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<P><B><A HREF="01tyt04.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/01tyt04.gif">Figure 1.4. A schematic of a hub network.</A></B>

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<P>Hub networks can be very large, using a high-speed fiber optic backplane and slightly slower Ethernet drops to hubs from which a workgroup can be supported. The hub network can also be small, with a couple of hubs supporting a few devices connected together by standard Ethernet cables. The hub network is scaleable (meaning you can start small and expand as you need to), which is part of its attraction.

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<P>Hub networks have become popular for large installations, in part because they are easy to set up and maintain. They also can be the least expensive system in many larger installations, which adds to their attraction. The backplane can extend across a considerable distance just like a bus network, whereas the ports, or connection points, are usually grouped in a set placed in a box or panel. There can be many panels or connection boxes attached to the backplane.

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<FONT SIZE=4 COLOR="#FF0000"><B>Wide Area Networks</B></FONT></CENTER></H4>

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<P>As I mentioned earlier, LANs can be combined into a large entity called a WAN. WANs are usually composed of LANs joined together by a high-speed link (such as a telephone line or dedicated cable). At the entrance to each LAN, one or more machines act as the link between the LAN and WAN: these are called gateways. I talk about gateways and the types of gateways used in a WAN in more detail on many of the following days, but for now you need to know only that a gateway is the interface between a LAN and a WAN. The same applies for any LAN that accesses the Internet: one machine usually acts as the gateway from the LAN to the Internet (which is really just a very large WAN).

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<P>Many terms other than <I>gateway</I> are also used. You will hear terms like <I>router</I> and <I>bridge</I>. They are all gateways, but they perform slightly different tasks. To understand their roles (which I mention many times in the next week's material), you need to take a quick look at how WANs are laid out.

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<P>LANs can be tied to a WAN through a gateway that handles the passage of data between the LAN and WAN backbone. In a simple layout, a router is used to perform this function. This is shown in Figure 1.5.

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<P><B><A HREF="01tyt05.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/01tyt05.gif">Figure 1.5. A router connects a LAN to the </B><B>backbone.</A></B>

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<P>Another gateway device, called a bridge, is used to connect LANs using the same network protocol. Bridges are used only when the same network protocol (such as TCP/IP) is on both LANs. The bridge does not care which physical media is used. Bridges can connect twisted-pair LANs to coaxial LANs, for example, or act as an interface to a fiber optic network. As long as the network protocol is the same, the bridge functions properly.

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<P>If two or more LANs are involved in one organization and there is the possibility of a lot of traffic between them, it is better to connect the two LANs directly with a bridge instead of loading the backbone with the cross-traffic. This is shown in Figure 1.6.

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<P><B><A HREF="01tyt06.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/01tyt06.gif">Figure 1.6. Using a bridge to connect two </B><B>LANs.</A></B>

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<P>In a configuration using bridges between LANs, traffic from one LAN to another can be sent through the bridge instead of onto the backbone, providing better performance. For services such as Telnet and FTP, the speed difference between using a bridge and going through a router onto a heavily used backbone can be significant.

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<P>WANs are an important subject, and I look at them again in more detail on Day 13, &quot;Managing and Troubleshooting TCP/IP.&quot;

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<FONT SIZE=5 COLOR="#FF0000"><B>Layers</B></FONT></CENTER></H3>

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<P>Suppose you have to write a program that provides networking functions to every machine on your LAN. Writing a single software package that accomplishes every task required for communications between different computers would be a nightmarish task. Apart from having to cope with the different hardware architectures, simply writing the code for all the applications you desire would result in a program that was far too large to execute or maintain.

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<P>Dividing all the requirements into similar-purpose groups is a sensible approach, much as a programmer breaks code into logical chunks. With open systems communications, groups are quite obvious. One group deals with the transport of data, another with the packaging of messages, another with end-user applications, and so on. Each group of related tasks is called a <I>layer.</I>

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<NOTE>

<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>The layers of an architecture are meant to be stand-alone, independent entities. They usually cannot perform any observable task without interacting with other layers, but from a programming point of view they are self-contained.</NOTE>

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<P>Of course, some crossover of functionality is to be expected, and several different approaches to the same division of layers for a network protocol were proposed. One that became adopted as a standard is the Open Systems Interconnection Reference Model (which is discussed in more detail in the next section). The OSI Reference Model (OSI-RM) uses seven layers, as shown in Figure 1.7. The TCP/IP architecture is similar but involves only five layers, because it combines some of the OSI functionality in two layers into one. For now, though, consider the seven-layer OSI model.

