7 OSI layers

Application

Closes to the end user; provides network services (e.g., file transfer, email, remote access). It handles human-readable data and directory services. It does not include applications themselves, but the interface that let them communicate

Presentation

Transforms data into a common format for layer 7 (e.g.,SSL/TLS, JPEG, MPEG). Performs data translation, encryption/decryption, and compression. it is often called the “Syntax layer”

Session

Controls dialogues between computers. This layer manages session establishment, synchronization, recovery, and termination. It enables checking points so a session can resume after interruption

This layer guarantees reliable end-to-end delivery (TCP) or provides best-effort delivery (UDP). It adds port numbers, performs segmentation, reassembly, flow control, and error handling. 

Network

This layer determines the optimal path for data across multiple networks. It handles logical addressing (IP), routing, packet fragemntation/reassembly, and traffic control 

Data link

This layer ensures error-free frame delivery on the same physical network. It is divded into logical link control (LLC) and Media access control (MAC) sublayers, its functions are framing, physical addressing (MAC), error detection, flow control, access control

Physical

This layer transmit raw bits over a physical medium. It defines bit rate, synchronization, transmission mode (Simple, half-duplex, full-duplex), and topology. It implements tandards sich as ethernet, USB, Bluetooth, and fiber optics.