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Application Layer of the OSI Model: Definition, Functions & Protocols

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  • 0:01 Application Layer Defined
  • 0:51 The Seven Layers
  • 2:12 Protocols & Functions
  • 6:34 Lesson Summary
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Lesson Transcript
Instructor: Jarvista Rivers
Learn about how the Application Layer of the OSI model allows us to perform basic daily activities. Go over the definition of the Application Layer as well as its various functions and protocols.

Application Layer Defined

We think nothing of following a star on Twitter, listening to music, or researching a paper online. But for us to see and accomplish these things, our electronic devices require services from a network. The Application Layer is where we find the final pieces necessary to accomplish these tasks.

The Application Layer is the ending of data transfer. When you get here, you will find applications like Microsoft Word or programs like Java. It is at this point that the data is in a visual form a user can truly understand, rather than binary zeroes and ones.

Networks are quite complex. The OSI (Open Systems Interconnection) model is used to understand how networks or systems operate. In trying to understand this model, it is best to look at it based on its seven layers. We'll take a look at the OSI model broken down into these seven layers, focusing specifically on the Application Layer.

The Seven Layers

The seven layers can be separated into two parts. Layers one through three are the Transportation set, consisting of the Physical Layer, the Data Link Layer and the Network Layer. The Physical Layer deals with transmitting raw bits over a communication channel. The second layer, the Data Link Layer, makes sure the appropriate physical protocol is assigned to the data. The Network Layer controls the operation of the subnet which are identifiable parts of the network. It decides how the data will be sent to the recipient device.

The fourth layer, the Transport Layer, sustains flow control of data. Error checking and recovery of data are also a part of this layer. It accepts data from above, splits it into small units, and ensures that the pieces all arrive correctly.

Layers five through seven make up the Application set. The Session Layer allows users on different machines to establish sessions between them. The sixth layer, the Presentation Layer, deals with the syntax and semantics of the information transmitted. It converts the data sent from the seventh layer into a standard format that the layers can understand.

The final layer is the Application Layer, and it contains many different protocols. This layer interfaces with the operating system and other applications and communicates data between files, messages, and other network activities.

Protocols and Functions

Now that we have a basic understanding of how the seven layers work, let's focus more on what the Application Layer does. In this layer, data can be generated using applications like Microsoft Excel or streaming audio and video data. But it is also used to transfer information across the Internet or through email. In order for any of these to work there must be protocols, or the set of rules or procedures for handling data or information transfer.

Since the OSI Reference Model does not have its own set of protocols for communication, the layers have their own. The Application layer has several protocols to help with information interchange. Some of the protocols are DNS (Domain Name System), HTTP (Hypertext Transfer Protocol), SOAP (Simple Object Access Protocol), IMAP (Internet Message Access Protocol), POP3 (Post Office Protocol Version 3), SMTP (Simple Mail Transfer Protocol), SIP (Session Initiation Protocol).

As you can see, there are several protocols for data generation and transfer in the Application Layer, so let's look at a few of them in more detail.

Domain Name System

The largest network in the world is the Internet. The information on the World Wide Web is stored as a special type of document file called a web page. The combination of numerous web pages makes up a website. The good thing about web pages is that on them you may find links to other websites or web pages. You get to the other pages by clicking on the hyperlink (highlighted text or image that takes you to another webpage). The hyperlink uses a Universal Resource Locator (URL) to name the pages on the web. The URL gives the name, location, and how to access a web page.

In order for the information to transfer across the Internet, a special set of protocols had to be created to reduce the complexity of Internet addresses. As the Internet became more of a vital part of education, business, and our personal lives, the need for more IP (Internet Protocol) addresses grew dramatically. The old way of using IP addresses would have caused confusion and possible duplication, so the need to evolve was urgent.

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