The seven layers are:
- Data link
Let’s discuss amongst ourselves.
Application (Layer 7)
When a program needs to send a network communication, it first interacts with the application layer. In the OSI context, “application” doesn’t mean Excel, Word, or their ilk. Instead, Layer 7 is the protocol a program like Outlook or Internet Explorer uses to send network communications (think SMTP and HTTP). For example, if you use a file transfer program to send a file to a coworker, the program interacts with the application layer and decides what protocol(such as FTP, TFTP or SMB) it will use to send your file.
Presentation (Layer 6)
The presentation layer transforms the data you send into a universally recognizable format. Different devices format data in different ways. If your computer tries to communicate with a different kind of computer, Layer 6 ensures that the other computer can understand the data you send. Layer 6 protocols include ASCII and MIDI. This layer also handles data encryption, when necessary.
Session (Layer 5)
If everyone talks at once, no one hears a thing. The session layer negotiates and maintains your connections to other devices. It makes sure that sending and receiving devices can communicate with each other without “talking” over one another. Layer 5 also handles dismantling the connection when your communication ends. Layer 5 protocols include NetBIOS and session establishment for TCP.
Transport (Layer 4)
The transport layer prepares your data for transmission across the network. Your computer communicates with the receiving computer to decide how to break up your data into separate pieces, how to make sure none of the fragments get lost, and how to verify all the fragments arrived. Layer 4 prepares data in this way using protocols such as TCP or UDP.
Network (Layer 3)
The network layer makes sure your data knows how to get from your network to the network you want to communicate with. In other words, Layer 3 handles Internet Protocol (IP) androuting. Any time an IP address shows up, think “Layer 3.” Layer 3 protocols include ICMP, IPSec, ARP, and BGP.
Data Link (Layer 2)
This layer makes sure that your computer’s abstract address (for example, its IP address) gets associated with your physical computer. It does so using protocols like MAC. Layer 2 protocols work within a network, but can’t travel out to other networks. (Doing so would require routing, which happens at Layer 3.) Layer 2 specifications include Ethernet, Token Ring, and PPP.
Physical (Layer 1)
This is the layer that handles the actual electrical and physical transmission of your data over some sort of network medium. The physical transmission might travel over different types of wires (e.g., shielded, unshielded, twisted pair), through the air (wireless), or even via light (fiber optic). What voltage? How many pins in the cable? Following Layer 1 specifications ensures the data physically gets from one end to the other. Layer 1 specs you might’ve heard of include RS-232, DSL, and 10Base-T.
When your computer needs to send a network communication, it hands its message to the application layer. The application layer selects a protocol, then passes the data to the presentation layer, and so on. Your data works down through the rest of the OSI layers until it ends up on Layer 1, whereupon it travels over some medium as electronic bits of data. Eventually, the receiving network gets your message and processes it in the reverse order, starting from the physical layer and working the data back up to the application layer. You can find a great graphical analogy to this process on Wikipedia’s OSI Model page. For fun, also check out Dick Lewis’s humorous James Bond analogy for the OSI model.
Credit :: Watchguard and Wikipedia