OSI Model Flashcards
Layer 7 – Application
The application layer of the OSI model essentially provides networking options to programs running on a computer. It works almost exclusively with applications, providing an interface for them to use in order to transmit data. When data is given to the application layer, it is passed down into the presentation layer.
Layer 6 – Presentation
The presentation layer receives data from the application layer. This data tends to be in a format that the application understands, but it’s not necessarily in a standardised format that could be understood by the application layer in the receiving computer. The presentation layer translates the data into a standardised format, as well as handling any encryption, compression or other transformations to the data. With this complete, the data is passed down to the session layer.
Layer 5 – Session
When the session layer receives the correctly formatted data from the presentation layer, it looks to see if it can set up a connection with the other computer across the network. If it can’t then it sends back an error and the process goes no further. If a session can be established then it’s the job of the session layer to maintain it, as well as co-operate with the session layer of the remote computer in order to synchronise communications
Layer 4 – Transport
The transport layer is a very interesting layer that serves numerous important functions. Its first purpose is to choose the protocol over which the data is to be transmitted. The two most common protocols in the transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol); with TCP the transmission is connection-based which means that a connection between the computers is established and maintained for the duration of the request. This allows for a reliable transmission, as the connection can be used to ensure that the packets all get to the right place
Layer 3 – Network
The network layer is responsible for locating the destination of your request. For example, the Internet is a huge network; when you want to request information from a webpage, it’s the network layer that takes the IP address for the page and figures out the best route to take.
Layer 2 – Data Link
The data link layer focuses on the physical addressing of the transmission. It receives a packet from the network layer (that includes the IP address for the remote computer) and adds in the physical (MAC) address of the receiving endpoint
Layer 1 – Physical
The physical layer is right down to the hardware of the computer. This is where the electrical pulses that make up data transfer over a network are sent and received. It’s the job of the physical layer to convert the binary data of the transmission into signals and transmit them across the network, as well as receiving incoming signals and converting them back into binary data.
