Question

Write notes on OSI Model ?

Answer 1

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1. Introduction

• Definition: A conceptual framework used to describe the functions of a networking system.
• Developer: International Organization for Standardization (ISO).
• Year: Published in 1984.
• Purpose: To guide product implementers so their products will work with other products (Interoperability). It standardizes how different computer systems communicate.

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2. The Structure (7 Layers)

The model divides network communication into seven layers.
* Control Flow: Data moves down the layers at the sender (Encapsulation) and up the layers at the receiver (Decapsulation).

Mnemonic to remember order (Bottom-up):
> Please Do Not Throw Sausage Pizza Away
> (Physical, Data Link, Network, Transport, Session, Presentation, Application)

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3. Layer-by-Layer Breakdown

Layer 7: Application Layer

• Function: The "User Interface" layer. It creates the data and interacts directly with the software application (like a web browser or email client).
• Key Actions: Network access for applications, resource sharing.
• Protocols: HTTP, HTTPS, FTP, SMTP, DNS, Telnet.
• PDU (Protocol Data Unit): Data.

Layer 6: Presentation Layer

• Function: The "Translator." Ensures data is in a usable format for the receiver.
• Key Actions:
  • Translation: (e.g., ASCII to EBCDIC).
  • Encryption/Decryption: (SSL/TLS happens here).
  • Compression: Reduces data size.
• PDU: Data.

Layer 5: Session Layer

• Function: The "Manager." Controls the conversation between two computers.
• Key Actions:
  • Establishing, maintaining, and terminating connections (Sessions).
  • Synchronization (checkpoints) to prevent data loss.
• PDU: Data.

Layer 4: Transport Layer

• Function: "End-to-End Delivery." Ensures data is transferred reliably and without errors.
• Key Actions:
  • Segmentation: Breaking data into chunks.
  • Flow Control: Managing data speed.
  • Error Control: Retransmitting lost data.
• Protocols: TCP (Reliable), UDP (Fast/Unreliable).
• PDU: Segment (TCP) or Datagram (UDP).

Layer 3: Network Layer

• Function: "Path Determination." Decides the physical path the data will take (Routing).
• Key Actions:
  • Logical Addressing: Assigning Sender/Receiver IP addresses.
  • Routing: Moving packets across different networks.
• Hardware: Routers, Layer 3 Switches.
• Protocols: IP (IPv4, IPv6), ICMP.
• PDU: Packet.

Layer 2: Data Link Layer

• Function: "Node-to-Node Transfer." Defines how data is formatted for transmission over the physical medium.
• Key Actions:
  • Physical Addressing: MAC Addresses.
  • Error Detection: Adds a Trailer (FCS) to check for corruption.
• Sub-layers:
  1. LLC (Logical Link Control): Talks to Network Layer.
  2. MAC (Media Access Control): Talks to Physical Layer.
• Hardware: Switches, Bridges, NIC (Network Interface Card).
• PDU: Frame.

Layer 1: Physical Layer

• Function: "Transmission." The actual hardware connection.
• Key Actions: Transmitting raw bitstreams (0s and 1s) over a physical medium.
• Components: Cables (Ethernet, Fiber), Voltage, Radio frequencies (Wi-Fi), Hubs, Repeaters.
• PDU: Bit.

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4. Summary Table

| Layer # | Layer Name | Main Responsibility | PDU (Unit) | Devices/Protocols |
| :--- | :--- | :--- | :--- | :--- |
| 7 | Application | Network Process to Application | Data | Chrome, Outlook, HTTP |
| 6 | Presentation | Encryption & Formatting | Data | JPEG, MP3, SSL |
| 5 | Session | Inter-host Communication | Data | NetBIOS, RPC |
| 4 | Transport | End-to-End Connections | Segment | TCP, UDP, Firewalls |
| 3 | Network | Routing & Logical Addressing | Packet | Routers, IP |
| 2 | Data Link | Physical Addressing (MAC) | Frame | Switches, Ethernet |
| 1 | Physical | Media, Signal, and Binary | Bit | Cables, Hubs |

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5. Key Concepts

• Encapsulation:
  As data moves down from Layer 7 to Layer 1, each layer adds a "Header" (and sometimes a "Trailer" at Layer 2) to the data. This is like putting a letter inside an envelope, then inside a box, then on a shipping truck.
• Decapsulation:
  As data moves up from Layer 1 to Layer 7 at the receiving end, the headers are stripped off layer by layer until only the original data remains.

6. Why use the OSI Model?

1. Troubleshooting: It helps isolate problems. (e.g., "Is this a physical cable issue (Layer 1) or an IP address issue (Layer 3)?")
2. Standardization: Allows hardware from different vendors (e.g., Cisco and Juniper) to work together.
3. Modular Engineering: A change in one layer (upgrading from IPv4 to IPv6) doesn't require changing the other layers (like your cables or applications).