The Network layer ensures that data reaches the receiver by being routed through intermediate nodes between the endpoints. This layer selects the best transmission path for data transfer based on network conditions, priorities, and other parameters. Information blocks carrying the network address are called **packets**. It handles **IP assignments** for connection between the devices where the data will be transmitted. It can be said that the internet backbone is built upon this layer, enabling communication between different networks.

2. Data Link Layer

The Data Link layer sets the rules for accessing and using the physical layer. At this layer, data is encoded by being converted into logical signals. It has the function of detecting errors at the bit level at the receiver and alerting the user to obtain the correct data if correction is not possible. The Data Link layer is used for direct connection between systems within a local area network (LAN). Therefore, LAN connections are generally at the Data Link layer level. Separate Data Link layer protocols have been created for LAN technologies such as **Ethernet** and **ATM**.

1. Physical Layer

The Physical layer provides the physical transmission of data over an electrical medium. Criteria such as whether the transmission medium will be wired or wireless, and if wired, whether it will be transmitted via Ethernet or fiber cable, are determined here. It ensures the transmission of data by converting it into **bits**. For the successful transmission of data, the sending device and the receiving device must be defined by the same protocols.

The **Application, Presentation, and Session** layers are mostly implemented with software and form the network superstructure. The **Transport** layer acts as a transition point, allowing the **Physical, Data Link, and Network** layers to form the network infrastructure. In our next article, we will examine the **TCP/IP protocol**, which is the internet access protocol.