OSI Model - 7 Layers

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.

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.

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).

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.

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.

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.

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.