Chapter 5 gives us a justification for the next seven chapters. The objectives important to this chapter are on page 5-1:
Concepts:This chapter has the task of convincing us that it is important to read and learn the next seven chapters, which teach us about the seven layers of a theoretical network model. This is not the practical knowledge of NetWare that we get from other courses in this sequence, but it gives us a framework in which to understand not only NetWare, but other network operating systems that are compared in later chapters. A series of questions appears on page 5-2, leading us to a discussion of how things happen on a network. The questions are meant to raise your interest about things like how, how much, when, what if, what if not... On page 5-3, you learn that the way a network works can be understood in terms of a model of a network that was created by the International Organization for Standardization, whose initials are ISO (well, we aren't told what language the initials are in.) Their model is called the Open Systems Interconnection (OSI) Reference Model, hence the ISO-OSI model. Once you understand this model, you will have a general, powerful reference for examining and comparing networks. The seven layers of the model are usually written in a list, numbering
the top as layer seven and the bottom as layer one.
Several mnemonic sentences exist to help us remember the proper order. I recommend "Please Do Not Throw Sausage Pizza Away", because this is in the correct numeric order (bottom to top). If you must have one that goes from top to bottom, try "All People Studying This Need Drastic Psychotherapy". Warning: you MUST remember the correct order, the correct numbers and the correct details for each layer. The processes that happen in each layer communicate with the next layer. Which way is next, up or down? It depends whether data is being passed out of the stack (down) or into it (up). Typically, a computer generates a request starting at the top layer, and working down. The request is passed across the network (probably to a server) and the received request is passed up the layers. When a response is generated, the process reverses. As a request is passed down the layers, it is like the graphic on page 5-5. At each new layer, a header is added to the request. The header holds information that is added to the request by processes at that layer. So, by the time the request leaves the computer, it has had six more parts added to it. When the request is received by the server, the receiving layers strip off the information added to the request by their counterpart layers, hence the illustration showing the request increasing in size as it leave the requester, and decreasing in size as it is processed by the receiver. This is a simplification to get the idea across to you, but then ,"it's only a model". (Patsy, Monty Python and the Holy Grail.) On page 5-6, we are given a bit of terminology about what the service
data units (globs of data) are called at each layer. Note that all
layers involve the term "packets", and most have other terms specific
to them as well:
Chuck Wineinger (a previous student) offers a mnemonic sentence for the major service data unit for each layer: By Finding Diamonds, Sally Preferred Picky Men. Make it "Dear Sally", if you want to remember that Datagrams fit on Layer 4, too. Finally, page 5-7 gives us another metaphor for the model. Consider each layer of the model as being like a shelf in a bookcase. There are seven shelves, and each shelf has several books on it. The books represent the topics that we discuss in our study of that layer. In fact, in each book are discussions of the various methods that relate to that topic. A method is a way of implementing a task the topic covers. For instance, the Physical layer includes the Connection Type topic (a task), and we will see that there are two classic methods for making connections (ways to do it). Using the metaphor of the bookcase, your author explains that the ISO might have put some books on one shelf or another, as they fit the subject matter of more than one shelf. This is to explain why some actual network examples we will see do not fit the model as cleanly as they might. There is no definitive rule that says either is right, only that each must follow its own standard. |