OSI Model - 7 Layers

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1. Physical Layer

• Responsible for the actual physical connection between devices.
• Handles transmission of raw binary data over the medium (e.g., copper wires, fiber optics).
• Deals with aspects like voltage levels, timing, and data rate (bit rate).
• Includes hardware components like cables, switches, and network interface cards (NICs).
• Concerned with modulation, signal strength, and noise immunity.

2. Data Link Layer

• Ensures reliable data transfer between two directly connected nodes (e.g., within a local network).
• Divided into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).
• LLC handles error detection and flow control.
• MAC addresses access to shared media and manages hardware (MAC) addressing.
• Responsible for framing, addressing, error detection (CRC), and retransmission of lost frames.

3. Network Layer

• Manages the delivery of packets across multiple networks (routing).
• Handles logical addressing through IP addresses (IPv4/IPv6).
• Determines the best path for data to travel across networks (routing algorithms, such as OSPF, RIP, BGP).
• Responsible for packet forwarding, fragmentation, and reassembly.
• Provides connectionless communication (e.g., the Internet).

4. Transport Layer

• Ensures end-to-end communication between devices.
• Handles segmentation and reassembly of data (breaks data into segments).
• Provides error recovery, flow control, and retransmission of lost packets.
• Protocols: Transmission Control Protocol (TCP – reliable, connection-oriented) and User Datagram Protocol (UDP – connectionless, faster but less reliable).
• Manages port addressing, allowing multiple applications to use the network simultaneously.

5. Session Layer

• Establishes, maintains, and terminates communication sessions between applications.
• Provides synchronization, session checkpoints, and recovery (useful in cases of disconnection).
• Manages sessions and dialogues between source and destination systems (e.g., handling sessions in video streaming or remote desktop connections).
• Ensures full-duplex, half-duplex, or simplex communication.

6. Presentation Layer

• Translates data between the application layer and the lower layers (data encoding, translation, and conversion).
• Handles data formats such as encryption, compression, and encoding (e.g., JPEG, PNG, ASCII, EBCDIC, encryption standards like SSL/TLS).
• Ensures that data sent by the application layer is readable by the receiving system, despite potential differences in data representation.

7. Application Layer

• Closest to the end user, this layer interacts directly with software applications that use the network.
• Provides network services to end-user applications (e.g., email, file transfer, web browsing).
• Handles application-specific protocols like HTTP, FTP, SMTP, DNS, POP3 etc.
• Acts as an interface for communication and ensures that the network services meet user needs.