OSI Model
The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to understand and standardize how different networking protocols interact in a network communication system. It divides network communication into seven distinct layers, each with specific responsibilities.
OSI Model: The 7 Layers
| Layer | Name | Function |
|---|---|---|
| 7 | Application | End-user interface & app services |
| 6 | Presentation | Data translation, encryption, compression |
| 5 | Session | Session control and synchronization |
| 4 | Transport | Reliable data transfer, segmentation |
| 3 | Network | Routing and logical addressing |
| 2 | Data Link | Physical addressing and error detection |
| 1 | Physical | Transmission of raw bits over media |
Detailed Breakdown of Each Layer
Layer 7: Application Layer
Closest to the user.
Provides network services directly to applications (e.g., web browsers, email clients).
Functions:
- Resource sharing
- Remote file access
- Network management
Examples:
- HTTP, FTP, SMTP, DNS, POP3, IMAP, SNMP
Layer 6: Presentation Layer
Translates data between application and network format.
Handles data encoding, encryption, compression.
Ensures that data is in a readable format for the Application Layer.
Functions:
- Character encoding (e.g., ASCII to EBCDIC)
- Data encryption/decryption (e.g., TLS, SSL)
- Data compression (e.g., JPEG, MPEG)
Layer 5: Session Layer
Manages sessions or connections between applications.
Responsible for:
- Establishing, maintaining, and terminating sessions
- Authentication and authorization
- Synchronization checkpoints
Examples:
- NetBIOS
- RPC (Remote Procedure Call)
Layer 4: Transport Layer
Responsible for reliable delivery of data.
Functions:
- Segmentation and reassembly
- Flow control
- Error detection and correction
Protocols:
- TCP (reliable, connection-oriented)
- UDP (unreliable, connectionless)
Layer 3: Network Layer
Deals with routing and logical addressing.
Determines best path for data delivery.
Functions:
- IP addressing
- Packet forwarding and routing
Protocols:
- IP (IPv4/IPv6)
- ICMP, ARP, RIP, OSPF, BGP
Layer 2: Data Link Layer
Provides node-to-node communication.
Responsible for framing, MAC addressing, and error detection.
Sub-layers:
- LLC (Logical Link Control) – Error checking, frame synchronization
- MAC (Media Access Control) – Controls how devices access the media
Examples:
- Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11)
- Switches operate here
Layer 1: Physical Layer
Concerned with the actual transmission of bits over physical media.
Functions:
- Defines hardware specifications (cables, connectors)
- Modulation and signal encoding
- Bit rate control
Examples:
- Cables (Ethernet, fiber)
- Radio frequencies
- Hubs, repeaters
Easy Mnemonic to Remember the Layers (Top to Bottom)
All People Seem To Need Data Processing
- A – Application
- P – Presentation
- S – Session
- T – Transport
- N – Network
- D – Data Link
- P – Physical
Encapsulation in the OSI Model
Each layer adds its own header (and sometimes trailer) to the data:
| Layer | Data Unit Name |
|---|---|
| Layer 7-5 | Data |
| Layer 4 | Segment (TCP) / Datagram (UDP) |
| Layer 3 | Packet |
| Layer 2 | Frame |
| Layer 1 | Bits |
Data goes down the stack (encapsulation), gets transmitted, then up the stack (decapsulation) at the receiver.
Real-World Device Mapping (Simplified)
| Layer | Devices Typically Operating Here |
|---|---|
| 7–5 | Computers, Smartphones (apps) |
| 4 | Firewalls, Load Balancers |
| 3 | Routers |
| 2 | Switches, Bridges |
| 1 | Hubs, Repeaters, Cables |