OSI model: Created by ISO ( international Standard Organization) in the 1980's
Application ------------- Data; end users services like HTTP, FTP, SMTP, etc
Presentation ----------- Data; how the data is presented; encryption, compression, JPEG, ASCI
Session ------------------ Data; takes the port numbers and delegates sessions on the said ports
Transport --------------- Segments; Transports via TCP or UDP and assigns port numbers
Network ----------------- Packets; IP addressing and Routing Protocols like RIP, OSPF, BGP, etc
Data-Link ---------------- Frames; Ethernet and ARP / MAC
Physical ----------------- Bits or Cables; Ethernet
The OSI, open system interconnect model is a model of how a network passes data and information. It is used to understand and troubleshoot a network.
A tip to remember the OSI model is People Do Not Throw Sausage Pizza Away.
Layer 1 = Physical; its all about media transmissions. Its related to the functions of activating, deactivating and maintaining a physical data link. Physical data links can be in the form of fiber, coaxial, wireless and even connectors / components that connect devices to physical media.
Layer 2 = Data Link; defines access to the media like Ethernet on a fiber cable, which is a protocol on this layer. It is also used to define the devices that are trying to access the media. A MAC address is an example of defining a device.
Layer 3 = Network; once the devices are interconnected via the data link layer, it is now time to connect to the global internet. This layer routes packets across the network to its intended destinations. This layer also defines the logical addressing aka IP addressing with the understanding of redundant paths in which the best path is selected to deliver data.
Layer 4 = Transport; deals with actual data transport from Host-to-Host. It hides the complexity of the network from the upper layers (5-7) so, application developers can build their applications without thinking about how they are going to deal with the network. It transports by dealing with the multiple paths by the network layer and using them to establish and maintain connections. It makes decisions on whether this communication is reliable or not. (Duplex)
Layer 5 = Session; this layer deals with establishing, maintaining and terminating sessions related on the hosts end. It will take the assigned port numbers from the transport layer and delegate them through multiple users / applications.
Layer 6 = Presentation; deals with formatting the data. Whether the data is in the form or photos, videos or translation of languages, this layer is all about how the data is presented to the user. (JPEG / ASCI)
Layer 7 = Application; closest to the user, defines the services being offered to the user like email, file transfers, voice and etc.
Original blog from: Pearson