Understanding the OSI Model: A Guide to Network Communication

Computer Networking : Part 2

Student: Whoa, this OSI model sounds really interesting! Can you tell me more about it?

\=> Absolutely! The OSI model, which stands for Open Systems Interconnection, is a conceptual framework that helps standardize and describe the functions of communication systems and networks. It's like a blueprint that guides the design and implementation of network protocols. The model consists of seven layers, and each layer has its specific tasks and responsibilities in the communication process.

OSI Model Layers and Protocols in Computer Network

Student: Can you explain each layer to me using an example?

\=> Sure! Let's take the example of online video streaming to understand how each layer functions in a real-life scenario. When you're about to watch your favorite TV series on an online video streaming platform, the OSI model comes into play behind the scenes. Let's break it down:

  1. Application Layer: At the application layer, the video streaming application on your device initiates a request to access the desired content. The Application Layer transfers data to Presentation Layer.

    The Application Layer: content requested and returned in required format

  2. Presentation Layer: The presentation layer ensures that the video data is properly encoded and formatted, ready for transmission over the network. The Presentation Layer converts these data machine representable binary format. From ASCII to EBCDIC. This is known as transportation. I also provide abstraction and SSL protocol is used.

    The Presentation Layer: encryption, compression, translation

  3. Session Layer: The session layer establishes a session between your device and the video streaming server, enabling continuous communication throughout the streaming process. It allows you to maintain a connection and interact with the server throughout the ordering process. The session layer protocols handle tasks like session initiation, maintenance, and termination. The session layer assumes that the layer below will do its work.

    The Session Layer: session of communication

  4. Transport Layer: The transport layer comes into play, breaking down the video data into smaller, manageable chunks called packets. It adds necessary information, such as sequence numbers and error detection codes, to ensure reliable transmission. TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are commonly used transport layer protocols. It is done in three ways:

    1. Segmentation and Reassembly: The Transport Layer breaks the data received from the upper layers into smaller units called segments or datagrams. Every segment will contain a source and destination port number and sequence number. (Sequence number helps to reassemble at the destination in the correct order.

    2. Flow Control: The transport layer controls the flow of data.

    3. Error Control Some data packets get lost or corrupted so the error is controlled.

      The Transport Layer: segment, transport, reassembly

  5. Network Layer: The network layer handles the addressing and routing of video packets across different networks. It determines the optimal path for the packets to travel from the streaming server to your device.

    The Network Layer: packets creation, transport, packets assembly

    It does logical addressing (IP addressing done in the network layer is known as logical addressing). We know that every computer has its IP address. Here, the network layer assigns the senders and receivers' IP addresses to every segment and it forms an IP packet(so that every packet reaches the correct destination), it also provides routing i.e. moving one data packet from source to destination.

  6. Data Link Layer: The data link layer takes care of the physical transmission of data packets over the network medium. It ensures error-free delivery by performing error detection and correction. The primary functions of the Data Link Layer include framing, physical addressing, error detection and correction, and flow control. Let's delve into each of these functions in more detail:

    • Framing: The Data Link Layer receives data from the Network Layer above it and divides it into manageable units called frames. A frame consists of a header, which contains control information such as source and destination addresses, and a trailer which includes error detection information.

      What is the OSI Model? | Cloudflare

      Framing ensures that the receiving device can accurately identify the boundaries of each frame, facilitating proper extraction and processing of data.

    • Physical Addressing: Every device connected to a network has a unique physical address, known as the MAC (Media Access Control) address. The Data Link Layer uses MAC addresses to identify the source and destination devices within the local network.

      What is a MAC Address? How to Find My MAC Address

      MAC addresses are typically assigned by the manufacturer and are stored in the network interface card (NIC) or network adapter of each device.

    • Error Control: The Data-link layer detects and corrects errors in transmitted data using error detection and correction techniques. Error detection bits are added to the frame header for the receiver to verify the correctness of received data.

    • The Data-link layer synchronizes the sending and receiving speeds of devices to avoid overflow in the receiver's buffer and prevent loss of frames. It establishes flow control mechanisms between the sender and receiver.

    • Access Control: When multiple devices share the same communication channel, the Data-link layer ensures access control to prevent collisions. Techniques like CSMA/CD (Carrier Sense Multiple Access with Collision Detection) and CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) are used to avoid collisions and frame loss.

      The Data-link layer consists of two sub-layers:

      1. Logical Link Control (LLC): This sublayer deals with data multiplexing, flow control, error messaging, and acknowledgements.

      2. Media Access Control (MAC): The MAC sublayer manages device interactions, including addressing frames and controlling physical media access.

  7. Physical Layer: The physical layer represents the actual physical medium, such as Ethernet cables or wireless signals, through which the video packets are transmitted. It handles the electrical and mechanical aspects of data transmission.

    What is the OSI Model? | Cloudflare

Student: Wow, that's a lot of layers and tasks! It's amazing how they all work together to make online video streaming possible.

\=> Absolutely! The collaboration of these layers ensures that data is transmitted efficiently and reliably across networks. Each layer focuses on specific aspects of communication, and together, they enable seamless data transmission from the streaming server to your device.

Student: This OSI model really simplifies the complexity of network communication. It's like breaking down a big task into smaller, more manageable steps.

\=> Exactly! The OSI model provides a structured approach to understanding network communication. By dividing the process into layers, it becomes easier to design, implement, and troubleshoot network protocols and systems.

Student: I'm starting to see the big picture now. This OSI model is an essential foundation for understanding how networks operate.

\=> Absolutely! It's a fundamental concept in the world of networking. Understanding the OSI model will greatly enhance your knowledge and proficiency in working with computer networks.

Working of OSI model with the help of a flow chart:

OSI 7 Layers Explained the Easy Way