OSI Model | Computer Networks - Computer Science Engineering (CSE) PDF Download

What is OSI Model? – Layers of OSI Model

The OSI Model, which stands for Open Systems Interconnection, refers to a non-proprietary framework consisting of seven layers, each designated with a specific function. These layers collaborate to facilitate data transmission from one individual to another worldwide. The OSI reference model was developed by the International Organization for Standardization (ISO) in 1984.

OSI Model | Computer Networks - Computer Science Engineering (CSE)

While the OSI model offers a theoretical foundation for comprehending network communication, it is typically not implemented in its entirety in real-world networking hardware or software. Instead, various protocols and technologies are designed based on the principles of the OSI model to ensure efficient data transmission and networking operations.

What Are The 7 Layers of The OSI Model?

The OSI model consists of seven layers, arranged from lowest to highest:

OSI Model | Computer Networks - Computer Science Engineering (CSE)

  1. Physical Layer
  2. Data Link Layer
  3. Network Layer
  4. Transport Layer
  5. Session Layer
  6. Presentation Layer
  7. Application Layer

1.Physical Layer – Layer 1

The Physical Layer is the lowest layer, responsible for the physical connection between devices, transmitting data as bits.

OSI Model | Computer Networks - Computer Science Engineering (CSE)

  • Functions:

    • Bit Synchronization: Provides bit-level synchronization using a clock.
    • Bit Rate Control: Defines the transmission rate (bits per second).
    • Physical Topologies: Specifies device arrangement (bus, star, mesh).
    • Transmission Mode: Defines data flow modes (simplex, half-duplex, full-duplex).
  • Devices: Hub, Repeater, Modem, Cables.

  • Note: Network Layer, Data Link Layer, and Physical Layer are known as Lower Layers or Hardware Layers.

2.Data Link Layer (DLL) – Layer 2

The Data Link Layer ensures error-free data transfer between nodes over the physical layer, using MAC addresses.

OSI Model | Computer Networks - Computer Science Engineering (CSE)

  • Functions:

    • Framing: Transmits meaningful bits by attaching special bit patterns.
    • Physical Addressing: Adds MAC addresses to frames.
    • Error Control: Detects and retransmits damaged or lost frames.
    • Flow Control: Coordinates data rate between sender and receiver.
    • Access Control: Determines device control over a shared communication channel.
  • Devices: Switch, Bridge.

  • Note: Data Link Layer packets are called Frames. Managed by NIC and device drivers.

3.Network Layer – Layer 3

The Network Layer handles data transmission between different networks, routing packets by selecting the optimal path.

OSI Model | Computer Networks - Computer Science Engineering (CSE)

  • Functions:

    • Routing: Determines the best route for packet transmission.
    • Logical Addressing: Uses IP addresses to uniquely identify devices.
  • Devices: Routers, Switches.

  • Note: Network Layer packets are called Packets.

4.Transport Layer – Layer 4

The Transport Layer provides end-to-end data delivery, acknowledging successful transmission and retransmitting data if errors occur.

OSI Model | Computer Networks - Computer Science Engineering (CSE)

  • Functions:

    • Segmentation and Reassembly: Breaks messages into segments and reassembles them at the destination.
    • Service Point Addressing: Uses port addresses to ensure data is delivered to the correct process.
  • Services:

    • Connection-Oriented Service: Reliable and secure transmission with acknowledgments.
    • Connectionless Service: Faster communication without acknowledgments.
  • Devices/Protocols: TCP, UDP, NetBIOS, PPTP.

  • Note: Transport Layer packets are called Segments. Managed by the OS.

5.Session Layer – Layer 5

The Session Layer manages session establishment, maintenance, and termination, ensuring secure communication.

OSI Model | Computer Networks - Computer Science Engineering (CSE)

  • Functions:

    • Session Establishment, Maintenance, Termination: Manages connection lifecycle.
    • Synchronization: Adds checkpoints for error identification and data re-synchronization.
    • Dialog Controller: Manages communication modes (half-duplex, full-duplex).
  • Devices/Protocols: NetBIOS, PPTP.

  • Note: Integrated with Presentation and Application layers in the TCP/IP model as the "Application Layer."

6,Presentation Layer – Layer 6

The Presentation Layer, also known as the Translation Layer, formats and encrypts data for network transmission.

OSI Model | Computer Networks - Computer Science Engineering (CSE)

  • Functions:

    • Translation: Converts data formats (e.g., ASCII to EBCDIC).
    • Encryption/Decryption: Translates data into ciphertext and back to plaintext.
    • Compression: Reduces the number of bits for transmission.
  • Devices/Protocols: JPEG, MPEG, GIF.

7.Application Layer – Layer 7

The Application Layer interfaces with network applications, generating data for transmission and displaying received information.

OSI Model | Computer Networks - Computer Science Engineering (CSE)

  • Functions:

    • Network Virtual Terminal (NVT): Allows remote host login.
    • File Transfer Access and Management (FTAM): Manages remote file access and control.
    • Mail Services: Provides email functionality.
    • Directory Services: Provides global information access.
  • Devices/Protocols: SMTP.

  • Note: Also known as the Desktop Layer.

Note: The OSI model is a reference model and is not implemented on the internet due to its late invention. The current model in use is the TCP/IP model.

The document OSI Model | Computer Networks - Computer Science Engineering (CSE) is a part of the Computer Science Engineering (CSE) Course Computer Networks.
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FAQs on OSI Model - Computer Networks - Computer Science Engineering (CSE)

1. What is the OSI Model and how many layers does it have?
Ans. The OSI Model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven different layers.
2. What are the seven layers of the OSI Model?
Ans. The seven layers of the OSI Model are: Physical Layer, Data Link Layer, Network Layer, Transport Layer, Session Layer, Presentation Layer, and Application Layer.
3. What is the purpose of the OSI Model in computer science engineering?
Ans. The OSI Model helps in understanding how different networking protocols and technologies interact with each other, providing a clear structure for network communication.
4. How does the OSI Model help in troubleshooting network issues?
Ans. The OSI Model allows network engineers to isolate problems by focusing on specific layers, making it easier to identify where the issue lies and how to resolve it efficiently.
5. Can the OSI Model be applied to real-world networking scenarios?
Ans. Yes, the OSI Model serves as a fundamental tool for designing, implementing, and troubleshooting network systems in various industries, making it a valuable concept for computer science engineering professionals.
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