OSI Model
The Open Systems Interconnection (OSI) model is a conceptual framework that standardizes network communication into 7 distinct layers. Each layer has specific responsibilities and communicates with adjacent layers.
Memory Trick (Top → Bottom)
All People Seem To Need Data Processing
Application · Presentation · Session · Transport · Network · Data Link · Physical
Application Layer
PDU: DataProvides network services directly to end-user applications. This is the layer users interact with.
Protocols
Key Functions
- File transfer
- Web browsing
Devices
Presentation Layer
PDU: DataTranslates data between application and network formats. Handles encryption, compression and encoding.
Protocols
Key Functions
- Data translation
- Encryption/Decryption
- Compression
Devices
Session Layer
PDU: DataManages sessions between applications. Establishes, maintains and terminates communication sessions.
Protocols
Key Functions
- Session establishment
- Session maintenance
- Session termination
Devices
Transport Layer
PDU: Segment (TCP) / Datagram (UDP)Provides end-to-end communication, error recovery and flow control between hosts.
Protocols
Key Functions
- Segmentation
- Flow control
- Error control
Devices
Network Layer
PDU: PacketHandles logical addressing and routing of packets between different networks.
Protocols
Key Functions
- Logical addressing (IP)
- Routing
- Path determination
Devices
Data Link Layer
PDU: FrameProvides node-to-node data transfer and handles error detection in the physical layer.
Protocols
Key Functions
- MAC addressing
- Error detection (CRC)
- Flow control
Devices
Physical Layer
PDU: BitsTransmits raw bits over a physical medium. Defines electrical, mechanical and timing specifications.
Protocols
Key Functions
- Bit transmission
- Physical topology
- Signal encoding
Devices
OSI vs TCP/IP Model
| Feature | OSI Model | TCP/IP Model |
|---|---|---|
| Layers | 7 layers | 4 layers |
| Type | Reference/Conceptual | Practical/Implementation |
| Developed by | ISO (1984) | DARPA (1970s) |
| Usage | Teaching & troubleshooting | Internet communication |
| Transport protocols | TCP, UDP, SCTP | TCP, UDP |
| Session & Presentation | Separate layers (5 & 6) | Merged into Application |
Viva / Interview Questions
Q: What does OSI stand for?
Open Systems Interconnection
Q: Which layer is responsible for routing?
Network Layer (Layer 3)
Q: What PDU does the Transport layer use?
Segment (TCP) or Datagram (UDP)
Q: Which layer handles encryption?
Presentation Layer (Layer 6)
Q: What devices operate at Layer 2?
Switches and Bridges
Q: What is the mnemonic for OSI layers (top to bottom)?
All People Seem To Need Data Processing