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The Seven Layer Burrito -- We're Not Talking Mexican Food Home networking seems to be all the buzz these days. It's even becoming a factor in the real estate market. People are installing LANs in their homes for sharing DSL and cable modem connections which are used as a selling point adds to the resale value of the home. In a previous article we discussed how to do some basic networking and some of the terms involved. But what is all of that stuff that is going over the wires? How does it work? What is it made of? It's all based on a model. Some refer to it as, "The Seven Layer Burrito." It's more commonly referred to as the Open Systems Interconnect, or OSI Model. This month we will do an overview of the model to lay some groundwork. Back in the days B.A. (before Apple) when the Internet , and consequently the earliest Local Area Networks (LANs), were being developed, different groups made up their own way of doing things. Xerox had their flavor of Ethernet; IBM was developing Token Ring; so on and so forth. Different hardware protocols were being developed and different routed and routing protocols (the methods by which information is sent over a network) were also being developed. Then the government got involved. They didn't want to have to manage all of this different stuff as they began to integrate networks into their offices so they declared (the way that they do) that a standard needed to be developed. Long story short, that standard ended up being the OSI model. The term "model" is applied because it is just that. It is something for developers to base their programs and systems on so that it will be compatible with other systems based on the same thing. So with that history behind us, lets dig in. The best way to picture it is like a stack of books. You may have heard the term "OSI stack" or "protocol stack." This stack is seven books high. From top to bottom they are: Layer 7: Application There are several ways to remember all of the layers in order. The one that sticks in my mind is, "All People Seem To Need Data Processing." This gives them to you in order from top to bottom. So what do all of these layers do?
Let's start from the bottom this time The Physical layer is just that; the physical medium that the data goes over. This layer defines the specifications of the medium. How many wires? Which wires are for what? What's the voltage? What grade of fiber optic cable? The Data Link layer is where things start to happen. Part of what this layer does is defines how the medium is accessed. It determines if your computer is supposed to listen for an opening or if it is just supposed to start sending data and hope everything goes OK. Another part of it's job is to handle the physical addressing of the devices on your network. This includes the network cards in the computers, the printers, your DSL or cable modem, etc. This address is called the MAC address. I was horribly confused being a Macintosh person the first time I heard this. "PCs can have a MAC address?" MAC stands for Media Access Control. Every network interface has an address. If you want to see the MAC address of your Macintosh you can open the TCP/IP Control Panel, go to the File menu and select Get Info. You should see a hardware address listed. This address comes "burned in" your Network Interface Card (NIC). There is a whole organization devoted to making sure that all NICs have a unique address so there isn't any confusion even on the Internet. The Network layer is where things may (or may not) start to become familiar. While the Data Link layer is concerned with the physical address of your computer, the Network layer is concerned with what network your computer is a part of. If you have DSL or cable modem access you should have heard the term IP address. This is the layer where this address exists. If you have a Mac on an AppleTalk network, your computer has an AppleTalk address. Depending on the kind of network you are on, the network layer is what makes sure that information gets sent in the right direction. The Transport Layer makes sure that the data gets to where it is going. Earlier we mentioned the TCP/IP control panel. The IP part of that name lives in the Network Layer. The TCP part lives in the Transport layer. TCP/IP stands for Transmission Control Protocol/Internet Protocol. The two major players in the TCP layer are TCP and UDP. The major difference being that TCP is self-correcting, meaning that as it is sending and receiving data, if there is a problem it will take steps to correct the bad data. UDP has no built in mechanism to do this, it relies on the layers above it to do this. Have you ever been using Netscape (not to pick on Netscape but it is the program where I saw this message the most) and seen the box pop up that says, "Error: broken socket connection."? This is the layer where that error occurs. The Session layer is that part that negotiates the connection you are making. It lays the ground rules for the kind of communication, how fast, and it also governs the ending of the connection. The Presentation layer is where data formatting is done. It's the one that determines if the file is a jpeg, a gif, a QuickTime movie, the kind of text encoding being used, etc. It is also where encryption is done. The Application layer is a bit deceiving in its title. This isn't an application like Internet Explorer, or Fetch, or an email client. The Application layer is what interfaces the program you are using with the OSI stack. The program you are using sends and received that data to/from the Application layer.
That's about enough of an overview. Over the next couple of months, we will take a couple of these at a time and go into them a bit more in-depth. Having a basic understanding of these things will make your patience with your ISPs tech support a little better. Knowing what can go wrong (or what has gone wrong) seems to make it easier to handle. When we go deeper into these topics it will be from the perspective of Ethernet since that is the most common kind of LAN in homes and small businesses.
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