Lecture 2: OSI Model and TCP/IP

The OSI Model (7 Layers) The Open Systems Interconnection (OSI) model divides network communication into seven specialized layers:

• Layer 7 - Application Layer: Closest to the user; provides network services to applications (e.g., HTTP, FTP, SMTP, DNS, Telnet).
• Layer 6 - Presentation Layer: Translates, formats, compresses, and encrypts/decrypts data (e.g., JPEG, ASCII, SSL).
• Layer 5 - Session Layer: Establishes, maintains, and terminates connections (dialog control) between devices.
• Layer 4 - Transport Layer: Ensures reliable process-to-process delivery, segmentation, flow control, and error recovery. Principal protocols include TCP (reliable, connection-oriented, slower) and UDP (unreliable, connectionless, faster, used for streaming).
• Layer 3 - Network Layer: Manages logical addressing (IP) and routing to find the best path from source to destination.
• Layer 2 - Data Link Layer: Manages node-to-node frame delivery and physical addressing (MAC). It has two sublayers: LLC (Logical Link Control) and MAC (Media Access Control).
• Layer 1 - Physical Layer: Transmits raw bits (0s and 1s) over physical media as electrical, optical, or wireless signals. Handles cables, interfaces, and voltage levels.

Encapsulation Process

• As data moves down the layers from source to destination, each layer adds its own specific control information, known as a Protocol Data Unit (PDU) (e.g., adding headers). The receiving device strips these off in a process called de-encapsulation.

TCP/IP Model

• A practical standard that condenses the OSI model into four layers: Application (incorporating OSI L5, L6, L7), Transport, Network (Internet), and Network Interface (combines Data Link and Physical).