OSI Reference Model
The Open Systems Interconnection (OSI) model is a conceptual framework that standardises the functions of a communication system into seven distinct layers. Each layer serves the layer above it and is served by the layer below it.
| Layer | Name | Description |
|---|---|---|
| 7 | Application | Network services directly to end-user applications. Protocols include HTTP, FTP, SMTP, DNS, Telnet, SNMP. |
| 6 | Presentation | Data translation, encryption, and compression. Ensures data from the application layer is readable by the receiving system. |
| 5 | Session | Establishes, manages, and terminates sessions between applications. Handles dialog control and synchronisation. |
| 4 | Transport | End-to-end data transfer, reliability, flow control, and error recovery. Protocols include TCP and UDP. |
| 3 | Network | Logical addressing and routing. Determines the best path for data delivery. Protocols include IP, ICMP, OSPF, BGP. |
| 2 | Data Link Layer | Physical addressing (MAC), frame synchronisation, error detection, and media access control. Protocols include Ethernet, PPP, Frame Relay. |
| 1 | Physical | Transmission and reception of raw bit streams over a physical medium. Defines cables, connectors, voltages, and signalling. |