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datalink layer entities exchange *frames*
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network layer entities exchange *packets*
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transport layer entities exchange *segments*
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application layer entities exchange *SDUs*
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Reference models
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Two reference models have been successful in the networking community : the OSI reference model and the TCP/IP reference model. We discuss them briefly in this section.
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The TCP/IP reference model
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the Application layer
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the Transport layer
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the Internet layer which is equivalent to the network layer of our reference model
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the Link layer which combines the functions of the physical and datalink layers of our five-layer reference model
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Besides this difference in the lower layers, the TCP/IP reference model is very close to the five layers that we use throughout this document.
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The OSI reference model
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Compared to the five layers reference model explained above, the :term:`OSI` reference model defined in [X200]_ is divided in seven layers. The four lower layers are similar to the four lower layers described above. The OSI reference model refined the application layer by dividing it in three layers :
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the `Application layer` that contains the mechanisms that do not fit in neither the Presentation nor the Session layer. The OSI Application layer was itself further divided in several generic service elements.
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Footnotes
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There is now a rough consensus for the greater use of the Unicode_ character format. Unicode can represent more than 100,000 different characters from the known written languages on Earth. Maybe one day, all computers will only use Unicode to represent all their stored characters and Unicode could become the standard format to exchange characters, but we are not yet at this stage today.
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An interesting historical discussion of the OSI-TCP/IP debate may be found in [Russel06]_
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