OSI is a reference model around which the networks are built. Generally it is used as a template and exists only on paper.
- The OSI model defines services, interfaces and protocols very clearly and makes clear distinction between them. It is protocol independent.
- In TCP/IP, services, interfaces and protocols are not clearly separated making it protocol dependent
- This means that OSI is universal for any type of network, whereas TCP/IP is compatible with a subset of network types and the protocols designed to work only with the suite/infrastructure (the Internet)
- The OSI model defines standards for:
- The way in which devices communicate between each other
- The means used to inform devices where to send data and where not to transmit data
- The methods which ensure that devices have a correct data flow rate
- The means used to ensure that devices have a correct data flow rate
- The means used to ensure that data is passed to, and received by the internet recipient
- The manner in which physical transmission media is arranged and connected
The 7 Layers of the OSI Model
- 7. Application layer: Data generated by and usable by software applications. The main protocol used at this layer is HTTP.
- 6. Presentation layer: Data is translated into a form the application can accept. Some authorities consider HTTPS encryption and decryption to take place at this layer.
- 5. Session layer: Controls connections between computers (this can also be handled at layer 4 by the TCP protocol).
- 4. Transport layer: Provides the means for transmitting data between the two connected parties, as well as controlling the quality of service. The main protocols used here are TCP and UDP.
- 3. Network layer: Handles the routing and sending of data between different networks. The most important protocols at this layer are IP and ICMP.
- 2. Data link layer: Handles communications between devices on the same network. If layer 3 is like the address on a piece of mail, then layer 2 is like indicating the office number or apartment number at that address. Ethernet is the protocol most used here.
- 1. Physical layer: Packets are converted into electrical, radio, or optical pulses and transmitted as bits (the smallest possible units of information) over wires, radio waves, or cables.