mirror of https://github.com/tLDP/LDP
73 lines
3.5 KiB
XML
73 lines
3.5 KiB
XML
<sect1 id="Foreward">
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<title>Foreward</title>
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<para>
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It is generally thought that a computer network is a group of two or
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more computers that communicate through a transmission medium with their
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main purpose being to share resources, both physical and logical,
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such as files, devices (such as printers and modems) and services.
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Basically, they consist of two fundamental things, devices and
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connections. However, even this will not be of any real practical
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use since there is no way in which for humans to use the network to receive
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or distribute data. Hence, the basic neccessities for a successful and
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useful computer network are a communication link, an interface to the
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link and software to access the link. Note that for the rest of this
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document, the words "link" and "connection" will be loosely synonymous.
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</para>
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<para>
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So how does the theory of information distribution actually apply in the
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real world?
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</para>
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<para>
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Well, firstly, communication can take a number of forms. Namely, simplex,
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half-duplex and duplex. In the case of simplex, this form of communication
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is when only one party can only listen and the other can only send. In the
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case of half-duplex, each party must be listening or sending but not both.
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Finally, duplex or full-duplex communication occurs when both parties can
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receive and send information at the same time.
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</para>
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<para>
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Secondly, a common standard must be agreed to as to who will take precedence.
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ie. a "common language" to determine whether there should be priority levels
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on messages (in case there is an emergency), what type of error correction
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should occur (ie. whether a simple mechanism such as even-parity will suffice
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or whether something more sophisticated is required such as CRC-32), what
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should happen when an error in communication does occur, etc.... In other
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words, a protocol must be formed. In the network setting, a protocol is an
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agreement between communicating parties/entities on how communication is to
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progress.
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</para>
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<para>
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Thirdly, it has been generally found that the most efficient and least error
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prone method by which to achieve these goals is through 'layering'. This idea
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will be explained further on in the sections entitled Layering and the
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OSI and TCP/IP network layering models but it basically involves abstracting
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information starting from the top where humans see it (messages) and breaking
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it down (ie. datagrames, packets, frames, cells and finally bits). As the
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information is sent up or down further information will be removed or added
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respectively to a 'header' which now forms a part of the original data. It is
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this header which contains the information necessary for error correction,
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message prioritisation, etc....
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</para>
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<para>
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Further, it is as a direct consequence of this abstraction process
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that problems associated with network usage, programming and troubleshooting
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can be eased. For example, to the programmer it allows them to be able to add
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information to the data being manipulated so that there is a lower chance of data
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corruption as it travels through the network, so that transportation of
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the data is sped up and also so that there is some flow control. To the
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system administrator, it can form a valuable idea as to how to fix any
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potential issues. For example, let's say that there seems to be a network
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connection problem. The administrator can proceed in one of either two ways.
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He can move upwards through the layer hierarchy or downwards. Either way, he is
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reducing the problem search space as he proceeds through the stack.
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</para>
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</sect1>
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