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The application layer of the seven-layer OSI model is the top layer that approaches protocols for application interaction with the network.
With a focus on end-user services, the application layer helps to facilitate process-to-process connections over Internet protocol.
In order to understand individual layers of the OSI model, it's important to know that this is much more of a framework than a concrete set of designations.
Some experts describe these abstraction layers as being like an onion — where each layer protects the layers underneath it, and drilling down into the seven-layer model means peeling away each subsequent layer to look inside.
First, the seven layers of the OSI model are identified this way — inside the application layer is the presentation layer. Then there's the session layer, the transport layer, the network layer, the data link layer, and at the bottom, the physical layer that deals with physical connections of hardware inside the network.
Abstraction is the principal where processes become built on simpler or more basic premises representing the inside of the onion, or in another metaphorical sense, the bottom block of a technological tower.
As for the application layer specifically, this layer is what deals with protocols such as FTP and Telnet that relate to the handling of IP traffic. Web browsers, SNMP protocols and HTTP protocols, or HTTP’s successor HTTPS, are other examples of application layer systems.
Another way to understand how the application layer works is to look at the presentation layer, which is often characterized as involving encryption and other formatting that will support the application layer itself. Experts describe the presentation layer as the “syntax layer” and explain that it transforms data into a form that the application layer can accept.
One more simplified example of characterizing the seven network layers, including the application layer, is to simply use the terminology. In other words, the session layer deals with sessions between network components — where the presentation layer, as mentioned, deals with encryption and formatting, and the application layer deals with application activities. Further in, the data link layer, for example, is primarily related to handling internal network activity between nodes.
Understanding the OSI seven-layer model better helps network administrators to refine network builds and implement specific functionality and protocols that will accommodate network performance. The application layer is useful in understanding how data flows through a network at the application level.
The use of application programming interfaces also relates to the use of the application layer. Some of the ways that applications “talk” to each other using APIs as a sort of connective tissue will be worked out on the application layer level.
The OSI layer 7, the application layer, could also have lots of applications to the Internet of Things (IoT) as IoT-connected devices proliferate. Some experts have predicted that in a few years, we will have 500 billion connected devices communicating.
Some of the application layer functionality of the OSI model may also apply to “Web 3.0,” tomorrow’s theorized semantic web, in which different and new kinds of connectivity will take place online.