The OSI model is a conceptual framework that describes how two systems communicate over a network. It breaks down the responsibility of data communication into seven layers.
The Seven Layers of the OSI Model
As I told you, the OSI model consists of 7 layers. Each layer takes data from the previous layer and encapsulates it to make its Protocol Data
Unit (PDU).
What is the PDU?
The PDU is used to describe the data at each layer. PDUs are also part of
TCP/IP.
Let’s explore the layers in a top-down approach.
1. Application Layer
The Application layer is the top layer of the OSI model. It provides network services directly to user applications. When you read this article, you are on the application layer. The biggest interface of this layer is the HTTP.
Example: Web browsers using HTTP, Sending emails using SMTP, etc.
2. Presentation Layer
The Presentation layer acts as a translator between the application and the network. It is responsible for:
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Data format translation (e.g., converting between ASCII and Unicode).
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Encryption and decryption for secure communication.
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Data compression to reduce transmission size.
Example: Secure Sockets Layer (SSL) and Transport Layer Security (TLS).
3. Session Layer
The Session layer is responsible for managing and controlling sessions between applications. Its main functions are:
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Establishing, maintaining, and terminating sessions.
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Synchronization and checkpointing during long data transfers.
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Managing multiple sessions simultaneously.
Example: Session management in video conferencing apps, state synchronization in multiplayer online games.
4. Transport Layer
The Transport layer ensures end-to-end communication and reliable data transfer. It provides services such as:
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Segmentation and reassembly of data.
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Flow control and error correction.
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Connection-oriented (TCP) or connectionless (UDP) communication.
Example: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
5. Network Layer
The Network layer determines the best path for data transfer and handles logical addressing. The key responsibilities of this layer are:
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Routing and forwarding data packets.
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IP addressing (IPv4/IPv6).
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Packet fragmentation and reassembly.
Example: The Internet Protocol (IP).
6. Data Link Layer
The Data Link layer ensures reliable data transfer between two physically connected devices. It is responsible for:
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Framing and error detection/correction.
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Media Access Control (MAC) addressing to identify devices.
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Flow control mechanisms.
Example: Ethernet and Wi-Fi protocols.
7. Physical Layer
The Physical layer deals with the transmission and reception of raw bit streams over a physical medium. This layer is concerned with:
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Hardware components like cables, switches, and network interface cards (NICs).
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Signal modulation, voltage levels, and data rates.
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Physical network topology and medium types (e.g., copper, fiber optics).
Example: Ethernet cables and fiber optic transmission.
Conclusion
The OSI model breaks down all the necessary functions to send a data packet over a network
between two hosts. Understanding this model is essential to learn data communication and networking.
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