Protocol Layers (OSI & TCP/IP)

Protocol Stack

A structured set of layers where each performs specific communication functions.

OSI Model

A 7 layer reference model for network communication.

Physical Layer (L1)

Handles the physical connection; defines electrical/mechanical properties of transmission.

Data Link Layer (L2)

Ensures reliable frame delivery over a physical link; manages error detection.

Network Layer (L3)

Handles logical addressing and routing of packets (IP layer).

Transport Layer (L4)

Provides end to end communication (TCP/UDP).

Session Layer (L5)

Manages sessions and dialogs between applications.

Presentation Layer (L6)

Performs data translation, encryption, and compression.

Application Layer (L7)

Provides network services such as HTTP, FTP, and SMTP.

TCP/IP Model

A 4layer model used by the Internet: Link, Internet, Transport, and Application layers.

Encapsulation

Process of wrapping data with protocol headers as it moves down the layers.

Decapsulation

Process of removing headers as data moves up the stack.

Layered Architecture

Each layer provides services to the one above and relies on the one below.

Why Layering?

Simplifies design, maintenance, and updates by separating responsibilities.

Example: Air Travel Analogy

Each step (ticketing, boarding, flying) represents a separate layer providing a service.

Service Models

Define what services each layer offers to the next.

Encapsulation Example

Application → Transport → Network → Link → Physical → Transmission → Reception (reverse on destination).