Thoughts on Using a Conceptual Model

Now, having read about each of the OSI layers , you still might wonder why this conceptual model is even necessary. You will find that when we start discussing actual network protocols, many of them do not map perfectly to the various layers of the OSI model. First of all, a conceptual model for network communications is useful in that it provides a context for discussing how data is generated by a user and then moved from one computer to another on a network. Breaking the different events involved in network communication into subsets also makes it generally easier to discuss and understand the entire process.

Another good reason for using a layered model is that programmers can actually work on modular protocols that "hook" into the overall conceptual model without really having to worry about the protocols that exist above or below the layer that they are working at. They know that protocols exist at these layers that will get the job done.

For example, even though TCP/IP does not have an individual protocol in its stack that maps to each and every layer in the OSI model, programmers have used this modular concept of different layers as new TCP/IP protocols have been developed. The Hypertext Transport Protocol (HTTP) was developed by programmers who knew that protocols already existed in the TCP/IP stack that took care of the Transport, Network, and Data Link layer functionalities.

HTTP is the protocol that provides us with the ability to use Web browsers to access Web sites on the World Wide Web. Programmers designed it to plug into the protocols already available in the TCP/IP stack.

Now that we've spent some time discussing the theory and concepts of network communication, we can take a look at the protocol stacks that are actually used. We will start with TCP/IP, which, due to the Internet, has become the worldwide standard protocol stack for computer networking.



The different frame types are actually defined by specifications devised by the IEEE (Institute of Electrical and Electronic Engineers). The IEEE also divided the Data Link layer of the OSI model into two sublayers : the Logical Link Control (LLC) layer and the Media Access Control (MAC) layer. The MAC sublayer is related to the hardware-addressing scheme that was devised by the IEEE. The LLC sublayer establishes and maintains the link between the sending and receiving computers as data moves across the network's physical media. The LLC sublayer also provides service access points (SAPs) , which are reference points that other computers sending information can refer to and use to communicate with the upper layers of the OSI stack on a particular receiving node.

Absolute Beginner's Guide to Networking
Absolute Beginners Guide to Networking (4th Edition)
ISBN: 0789729113
EAN: 2147483647
Year: 2002
Pages: 188
Authors: Joe Habraken © 2008-2017.
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