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Each layer here interacts only with the layer above and below it.

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But there must be some external interaction with the stacks here.

3
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So here I draw some diagrams that will show you two external connections and the link layer interacts

4
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with the physical network connections, transmitting the data in a physical medium.

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Right?

6
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So such as a process of electricity or light and the application layer interacts with the other application.

7
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And an application is a collection of related functionality that provides a service to a user.

8
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And in this figure you are seeing an example of an application that processes an email.

9
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And the service provided by the email application is sending and receiving of messages over a network.

10
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And here typically applications contains these components, which is the network communication.

11
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Uh, this component communicates over the network and processes incoming and outgoing data for a email

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application.

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The network communication is most likely standard protocol such as SMTP, POP3, IMAP or others.

14
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And we also have the content parsers here.

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Oops.

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Actually we are here.

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Content parsers.

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So data transferred over a network usually contains content that must be extracted and processed.

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So content might include textual data such as the body of an email, or it might be pictures or video.

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And we also have the user interface and this is the user interface that user interacts to it.

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So the UI allows the user to we received emails and, and to create a new emails for transmission In

22
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an email application.

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The UI might display emails using the HTML in a web browser and then I will also notify you that the

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user interacting with the UI doesn't have to be a human being.

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It could be another application that automates the sending and receiving of emails through a command

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line tool.