What Is the OSI Model? All 7 Layers Explained

The OSI model is a 7-layer framework that describes how data travels from one device to another across a network. Understanding each layer helps you troubleshoot problems, understand protocols, and think like a network engineer.

What Is the OSI Model?

The OSI (Open Systems Interconnection) model is a conceptual framework that standardises the functions of a communication system into seven distinct layers. Created by the International Organisation for Standardisation (ISO) in 1984, the OSI model gives engineers and developers a common language to describe how data moves from one device to another across a network.

Think of it as a blueprint — not the actual implementation, but the standard that guides how networking protocols are designed to work together.

## The 7 Layers of the OSI Model

Data travels down the layers on the sender's side and up the layers on the receiver's side.

  Sender                          Receiver
  ┌─────────────────────┐        ┌─────────────────────┐
  │  7. Application     │ ─────→ │  7. Application     │
  │  6. Presentation    │        │  6. Presentation    │
  │  5. Session         │        │  5. Session         │
  │  4. Transport       │        │  4. Transport       │
  │  3. Network         │        │  3. Network         │
  │  2. Data Link       │        │  2. Data Link       │
  │  1. Physical        │ ─────→ │  1. Physical        │
  └─────────────────────┘        └─────────────────────┘

## Layer 1 — Physical Layer

The physical layer deals with the raw transmission of bits over a physical medium — electrical signals, light pulses, or radio waves.

What it does:

Defines cables, connectors, and signal voltages
Controls bit rate and timing
Converts digital bits into physical signals and back

Examples: Ethernet cables (Cat5e, Cat6), fibre optic, Wi-Fi radio signals, USB, Bluetooth

Devices at this layer: Hubs, repeaters, network adapters, modems

## Layer 2-address-resolution-protocol-explained) — Data Link Layer

The data link layer handles node-to-node delivery of data on the same network. It packages bits into frames and manages error detection and physical addressing.

Key sub-layers:

LLC (Logical Link Control) — flow control and error checking
MAC (Media Access Control) — physical addressing using MAC addresses

Examples: Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11), ARP, PPP

Devices: Switches, bridges, network interface cards (NICs)

Data unit: Frame

## Layer 3 — network layer

The network layer is responsible for logical addressing and [Routing](/learning-center/what-is-the-network-layer-osi-model-layer-3) — getting packets across different networks from source to destination.

Examples: IP (IPv4-and-addressing-explained), IPv6), ICMP-and-network-diagnostics-explained), OSPF-border-gateway-protocol-explained), BGP, [Routing](/learning-center/what-is-the-network-layer-osi-model-layer-3) tables

Devices: Routers, Layer 3 switches

Data unit: packet-the-internets-core-protocol)-ip-the-internets-core-protocol)

> This is the layer where IP addresses live.

## Layer 4 — transport layer-the-internets-core-protocol)-the-backbone-of-reliable-internet-communication)

The transport layer-the-internets-core-protocol)-the-backbone-of-reliable-internet-communication) ensures reliable end-to-end communication between applications. It handles segmentation, flow control, error recovery, and reassembly.

Two key protocols:

| Feature | TCP-the-internets-core-protocol) | UDP | |---------|-----|-----| | Connection | Connection-oriented | Connectionless | | Reliability | Guaranteed delivery | No guarantee | | Speed | Slower | Faster | | Use case | Web browsing, email | Video streaming, gaming |

Examples: TCP-the-internets-core-protocol), UDP, TLS (partially)

Data unit: Segment (TCP-the-internets-core-protocol)) / Datagram (UDP)

[port-the-internets-core-protocol)-and-UDP-ports-explained) numbers](/learning-center/what-is-tcp-ip-the-internets-core-protocol) operate at this layer — allowing multiple applications to run simultaneously on one device.

## Layer 5 — session-how-the-world-wide-web-works) Layer

The session-how-the-world-wide-web-works) layer establishes, manages, and terminates sessions (conversations) between applications. It handles authentication and reconnection.

Examples: NetBIOS, RPC (Remote Procedure Call), SQL session-how-the-world-wide-web-works) management, SIP (used in VoIP)

Key functions:

session-how-the-world-wide-web-works) establishment, maintenance, and termination
Checkpointing and recovery (resuming large file transfers)

## Layer 6 — Presentation Layer

The presentation layer acts as a translator between the application and network formats. It handles data formatting, encryption/decryption, and compression.

What it does:

Translation — converts EBCDIC to ASCII, for example
encryption — SSL-how-https-keeps-your-data-safe)/TLS encryption/decryption happens here
Compression — reduces data size for efficient transmission

Examples: SSL-how-https-keeps-your-data-safe)/TLS, JPEG, MP4, ASCII, Unicode, MIME types

## Layer 7 — Application Layer

The application layer is the topmost layer, closest to the end user. It provides network services directly to applications and handles high-level protocols.

Examples: HTTP/HTTPS, DNS, SMTP, FTP, SSH, DHCP, SNMP

Key point: The application layer does not refer to the application itself (like Chrome or Outlook), but to the protocols those applications use to communicate.

## A Handy Mnemonic

To remember the layers from top (7) to bottom (1):

> All People Seem To Need Data Processing

| Letter | Layer | |--------|-------| | A | Application | | P | Presentation | | S | session-how-the-world-wide-web-works) | | T | Transport | | N | Network | | D | Data Link | | P | Physical |

## OSI vs [TCP-the-internets-core-protocol)/IP](/learning-center/what-is-tcp-ip-the-internets-core-protocol) Model

The real-world internet uses the [TCP-the-internets-core-protocol)/IP](/learning-center/what-is-tcp-ip-the-internets-core-protocol) model (4 layers), which maps to the OSI model like this:

| [TCP-the-internets-core-protocol)/IP](/learning-center/what-is-tcp-ip-the-internets-core-protocol) Layer | OSI Equivalent | |--------------|----------------| | Application | Application + Presentation + session-how-the-world-wide-web-works) | | Transport | Transport | | Internet | Network | | Network Access | Data Link + Physical |

The OSI model is primarily used as a teaching tool and troubleshooting framework. When a network engineer says "this is a Layer 3 issue", they are using OSI terminology to quickly communicate where a problem lies.

## Quick Recap

The OSI model has 7 layers: Physical, Data Link, Network, Transport, session-how-the-world-wide-web-works), Presentation, Application
Data flows down the stack on the sender and up on the receiver
Each layer adds its own header (encapsulation) as data goes down
The OSI model is a reference standard — the actual internet runs on the [TCP-the-internets-core-protocol)/IP](/learning-center/what-is-tcp-ip-the-internets-core-protocol) model
Network professionals use OSI layer numbers daily for troubleshooting and communication