Explain the OSI reference model of networking. Describe all seven layers with their functions.

The OSI (Open Systems Interconnection) model is a conceptual framework developed by the International Organization for Standardization (ISO) in 1984. It standardizes the functions of a communication system into seven distinct layers.

Seven Layers of the OSI Model:

Layer	Name	Function	Protocol Examples
7	Application	Provides network services directly to end-user applications	HTTP, FTP, SMTP, DNS
6	Presentation	Data translation, encryption, and compression	SSL/TLS, JPEG, ASCII
5	Session	Establishes, manages, and terminates sessions	NetBIOS, RPC
4	Transport	Reliable data transfer, flow control, error recovery	TCP, UDP
3	Network	Logical addressing and routing of data packets	IP, ICMP, ARP
2	Data Link	Physical addressing (MAC), error detection, framing	Ethernet, PPP, Wi-Fi
1	Physical	Transmission of raw bit streams over physical medium	Cables, Hubs, Fiber

Key Points:

• Data flows down from Layer 7 to Layer 1 at the sender and up from Layer 1 to Layer 7 at the receiver.
• Each layer adds its own header (encapsulation) during transmission.
• The model separates concerns so each layer can be developed and troubleshot independently.