The International Organization for Standardization came up with the Open Systems Interconnection (OSI) conceptual model which provides a standard for diverse computer systems to be able to communicate with each other using standard protocols. The OSI Model can be seen as a universal language for computer networking. It essentially splits the network communication system in to seven stacked abstract layers. Each layer performs a specific function and employs a different protocol. A good understanding of the OSI layers makes it easy to troubleshoot complex network issues.
| Layer | Function |
|---|---|
| 7. Application Layer | – End User Layer, this is where the human-computer interactions happen. – Note that the applications (eg: browser, mail clients) itself are not port of the Application Layer, the layer is only responsible for the protocols and data manipulation that the software relies on to present meaningful data to the user. – HTTP, FTP, IRC, SSH, DNS, SMTP |
| 6. Presentation Layer | – Syntax layer, ensures that the data is in the right format – Encryption, translation, and compression (compresses data received from layer 7 before sending to layer 5) happens here – SSL, SSH, IMAP, FTP, MPEG, JPEG |
| 5. Session Layer | – Responsible for opening and closing communication between the two devices in the network. – Controls ports. – Also synchronizes data transfer with checkpoints. eg: when a large file is being transferred, the session layer could set a checkpoint every 10 MBs. If there is a problem after transferring half data, the session could be resumed from the last checkpoint. Without the checkpoints, this would require a full retransmission. – APIs, Sockets, WinSock |
| 4. Transport Layer | – End-to-End connections – Breaks up (and re-assembles) data received from layer 3 into Segments. – Responsible for flow control and error control – TCP, UDP |
| 3. Network Layer | – Controls the physical path that the data will take (routing). – Connects two ‘networks’ (not devices) – Splits segments in to smaller units called Packets. – IP, ICMP, IPSec, IGMP |
| 2. Data Link Layer | – Facilitates data transfer between two devices on the SAME network – Breaks Packets into Frames – Manages flow control and error control in with in a network. – Works at MAC level – Ethernet, PPP, Switch, Bridge |
| 1. Physical Layer | – Physical structure of the network – cables, switches etc. – Data converted to bits/signals – Coax, Fiber, Wireless, Hubs, Repeaters |
