The Open Systems Interconnection (OSI) model is a conceptual framework that describes the functions of a communications system. It divides the communication process into seven layers, each with specific tasks and responsibilities.

Here’s a breakdown of each layer:

1. Physical Layer

• Purpose: Handles the transmission of raw bits over a physical medium.
• Functions:
  • Defines physical characteristics of interfaces and connectors.
  • Specifies signaling methods and bit encoding schemes.
  • Handles bit synchronization and error detection.
  • Transmits and receives raw data.
• Examples: Ethernet cables, fiber optic cables, wireless networks.

2. Data Link Layer

• Purpose: Ensures reliable data transfer between nodes on a network.
• Functions:
  • Frames data into packets for transmission.
  • Detects and corrects errors in data transmission.
  • Handles flow control to prevent overwhelming the receiver.
  • Manages access to the shared medium (e.g., Ethernet).
• Examples: Ethernet protocols (IEEE 802.3), Point-to-Point Protocol (PPP).

3. Network Layer

• Purpose: Routes data packets across the network.
• Functions:
  • Addresses data packets with logical addresses (IP addresses).
  • Determines the best path for data packets to reach their destination.
  • Fragments and reassembles data packets if necessary.
  • Handles routing protocols (e.g., RIP, OSPF, BGP).
• Examples: Internet Protocol (IP), IPv6, routing protocols.

4. Transport Layer

• Purpose: Provides reliable end-to-end communication between applications.
• Functions:
  • Establishes, maintains, and terminates connections between applications.
  • Segments data into smaller units for transmission.
  • Ensures data delivery and error recovery.
  • Provides flow control and congestion control.
• Examples: Transmission Control Protocol (TCP), User Datagram Protocol (UDP).

5. Session Layer

• Purpose: Manages communication sessions between applications.
• Functions:
  • Establishes, manages, and terminates sessions.
  • Synchronizes data exchange between applications.
  • Provides checkpointing and recovery mechanisms.
• Examples: Network File System (NFS), Remote Procedure Call (RPC).

6. Presentation Layer

• Purpose: Translates data into a format suitable for application processes.
• Functions:
  • Encodes and decodes data into different formats (e.g., ASCII, Unicode).
  • Handles data compression and encryption.
  • Converts data between different representations.
• Examples: Encryption algorithms (AES, RSA), data compression algorithms (ZIP, GZIP).

7. Application Layer

• Purpose: Provides services to user applications.
• Functions:
  • Interacts with user applications to provide network services.
  • Implements specific network protocols (e.g., HTTP, FTP, SMTP).
  • Handles user interfaces and data presentation.
• Examples: Web browsers, email clients, file transfer protocols.

Key Points:

• Each layer builds upon the services provided by the lower layers.
• Data flows from the top layer (Application) down to the bottom layer (Physical) and back up.
• Protocols are defined at each layer to govern communication and data exchange.
• The OSI model is a conceptual framework and does not represent the actual implementation of network protocols.