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<P><B><A HREF="01tyt07.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/01tyt07.gif">Figure 1.7. The OSI Reference Model showing all </B><B>seven layers.</A></B>

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<P>The application, presentation, and session layers are all application-oriented in that they are responsible for presenting the application interface to the user. All three are independent of the layers below them and are totally oblivious to the means by which data gets to the application. These three layers are called the upper layers.

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<P>The lower four layers deal with the transmission of data, covering the packaging, routing, verification, and transmission of each data group. The lower layers don't worry about the type of data they receive or send to the application, but deal simply with the task of sending it. They don't differentiate between the different applications in any way.

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<P>The following sections explain each layer to help you understand the architecture of the OSI-RM (and later contrast it with the architecture of TCP/IP).

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<FONT SIZE=4 COLOR="#FF0000"><B>The Application Layer</B></FONT></CENTER></H4>

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<P>The application layer is the end-user interface to the OSI system. It is where the applications, such as electronic mail, USENET news readers, or database display modules, reside. The application layer's task is to display received information and send the user's new data to the lower layers.

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<P>In distributed applications, such as client/server systems, the application layer is where the client application resides. It communicates through the lower layers to the server.

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<FONT SIZE=4 COLOR="#FF0000"><B>The Presentation Layer</B></FONT></CENTER></H4>

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<P>The presentation layer's task is to isolate the lower layers from the application's data format. It converts the data from the application into a common format, often called the <I>canonical representation.</I> The presentation layer processes machine-dependent data from the application layer into a machine-independent format for the lower layers.

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<P>The presentation layer is where file formats and even character formats (ASCII and EBCDIC, for example) are lost. The conversion from the application data format takes place through a &quot;common network programming language&quot; (as it is called in the OSI Reference Model documents) that has a structured format.

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<P>The presentation layer does the reverse for incoming data. It is converted from the common format into application-specific formats, based on the type of application the machine has instructions for. If the data comes in without reformatting instructions, the information might not be assembled in the correct manner for the user's application.

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<FONT SIZE=4 COLOR="#FF0000"><B>The Session Layer</B></FONT></CENTER></H4>

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<P>The session layer organizes and synchronizes the exchange of data between application processes. It works with the application layer to provide simple data sets called <I>synchronization points</I> that let an application know how the transmission and reception of data are progressing. In simplified terms, the session layer can be thought of as a timing and flow control layer.

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<P>The session layer is involved in coordinating communications between different applications, letting each know the status of the other. An error in one application (whether on the same machine or across the country) is handled by the session layer to let the receiving application know that the error has occurred. The session layer can resynchronize applications that are currently connected to each other. This can be necessary when communications are temporarily interrupted, or when an error has occurred that results in loss of data.

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<FONT SIZE=4 COLOR="#FF0000"><B>The Transport Layer</B></FONT></CENTER></H4>

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<P>The transport layer, as its name suggests, is designed to provide the &quot;transparent transfer of data from a source end open system to a destination end open system,&quot; according to the OSI Reference Model. The transport layer establishes, maintains, and terminates communications between two machines.

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<P>The transport layer is responsible for ensuring that data sent matches the data received. This verification role is important in ensuring that data is correctly sent, with a resend if an error was detected. The transport layer manages the sending of data, determining its order and its priority.

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<FONT SIZE=4 COLOR="#FF0000"><B>The Network Layer</B></FONT></CENTER></H4>

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<P>The network layer provides the physical routing of the data, determining the path between the machines. The network layer handles all these routing issues, relieving the higher layers from this issue.

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<P>The network layer examines the network topology to determine the best route to send a message, as well as figuring out relay systems. It is the only network layer that sends a message from source to target machine, managing other chunks of data that pass through the system on their way to another machine.

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<FONT SIZE=4 COLOR="#FF0000"><B>The Data Link Layer</B></FONT></CENTER></H4>

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<P>The data link layer, according to the OSI reference paper, &quot;provides for the control of the physical layer, and detects and possibly corrects errors that can occur.&quot; In practicality, the data link layer is responsible for correcting transmission errors induced during transmission (as opposed to errors in the application data itself, which are handled in the transport layer).

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<P>The data link layer is usually concerned with signal interference on the physical transmission media, whether through copper wire, fiber optic cable, or microwave. Interference is common, resulting from many sources, including cosmic rays and stray magnetic interference from other sources.

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