|
|
|
|
||
        |
|
 |
|||||||||||||||||||||||||||||||||
 |
 |
|||||||||||||||||||||||||||||||||
|
The Seven Layer Burrito: Part Two Last month we went through an overview of the OSI model upon which all networking today is based on. It's a model that software/hardware developers use when creating their products to work with a network. Here is a quick review of the seven layers: Layer 7: Application This month we are going to take a closer look at the top four layers.
Layer 7: Application The term "application" can be a bit confusing. It took me a while to figure it out when I first got into all of this. Application does not directly refer to the program you are running, like Internet Explorer. Rather, "application" refers to services that interface with the program you are working in. Since the Internet is probably the most commonly used network we will refer to services the Internet provides. Examples of applications in the Application layer are Simple Mail Transfer Protocol (SMTP), File Transfer Protocol (FTP), Telnet, and Hypertext Transfer Protocol (HTTP). All of this things live in the Application layer of the OSI model. They are the interface point between the rest of the OSI stack and the program you as the user are operating. At this point the form that the information is in is referred to as "data". As it goes down the OSI stack the information has different forms or is referred to differently.
Layer 6: Presentation Once the Application layer has created the data it is handed down to the Presentation layer. This layer of the stack takes care of formatting the data. If it is text, it encodes the data in a known format such as ASCII. If it is graphical, it takes care of the formatting for JPEG, GIF, TIFF, and all of the other various graphical formats. The same is true for QuickTime formatting, MPEG, anything along those lines. The Presentation layer is also where any encryption takes place. At this layer the information is still referred to as "data." At this point, another process is started called "encapsulation." When the Application layer hands the data down to the Presentation layer, the Presentation layer wraps up the data with it's own header which identifies its information. As the information goes down the stack the layer below takes the data from the layer above and wraps it in its own header and footer. Remember that I Love Lucy episode where Lucy and Ethel worked in the candy shop? The candy would come down the conveyer belt and their job was to wrap it in the wrappers. That is essentially what encapsulation is. Imagine many people on the conveyer belt each wrapping the candy in their own wrapper as it comes down the line, adding layer upon layer of wrapper.
Layer 5: Session Once the Presentation layer is done it hands the data down to the Session layer. The Session layer is the part of the stack that controls the initiation of the communication between the two end points. The next time you use Remote Access to connect to your Internet Service Provider open the control panel and watch what happens. You will see it go through the process of establishing a connection, stating the protocols, etc. That is the Session layer in action. Not only does it initiate the communication, it lays down the rules for the communication, maintains the communication, and is responsible for ending it in an orderly manner. Think of it as a mediator between your computer and the end system you are communicating with. Once again the Session layer encapsulates the data from above and sends it down to the layer below it, the Transport Layer.
Layer 4: Transport Two of the major functions of the Transport layer are error detection and flow control. Error detection is just what it sounds like. However, that isn't necessarily the same thing as error correction. Depending on the situation the Transport layer may or may not provide for any error correction. In some cases, it will pass on the message that an error was detected and one of the upper layers of the model will take care of the correction. Flow control is essentially a valve that either opens or closes to allow or restrict the flow of data, depending on the situation. This process is called "windowing." The sender and receiver agree to how much the window should be open initially. As traffic moves they send a series of acknowledgments to say "I received this and am expecting that next." As the data is sent conditions can change resulting in the window opening more to allow more data through. So when you are trying to download that huge file over your modem and its just dragging its feet even though you seem to have a fast connection, this is why.
That's enough techno-bable for today. Next month, we will look at the bottom three layers of the stack. If you would like some more information on these topics, there are some great web resources out there. For some more basic information and links to other resources you can go to http://www.whatis.com. This is a great resource for all things technical for the not-so-technical. For more in-depth information you can surf your way around Cisco's web site (http://www.cisco.com). Just do a search for something like Application, TCP, or other things mentioned in this article. Happy Networking!
|
||||||||||||||||||||||||||||||||||
|
 |
|||||||||||||||||||||||||||||||||
|
