The OSI Model

The Open Systems Interconnection (OSI) model is a conceptual framework that standardizes how network communication should be organized. Created by the International Organization for Standardization (ISO) in 1984, it divides networking into seven distinct layers.

The Seven Layers

┌───────────────────────────────────────────────────────────────┐
│  Layer 7: Application     │  HTTP, FTP, SMTP, DNS            │
├───────────────────────────────────────────────────────────────┤
│  Layer 6: Presentation    │  Encryption, Compression, Format │
├───────────────────────────────────────────────────────────────┤
│  Layer 5: Session         │  Session Management, RPC         │
├───────────────────────────────────────────────────────────────┤
│  Layer 4: Transport       │  TCP, UDP                        │
├───────────────────────────────────────────────────────────────┤
│  Layer 3: Network         │  IP, ICMP, Routing               │
├───────────────────────────────────────────────────────────────┤
│  Layer 2: Data Link       │  Ethernet, WiFi, MAC addresses   │
├───────────────────────────────────────────────────────────────┤
│  Layer 1: Physical        │  Cables, Radio waves, Voltages   │
└───────────────────────────────────────────────────────────────┘

Layer 1: Physical Layer

The physical layer deals with the actual transmission of raw bits over a physical medium.

Responsibilities:

• Defining physical connectors and cables
• Encoding bits as electrical signals, light pulses, or radio waves
• Specifying transmission rates (bandwidth)
• Managing physical topology (how devices connect)

Examples:

• Ethernet cables (Cat5, Cat6)
• Fiber optic cables
• WiFi radio signals
• USB connections

What it looks like:

Bit stream: 10110010 01001101 11010010 ...
            ↓
Physical:   ▁▁▔▔▁▔▁▁ ▁▔▁▁▔▔▁▔ ▔▔▁▔▁▁▔▁
            (voltage levels on copper wire)

Layer 2: Data Link Layer

The data link layer handles communication between directly connected devices on the same network segment.

Responsibilities:

• Framing: Organizing bits into frames
• MAC (Media Access Control) addressing
• Error detection (not correction)
• Flow control between adjacent nodes

Key Concepts:

• MAC Address: A 48-bit hardware address (e.g., 00:1A:2B:3C:4D:5E)
• Frame: The unit of data at this layer

Ethernet Frame Structure:

┌──────────┬──────────┬──────┬─────────────────┬─────┐
│ Dest MAC │ Src MAC  │ Type │     Payload     │ FCS │
│ (6 bytes)│ (6 bytes)│(2 B) │  (46-1500 B)    │(4 B)│
└──────────┴──────────┴──────┴─────────────────┴─────┘

FCS = Frame Check Sequence (error detection)

Layer 3: Network Layer

The network layer enables communication across different networks—it’s what makes “inter-networking” (the Internet) possible.

Responsibilities:

• Logical addressing (IP addresses)
• Routing packets between networks
• Fragmentation and reassembly
• Quality of Service (QoS)

Key Protocols:

• IP (Internet Protocol): The primary protocol
• ICMP (Internet Control Message Protocol): Error reporting and diagnostics
• ARP (Address Resolution Protocol): Maps IP to MAC addresses

Routing Decision:

   Source: 192.168.1.100
   Destination: 8.8.8.8

   Is destination on local network? NO
   ↓
   Send to default gateway (router)
   ↓
   Router examines destination, forwards to next hop
   ↓
   Process repeats until packet reaches destination

Layer 4: Transport Layer

The transport layer provides end-to-end communication services, handling the complexities of reliable data transfer.

Responsibilities:

• Segmentation and reassembly
• Connection management
• Reliability (for TCP)
• Flow control
• Multiplexing via ports

Key Protocols:

• TCP (Transmission Control Protocol): Reliable, ordered delivery
• UDP (User Datagram Protocol): Fast, connectionless delivery

Port Multiplexing:

Single IP address, multiple applications:

   IP: 192.168.1.100
   ├── Port 80:   Web Server
   ├── Port 443:  HTTPS Server
   ├── Port 22:   SSH Server
   └── Port 3000: Development Server

Layer 5: Session Layer

The session layer manages sessions—ongoing dialogues between applications.

Responsibilities:

• Establishing, maintaining, and terminating sessions
• Session checkpointing and recovery
• Synchronization

In Practice: This layer is often merged with the application layer in real implementations. Few protocols exist purely at this layer.

Examples:

• NetBIOS
• RPC (Remote Procedure Call)
• Session tokens in web applications (conceptually)

Layer 6: Presentation Layer

The presentation layer handles data representation—how information is formatted, encoded, and encrypted.

Responsibilities:

• Data translation between formats
• Encryption and decryption
• Compression and decompression
• Character encoding (ASCII, UTF-8)

In Practice: Like the session layer, this is often absorbed into the application layer. TLS can be considered a presentation layer protocol.

Examples:

• SSL/TLS (encryption)
• JPEG, GIF (image formatting)
• MIME types

Layer 7: Application Layer

The application layer is where network applications and their protocols operate. This is the layer developers interact with most directly.

Responsibilities:

• Providing network services to applications
• User authentication
• Resource sharing

Examples:

• HTTP/HTTPS (web)
• SMTP, POP3, IMAP (email)
• FTP, SFTP (file transfer)
• DNS (name resolution)
• SSH (secure shell)

How Data Flows Through Layers

When you send data, it travels down the stack on your machine, across the network, and up the stack on the destination:

   Sender                                    Receiver
┌───────────┐                            ┌───────────┐
│Application│ ──────────────────────────>│Application│
├───────────┤                            ├───────────┤
│Presentation─────────────────────────────Presentation
├───────────┤                            ├───────────┤
│  Session  │ ────────────────────────────  Session  │
├───────────┤                            ├───────────┤
│ Transport │ ────────────────────────────│ Transport │
├───────────┤                            ├───────────┤
│  Network  │ ────────────────────────────│  Network  │
├───────────┤      ┌─────────┐           ├───────────┤
│ Data Link │ ─────│ Router  │───────────│ Data Link │
├───────────┤      └─────────┘           ├───────────┤
│ Physical  │ ═══════════════════════════│ Physical  │
└───────────┘     Physical Medium        └───────────┘

Each layer adds its own header (and sometimes trailer) to the data—a process called encapsulation.

OSI in the Real World

Here’s an important truth: the OSI model is a teaching tool, not a strict blueprint.

The internet wasn’t built on OSI—it was built on TCP/IP, which predates OSI and uses a simpler four-layer model. Real protocols often don’t fit neatly into single layers:

• TLS spans presentation and session layers
• HTTP is application layer but handles some session concerns
• TCP handles some session-layer functions

The OSI model is valuable for:

• Learning and discussing networking concepts
• Troubleshooting (“Is this a Layer 2 or Layer 3 problem?”)
• Understanding where protocols fit conceptually

But don’t expect real-world protocols to follow it rigidly.

Memorization Tricks

Many people use mnemonics to remember the layers. From Layer 1 to 7:

• Please Do Not Throw Sausage Pizza Away
• Physical, Data Link, Network, Transport, Session, Presentation, Application

Or from 7 to 1:

• All People Seem To Need Data Processing

Summary

The OSI model provides a framework for understanding network communication:

In the next section, we’ll look at the TCP/IP model—what the internet actually uses.