Osi Modell Cheatsheet by Suhail Bais Ananou

OSI Model Layers Overview

Mnemonic Devices

Remember the order:

All People Seem To Need Data Processing
Please Do Not Throw Sausage Pizza Away

Layer Breakdown

The OSI (Open Systems Interconnection) model is a conceptual framework used to describe the functions of a networking system. It’s divided into seven layers, each with specific responsibilities.

Layers from top to bottom:
7. Application
8. Presentation
9. Session
10. Transport
11. Network
12. Data Link
13. Physical

Encapsulation

Data travels down the OSI model during transmission, each layer adding its own header (encapsulation). On the receiving end, the headers are removed (decapsulation) as data moves up the layers.

Layers 7-4: Application to Transport

Layer 6: Presentation

Function: Data representation, encryption, and decryption.
Protocols: TLS/SSL, MIME, XDR

Example: Encoding data for proper display (e.g., ASCII, UTF-8), encrypting data for secure transmission (SSL/TLS).

Layer 5: Session

Function: Manages connections between applications.
Protocols: NetBIOS, SAP, PPTP, L2TP

Example: Starting, maintaining, and terminating sessions between a client and a server.

Layer 7: Application

Function: Provides network services to applications.
Protocols: HTTP, SMTP, FTP, DNS, DHCP, Telnet, SNMP, POP3, IMAP, SSH, NTP

Example: A web browser (HTTP), email client (SMTP, POP3, IMAP)

Layer 4: Transport

Function: Reliable data transfer, segmentation, and flow control.
Protocols: TCP, UDP

Example: TCP ensures reliable, ordered delivery of data. UDP offers faster, connectionless service.

Layers 3-1: Network to Physical

Layer 3: Network

Function: Logical addressing and routing of data packets.
Protocols: IP, ICMP, IGMP

Example: Routers use IP addresses to forward packets across networks.

Layer 2: Data Link

Function: Physical addressing and error detection for direct connections.
Protocols: Ethernet, Wi-Fi, PPP, Frame Relay, ARP

Example: Ethernet uses MAC addresses for device identification on a local network. Switches operate at this layer.

Layer 1: Physical

Function: Physical transmission of data bits over a communication channel.
Protocols: Ethernet cables, Fiber optics, Wireless signals

Example: Cables, connectors, voltage levels, and radio frequencies.

Key Concepts and Protocols

TCP/IP vs. OSI

TCP/IP Model:
A practical implementation of the OSI concepts, but with fewer layers (4 layers).
Layers: Application, Transport, Internet, Network Access.

OSI Model:
A theoretical model with seven layers, providing a detailed framework for network communication. It serves as a reference for understanding network protocols and functions.

Common Protocols by Layer

Application Layer: HTTP, FTP, SMTP, DNS
Transport Layer: TCP, UDP
Network Layer: IP, ICMP
Data Link Layer: Ethernet, Wi-Fi, ARP

Troubleshooting with OSI

The OSI model aids in network troubleshooting by allowing you to isolate problems to a specific layer. For instance, if you can’t access a website (Application Layer), check lower layers like Network (IP configuration) and Data Link (physical connection).