OSI (Open Systems Interconnection)

The Open Systems Interconnection (OSI) model is a conceptual framework that divides network communications functions into seven layers. Sending data over a network is complex because various hardware and software technologies must work cohesively across geographical and political boundaries.

Reference Model is a conceptual framework used to understand and standardize the functions of a telecommunication or computing system, without regard to its underlying internal structure and technology. It divides network communication into seven distinct layers, each with specific functions. Here’s an overview of each layer:

1. Physical Layer (Layer 1)

Function: Handles the physical connection between devices. This includes the transmission of raw bitstreams over a physical medium (like cables, switches, and connectors).

Examples: Ethernet cables, fiber optics, radio frequencies, and hardware interfaces.

2. Data Link Layer (Layer 2)

Function: Provides node-to-node data transfer and handles error detection and correction from the Physical Layer. It ensures reliable communication across the physical network.

Examples: Ethernet, Wi-Fi, and Frame Relay. Protocols such as ARP (Address Resolution Protocol) operate here.

3. Network Layer (Layer 3)

Function: Manages routing and forwarding of data packets across the network. It is responsible for logical addressing and routing decisions to ensure data reaches the correct destination.

Examples: IP (Internet Protocol), ICMP (Internet Control Message Protocol), and routing protocols like OSPF (Open Shortest Path First) and BGP (Border Gateway Protocol).

4. Transport Layer (Layer 4)

Function: Ensures reliable data transfer between systems. It manages end-to-end communication, data flow control, and error recovery.

Examples: TCP (Transmission Control Protocol) for reliable connections, and UDP (User Datagram Protocol) for simpler, connectionless communication.

5. Session Layer (Layer 5)

Function: Manages sessions or connections between applications. It handles the setup, coordination, and termination of sessions.

Examples: APIs and protocols that support session management, such as RPC (Remote Procedure Call) and SMB (Server Message Block).

6. Presentation Layer (Layer 6)

Function: Translates, encrypts, and compresses data. It ensures that data is in a readable format for the application layer. This layer handles data representation and encoding.

Examples: Data format conversion (e.g., from ASCII to EBCDIC), encryption (e.g., SSL/TLS), and compression (e.g., JPEG, GIF).

7. Application Layer (Layer 7)

Function: Provides network services directly to end-user applications. It interacts with software applications to provide network services.

Examples: HTTP (Hypertext Transfer Protocol), FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol), and DNS (Domain Name System).

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