OSI Model Cheat Sheet
Here is an illustrated cheat sheet for the 7 layers of the OSI Model as used in computer networking. The OSI Model is a conceptual model that describes the universal standard of communication functions of a telecommunication system or computing system, without any regard to the system’s underlying internal technology and specific protocol suites.
A common technical interview question for would-be Network Administrators and Network Engineers is to explain or give a brief overview of OSI Model’s 7 layers. This cheat sheet is a great reference to have saved or to hang near by to quickly reference.
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OSI Model Brief Overview
Developed by the International Organization for Standardization (ISO) in the late 1970s and early 1980s, its primary purpose is to guide product developers and facilitate communication and interoperability between different systems and networks.
Here’s a brief breakdown of the seven layers:
Layer 1 – Physical Layer:
- Concerned with the transmission and reception of raw data bits over a physical medium like a wire.
- Defines the hardware elements involved such as cables, switches, and network interface cards.
- Examples: Copper wires, fiber optics, USB, Hubs, Repeaters
Layer 2 – Data Link Layer:
- Handles the physical addressing (MAC addresses) of frames and the error detection of data packets being sent and received.
- Divided into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC).
- Examples: Ethernet, PPP, Frame Relay, ARP, MAC addresses
- Bridges, Switching
Layer 3 – Network Layer:
- Determines the best path to transfer data from source to destination using logical addressing (like IP addresses).
- Handles packet forwarding and routing.
- Examples: IP, ICMP, OSPF, BGP, RIP
- Routing
Layer 4 – Transport Layer:
- Ensures that data transfer is reliable and error-free.
- Determines how much data to send, at what rate, and in what order.
- Examples: TCP (reliable, connection-oriented) and UDP (unreliable, connectionless), SPX
Layer 5 – Session Layer:
- Manages and controls the dialog between two devices.
- Determines whether communication will be half-duplex or full-duplex.
- Examples: RPC, PPTP, NetBIOS
Layer 6 – Presentation Layer:
- Translates the data format from one system to another.
- Deals with encryption, decryption, compression, and decompression.
- Examples: SSL/TLS, JPEG, MPEG.
Layer 7 – Application Layer:
- The layer where most end-user software operates.
- Provides network services to applications and ensures effective communication between software and lower OSI layers.
- Examples: HTTP, FTP, SMTP, SNMP, DHCP
The OSI model is largely theoretical in that no networking protocol suite operates strictly according to its seven-layer framework. However, it remains an essential tool for teaching and understanding the various tasks involved in data communication.
The more practical TCP/IP model, which is the basis of the internet, condenses these functions into fewer layers but is conceptually similar to the OSI model.
OSI Model vs TCP/IP Model
Both models use a layered approach, allowing for easier troubleshooting and understanding of where different protocols operate.
Both models describe end-to-end data communication from the source to the destination, ensuring data integrity and delivery.
OSI model has seven layers, while the TCP/IP model has four.
The OSI model was developed as a theoretical model, and while some protocols were developed to fit its structure, it was never widely adopted in its entirety. The TCP/IP model, on the other hand, was developed based on protocols already in use, making it more practical and widely used, especially with the rise of the internet.
In the TCP/IP model, the Application layer encompasses the responsibilities of three OSI layers: Application, Presentation, and Session. Similarly, the Network Access layer of TCP/IP amalgamates the functions of the Physical and Data Link layers of the OSI model.
The TCP/IP model was specifically designed around the TCP/IP protocol suite, making it more practical for real-world use. The OSI model, however, was created as a more general framework.
