OSI Model Reference
Click any layer to see PDU, protocols, devices, and exam tips
Click any layer to see protocols, devices, and exam tips.
OSI vs TCP/IP model
Frequently asked questions
What is the OSI model?
The OSI (Open Systems Interconnection) model is a conceptual framework that standardises how different network systems communicate. It divides networking into 7 layers, each with specific functions. It is used for troubleshooting and understanding how protocols relate to each other.
What mnemonic helps remember the OSI layers?
Top to bottom (7→1): 'All People Seem To Need Data Processing' (Application, Presentation, Session, Transport, Network, Data Link, Physical). Bottom to top (1→7): 'Please Do Not Throw Sausage Pizza Away'.
What is the difference between the OSI model and the TCP/IP model?
The TCP/IP model has 4 layers: Application (combines OSI layers 5–7), Transport, Internet (= Network), and Network Access (combines OSI layers 1–2). The OSI model is used for conceptual understanding and troubleshooting; the TCP/IP model reflects how internet protocols actually work.
At which layer do switches operate?
Standard switches operate at Layer 2 (Data Link) using MAC addresses to forward frames. Layer 3 switches can also route packets using IP addresses, operating at both Layer 2 and Layer 3.
What is a PDU?
PDU stands for Protocol Data Unit — the name for the data unit at each OSI layer. Layer 1: Bits. Layer 2: Frames. Layer 3: Packets. Layer 4: Segments (TCP) or Datagrams (UDP). Layers 5–7: Data.