The OSI (Open Systems Interconnection) model was developed by the International Organization for Standardization (ISO) to define how networks should be designed and how devices on a network talk to each other.
Learning the OSI seven layer model is essential to understanding networking. It is a core part of training for Cisco's entry-level certification, the Cisco Certified Network Associate (CCNA).
Each layer in the OSI model works directly with the layer above or below it and has a set function for moving communication across a network. This is done through the use of network protocols, such as TCP (Transmission Control Protocol, which ensures data is delivered correctly), IP (Internet Protocol, which assigns each networked device a logical address) , HTTP (hyper text transfer protocol, used with web browsers and web sites), and countless others.
The OSI Seven Layer Model
The OSI layers and their functions are:
- Application -- Works directly with applications, such as email programs, photo applications and web browsers.
- Presentation -- Works with the Application layer for functions such as encryption, compression and formatting.
- Session -- This layer manages the connections between applications at each end, such as setting up telnet connections and ensuring communication is synchronized.
- Transport -- Handles the actual transfer of data. This layer separates the application-related tasks from the network-related tasks, essentially acting as a buffer between the 2 sides of the model. Specifically, the Transport layer ensures the data is sent correctly and in its entirety.
- Network -- Network, or logical, addressing (such as IP addressing) is handled at this layer, along with routing, congestion control, error control, and general network traffic management.
- Data Link -- This layer is split into 2 sub-layers: Media Access Control (MAC) and Logical Link Control (LLC). The MAC controls a computers access to the data itself, while the LLC handles the actual framing and transport control of that data.
- Physical -- Often refers to the actual Physical equipment of the networked device, such as network ports, cables and power.
Key layers and focus in CCNA training and certification are the Network, Data Link and Transport layers, but understanding all seven layers is essential.
Troubleshooting with the Seven Layers of the OSI Model
The OSI model is also used for troubleshooting network issues, using what's called either top-down troubleshooting (starting from the top layer of the OSI model, or the application on the computer or server), or bottom-up troubleshooting (checking the device for physical problems such as no power, a bad cable or other physical problems).
This is where basic network tools such as ping and telnet come in handy. Ping is a simple network utility that sends a small amount of data over a network and waits for a response from the other device. If there is a response from the other device, then network connectivity has been confirmed and it is usually safe to assume connectivity up to the Network layer, and focus troubleshooting on the upper four layers of the OSI.
Since telnet operates at a even higher layer than ping (the Transport layer) it is also an excellent troubleshooting tool when available. Telnet actually opens up a connection with a device at the other end and depending on how it has been configured, can grant a user access to the other device.
